Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Leaf anatomy and photosynthesis : unravelling the CO2 diffusion pathway in C3 leaves
Berghuijs, H.N.C. - \ 2016
Wageningen University; KU Leuven. Promotor(en): Paul Struik; Bart M. Nicolaï, co-promotor(en): Xinyou Yin. - Wageningen : Wageningen University - ISBN 9789462577947 - 286
leaves - plant anatomy - photosynthesis - mesophyll - photorespiration - carbon pathways - solanum lycopersicum - bladeren - plantenanatomie - fotosynthese - bladmoes - fotorespiratie - koolstofpathways - solanum lycopersicum

Keywords: CO2 diffusion, C3 photosynthesis, mesophyll conductance, mesophyll resistance, re-assimilation, photorespiration, respiration, tomato

Herman Nicolaas Cornelis Berghuijs (2016). Leaf anatomy and photosynthesis; unravelling the CO2 diffusion pathway in C3 leaves. PhD thesis. Wageningen University, Wageningen, The Netherlands, with summaries in English and Dutch. 286 pages

Optimizing photosynthesis can contribute to improving crop yield, which is necessary to meet the increasing global demand for food, fibre, and bioenergy. One way to optimize photosynthesis in C3 plants is to enhance the efficiency of CO2 transport from the intercellular air space to Rubisco. The drawdown of CO2 between these locations is commonly modelled by Fick's first law of diffusion. This law states that the flux from the air spaces to Rubisco is proportional to the difference in partial pressure between these locations. The proportionality constant is the mesophyll conductance. Its inverse is mesophyll resistance. Mesophyll resistance is a complex trait, which lumps various structural barriers for CO2 transport and processes that add or remove CO2 along the diffusion pathway. In order to better understand how and to what extent these factors affect photosynthesis, it is necessary to find a more mechanistic description of CO2 transport in the mesophyll. The aim of this dissertation is to investigate how leaf anatomical properties and CO2 sources and sinks along the CO2­ diffusion pathway in C3 leaves affect the photosynthetic capacity of these leaves. In this study, Solanum lycopersicum was used as a model organism. In a first approach, we developed a model in which we partitioned mesophyll resistance into two sub-resistances. The model assumed that CO2 produced by respiration and photorespiration was released between the two sub-resistance components. By quantifying these resistances using measured thicknesses, exposed mesophyll and chloroplast surfaces, and assumed diffusive properties, we were able to simulate the effect of various anatomical properties on photosynthesis. A disadvantage of this two-resistance approach is that it assumes either that (photo)respiratory CO2 release takes place in the outer cytosol or that there is no CO2 gradient in the cytosol. Therefore, in a second approach we modelled CO2 transport, production and consumption by use of a reaction-diffusion model. This model is more flexible in terms of determining the location of CO2 sources and sinks. We developed methods to estimate physiological parameters of this model using combined gas exchange and chlorophyll fluorescence measurements on leaves. The results suggest that the rate of respiration depends on the oxygen partial pressure, which is often not considered in previous photosynthesis models. We also presented a method to calculate the fraction of (photo)respiratory CO2 that is re-assimilated. We found that this fraction strongly depends on both environmental factors (CO2, irradiance), the location of mitochondria relative to the chloroplast, stomatal conductance and various physiological parameters. The reaction-diffusion model and associated methods presented in this study provide a more mechanistic framework to describe the CO2 diffusion pathway in C3 leaves. This model could, therefore, contribute to identifying targets to increase mesophyll conductance in future research.

Ecology of lianas
Schnitzer, S.A. ; Bongers, F. ; Burnham, R.J. ; Putz, F.E. - \ 2015
Oxford : Wiley-Blackwell - ISBN 9781118392492 - 504
klimplanten - plantenecologie - plantenanatomie - plantenfysiologie - evolutie - tropische bossen - bossen - climbing plants - plant ecology - plant anatomy - plant physiology - evolution - tropical forests - forests
A liana is a long-stemmed, woody vine that is rooted in the soil at ground level and uses trees to climb up to the canopy to get access to well-lit areas of the forest. The main goal of this book is to present the current status of liana ecology in tropical and temperate forests. In essence, it is a forum to summarize and synthesize the most recent research in liana ecology and to address how this research fits into the broader field of ecology.
Dendrochronology and bark anatomy of the frankincense tree
Tolera Feyissa, M. - \ 2013
Wageningen University. Promotor(en): Frans Bongers, co-promotor(en): Ute Sass-Klaassen; Frank Sterck. - S.l. : s.n. - ISBN 9789461736444 - 136
boswellia - dendrochronologie - groei - plantenanatomie - harsgangen - boswellia - dendrochronology - growth - plant anatomy - resin canals

Boswellia papyrifera(Burseraceae) trees grow in drylands south of the Sahara. In Ethiopia, it grows in seasonally dry Combretum-Terminalia woodlands. It is a source of frankincense, an economically important olio-gum resin used for cultural and religious ceremonies throughout the world and as raw material in several industries. Ethiopia is a major exporter of frankincense. Currently, the populations of this species are threatened by farmland expansion, fire, overgrazing, improper tapping techniques and possibly also by climate change. Focussing on tree ring analyses and resin-production related bark anatomical features, this study had two objectives.

The first objective was to quantify the status of B. papyrifera populations with respect to radial stem-growth dynamics and size and age structure. Based on analysis of wood structure and crossdating of tree-rings series, it is shown that B. papyriferaforms annual growth rings and that the average age of sampled B. papyrifera trees is 76 years.More importantly, it is shown that the B. papyrifera populations lack trees that recruited over the last 55 years (1955-2010), and that the remnant trees established continuously between 1903 and 1955. This lack of successful recruitment for such a long period of time is attributed to continuous disturbances, such as fire and grazing accompanying new settlements of people into the area over the past decades. Radial growth patterns over decades suggest effects of heavy disturbances that the trees were experiencing. Remarkably, B. papyrifera trees showed a 2-3 year cycle in annual radial growth, and responded significantly to climate. As expected, radial growth increased with rainfall. An increase in ring width with maximum temperature may reflect radiation limits on growth. Radial growth decreased with increasing minimum temperatures, which may reflect temperature impacts on respiration. Overall, the predicted increase in temperature and rainfall for Ethiopia may not pose a direct threat for this species.

The second objective of this study was to describe the resin-secretory structure in the bark of B. papyrifera. The aim was to understand the relationship between structure and functioning of the secretory system with special reference to implications for frankincense yield and improvements of current tapping techniques. Resin canals of B. papyrifera form a three-dimensional network within the inner bark. In the wood, only few radial resin canals were encountered. The intact resin-producing and transporting network is on average limited to the inner 6.6 mm of the inner bark. Within the inner bark, the density of non-lignified axial resin canals decreases from the vascular cambium towards the outer bark. We also show that whole tree properties, such astotal resin-canal area in the bark, stem diameter, tree age, and the number of leaf apices impact frankincense yield.

Finally, this study provides recommendations for improving the existing tapping practice, aiming at maximization of frankincense yield at minimum damage costs to the trees. The new insights can also be used for selection and propagation of trees which are well suited for frankincense production. The information generated in this study is vital for planning sustainable management of the remnant trees and populations of B. papyrifera and the widely demanded frankincense.

Stomatal response characteristics as affected by long-term elevated humidity levels
Fanourakis, D. - \ 2011
Wageningen University. Promotor(en): Olaf van Kooten, co-promotor(en): Ep Heuvelink; Susana Pinto de Carvalho. - [S.l.] : S.n. - ISBN 9789461730015 - 169
huidmondjes - vochtigheid - tuinbouw - vaasleven - plantenfysiologie - abscisinezuur - cuticula bij planten - plantenanatomie - rosa - stomata - humidity - horticulture - vase life - plant physiology - abscisic acid - plant cuticle - plant anatomy - rosa

Restriction of leaf water loss, by stomatal closure, is decisive for plant survival, especially under conditions of water deficit. This sensitivity of stomata to low water potential is attenuated by high relative air humidity (RH ≥ 85%) during growth, which impedes the plant’s ability to survive when subsequently exposed to lower humidities due to a negative water balance. This thesis focuses on the extent of the existing variation and the reasons underlying cultivar differences in their tolerance to high RH, as well as the rate and reversibility of stomatal adaptation to elevated RH in the course of leaf ontogeny.

Cut rose was used as a model plant. An experiment on the postharvest water relations of three contrasting cultivars in their sensitivity to high RH showed that the sensitive cultivar (i.e. steepest decrease in the cut flower longevity) underwent a higher increase in the water loss compared to the tolerant cultivars. Preventing vascular occlusion considerably extended the time to wilting in the sensitive cultivar grown at high RH, showing that the high rate of water loss, as a result of plant growth at high RH, can only be detrimental for keeping quality under limiting water uptake conditions. Further investigation showed a large genotypic variation in the regulation of water loss, as a result of leaf development at high RH, and stomatal closing capacity was the key element in this process. The degree to which the stomatal anatomical features were affected and the extent that their functionality was impaired were not correlated. However, higher stomatal density, longer pore length and depth contributed to the higher water loss of high RH-grown leaves (16–30% of the effect depending on the cultivar). Reciprocal change in RH showed that stomatal functioning was no longer affected by the RH level after full leaf expansion. However, expanding leaves were always able to partly adapt to the new RH level. For leaves that started expanding at high RH but completed their expansion after transfer to moderate RH, the earlier this switch took place the better the regulation of leaf water loss. This behaviour of expanding leaves experiencing a shift from high to moderate RH was related with the increasing population of stomata exceeding a critical stomatal length. Contrary to this, leaves initially expanding at moderate RH and transferred to high RH exhibited poor stomatal functioning, even when this transfer occurred very late during leaf expansion. This suggests that stomata at various developmental stages were similarly prone to loss of closing ability, when these had been exposed to high RH prior to full leaf expansion.

Key words: abscisic acid, cuticular permeability, heterogeneity, hydraulic conductivity, pore aperture, relative air humidity, Rosa hybrida, stomatal anatomy, stomatal conductance, stomatal growth, stomatal initiation, stomatal malfunctioning, stomatal population, stomatal proximity, vase life.

Knoponderzoek Lisianthus : knoponderzoek ten behoeve van de sturing van de oogst van Lisianthus
Labrie, C.W. ; Kersten, M. ; Heij, G. - \ 2008
Wageningen : Wageningen UR, Glastuinbouw (PPO 3242000147) - 27
eustoma - eustoma grandiflorum - snijbloemen - knoppen - plantenanatomie - opbrengstregeling - teelt onder bescherming - cut flowers - buds - plant anatomy - yield regulation - protected cultivation
Teelthandleiding graszaad - plant en gewas
Borm, G.E.L. - \ 2005 2005 (2005)15 juni.
grassen - zaadbehandeling - zaadproductie - plantenanatomie - plantenontwikkeling - gewasopbrengst - akkerbouw - graszaden - teelthandleidingen - grasses - seed treatment - seed production - plant anatomy - plant development - crop yield - arable farming - grass seeds - cultivation manuals
Bij een grasplant komen verschillende organen voor zoals blad, wortel, stengel, bloem en zaad. Spruiten en bloeiwijzen zijn opgebouwd uit meerdere organen.
Teelthandleiding vezelvlas : morfologie
Paauw, J.G.M. - \ 2005 2005 (2005)15 april.
teelt - linum usitatissimum - vlas - plantenmorfologie - plantenanatomie - plantenfysiologie - vezelgewassen - akkerbouw - teelthandleidingen - cultivation - flax - plant morphology - plant anatomy - plant physiology - fibre plants - arable farming - cultivation manuals
In deze teelthandleiding wordt ingegaan op de kieming, vlasstengel, vertakking, de bloem, zaadbollen, zaad, afrijping en de vezel- en bladvorming bij vezelvlas.
Kromme vruchten in de doorteelt van aardbei
Booij, R. ; Meurs, E.J.J. ; Evenhuis, E. - \ 2002
Wageningen : Plant Research International - 24
fragaria - aardbeien - afwijkingen, planten - plantenontwikkeling - plantenmorfologie - plantenanatomie - teeltsystemen - nederland - aardbei - strawberries - plant disorders - plant development - plant morphology - plant anatomy - cropping systems - netherlands
Setaria faberi Herrm. (Chinese naaldaar) in Nederland over 't hoofd gezien
Dirkse, G.M. ; Reijerse, A.I. ; Abbink-Meijerink, C.G. - \ 2001
Gorteria 27 (2001)5. - ISSN 0017-2294 - p. 109 - 114.
setaria faberi - wilde planten - onkruiden - bouwland - akkergronden - flora - identificatie - karakteristieken - determinatietabellen - plantengeografie - plantenanatomie - plantenmorfologie - akkeronkruid - vegetatie - wild plants - weeds - arable land - arable soils - identification - characteristics - keys - phytogeography - plant anatomy - plant morphology
Setaria faberi, een adventief uit Oost- en Zuidoost-Azië, is volledig ingeburgerd als akkeronkruid in maïsakkers maar werd lange tijd over het hoofd gezien wegens de gelijkenis met S. viridis. Beschrijving van de soort, voorkomen binnen en buiten Nederland, en een nieuwe sleutel voor de determinatie van Setaria-soorten
Functional anatomy of the water transport system in cut chrysanthemum
Nijsse, J. - \ 2001
Wageningen University. Promotor(en): O. van Kooten; U. van Meeteren; C.J. Keijzer. - S.l. : S.n. - ISBN 9789058084491 - 148
plantenanatomie - wateropname (planten) - stofverplaatsing - water - plant-water relaties - snijbloemen - chrysanthemum - vaasleven - plant anatomy - water uptake - translocation - water - plant water relations - cut flowers - chrysanthemum - vase life

Cut flowers show a wide variance of keepability. The market demands more and more a guaranteed quality. Therefore, methods must be developed to predict vase life of cut flowers. Chrysanthemum ( Dendranthema x grandiflorum Tzvelev) and some other cut flowers suffer from unpredicted early leaf wilting during vase life. Researchers from Wageningen University and from the Research Station for Floriculture and Glasshouse Vegetables started a joint project to investigate the problem of early leaf wilting and to come to a better prediction of vase life of cut flowers. Preliminary experiments pointed out that early leaf wilting is caused by a decrease of the water uptake due to embolisms that are induced at the cut surface. This thesis reflects a part of the project on early leaf wilting and is focussed on the anatomical aspects of the stem water transport system.

Chrysanthemums are propagated by stem cuttings, which grow in about 12 weeks to commercial maturity. Flowering is induced by a short day treatment. Cut chrysanthemums have an erect stem and the leaves are helically arranged along the stem. The primary vessel bundle network was elucidated, revealing that leaves have their direct water supply from different vascular bundles, which are positioned around nearly half of the circumference of the stem (2.1). The xylem water transport system consists of primary xylem and secondary xylem. The older the stem part (i.e. the lower in the stem) the higher the relative amount of secondary tissue.

Digital image analysis procedures were constructed to enable the quantification of large amounts of anatomical data. Xylem vessel characteristics along the chrysanthemum stems were thus quantified and a mathematical description of the vessel characteristics was developed (2.2). Hydraulic conductivity, amount of vessels, average diameter of vessels, and vessel length all showed a gradual exponential decrease from the base to higher up the stem. The hydraulic resistivity calculated from vessel lumina was 30% lower than the experimentally measured resistivity, irrespective of the position in the stem. The remaining 30% is at least partly caused by the resistance of the intervessel pits.

A new theory was developed to explain the regulation of vessel lengths in plants (2.3). Vessel length depends on the amount of fused tracheary elements. The only assumption in the theory is that each element has the same chance to be the end of the vessel during vessel formation. This results in an exponential vessel length distribution, which indeed is always found in our chrysanthemum stems. The plant can thus determine its vessel length distribution by just steering the chance factor. This theory provides the most simple mechanism that enables plants to regulate the length of xylem vessels. Stochastic regulation of biological processes might be widely present in nature.

We reviewed and refined a method to obtain flat planes in all desired directions through frozen hydrated (biological) specimens (Chapter 3). A combination of proper sample preparation, trimming with a circular diamond saw, tight mounting using indium, and planing with a diamond knife proved to be a useful method to prepare stem xylem tissue for observation in a cryo-scanning electron microscope (cryo-SEM). This planing method is useful for a wide range of other applications of cryo-scanning electron microscopy.

We used cryo-SEM to study the dynamics of emboli in stem xylem vessels (Chapter 4). In accordance with our theoretical assumptions, wide vessels that were cut open at the cut surface appeared to embolise at a lower hydraulic tension than narrower vessels (4.1). The tension needed to embolise all vessels is easily reached in cut flowers, and it therefore can be assumed that under normal postharvest conditions all cut open vessels embolise. Our cryo-SEM results agree with the hypothesis that refilling of embolised vessels is needed to restore the fresh weight of cut chrysanthemums on vase after a dehydration treatment (4.2). Under normal post-harvest conditions air only enters cut open xylem vessels (4.3). The blockage of the xylem water flow due to emboli in cut chrysanthemums is therefore located in the base of the stem in the cut open vessels.

We developed a physical explanation of the mechanisms of induction and removal of emboli in cut open vessels of cut flowers (5.1). The refilling of embolised vessels after dehydration takes place in two phases:

  1. initial rise of vase water into the vessels, resulting in redistribution and compression of the thus entrapped air, and
  2. dissolution of the entrapped air into the surroundings.

It was concluded that the anatomy of the xylem vessels plays an important role in rehydration capability of cut flowers after air aspiration. A compact vessel system (narrow, short and well connected vessels) restores better from emboli than a vessel system consisting of wide, long and loosely connected vessels.

Knowing that the stem anatomy is important with respect to embolism repair, we tried to find an accurate, but easy method to test the stem anatomy on its sensitivity to early leaf wilting (5.2). Within our experiments we found that plants with wider vessels were more sensitive to early leaf wilting, but no absolute thresholds were found at comparing over different experiments. It was confirmed that the anatomy of the stem water transport system is important with respect to vase life quality. However, the combination with several other factors ultimately determines the vase life quality. Xylem thickness and some other stem tissue dimensions seem to be good indicators of the quality of the xylem vessel system and its sensitivity to embolism. A more extensive study is needed to prove the practical value of the use of these indicators. Breeders and growers may use our findings and optimise genotype and growing conditions to obtain chrysanthemums with narrower and shorter vessels at the cut surface in order to prevent the problem of early leaf wilting.

Modelstructuren voor het schatten van het volume c.q. gewicht van komkommers met computer vision
Langers, R.A. - \ 1998
Wageningen : IMAG-DLO (Nota / Dienst Landbouwkundig Onderzoek, Instituut voor Milieu- en Agritechniek P98-25) - 20
cucumis sativus - komkommers - plantenfysiologie - plantenanatomie - plantenmorfologie - modellen - theorie - onderzoek - machine vision - cucumbers - plant physiology - plant anatomy - plant morphology - models - theory - research
Amaranthus bouchonii Thell. (Franse amarant) en Amaranthus hybridus L. (groene amarant) in Nederland
Dirkse, G.M. ; Barendse, R. ; Abbink-Meijerink, C.G. - \ 1998
Gorteria 24 (1998)3/4. - ISSN 0017-2294 - p. 69 - 80.
amaranthaceae - plantenanatomie - plantenmorfologie - planten - habitats - milieu - flora - plantengeografie - taxonomie - plantkunde - nederland - plant anatomy - plant morphology - plants - environment - phytogeography - taxonomy - botany - netherlands
Een onderzoek naar de taxonomie van A. bouchonii en het A. hybridus-complex: areaal, verspreiding in Nederland, habitat en ecologie, morfologie en diagnostische kenmerken
Aster (2): keuringsrapport kleinbloemige herfstasters
Hoffman, M.H.A. - \ 1996
Dendroflora 33 (1996). - ISSN 0374-7247 - p. 3 - 26.
rassen (planten) - cultivars - rassen (taxonomisch) - overblijvende planten - taxonomie - classificatie - biologische naamgeving - plantenplagen - zaden - zaadcontrole - plantenanatomie - plantenmorfologie - herfst - varieties - races - perennials - taxonomy - classification - biological nomenclature - plant pests - seeds - seed testing - plant anatomy - plant morphology - autumn
In het eerste deel in Dendroflora nr. 32 (pag. 6-23) is een overzicht van asters gegeven en zijn de belangrijkste soorten en cultivargroepen beschreven. Als vervolg hierop worden in dit artikel de cultivars van kleinbloemige herfstasters behandeld met gegevens over gebruikswaarde, ziekten en plagen
Aster (1): sortimentsonderzoek herfstasters
Hoffman, M.H.A. - \ 1995
Dendroflora 32 (1995). - ISSN 0374-7247 - p. 6 - 23.
biologische naamgeving - classificatie - cultivars - flora - overblijvende planten - plantengeografie - plantenanatomie - plantenmorfologie - rassen (taxonomisch) - taxonomie - rassen (planten) - herfstteelt - biological nomenclature - classification - perennials - phytogeography - plant anatomy - plant morphology - races - taxonomy - varieties - autumn cultivation
Aster : verspreidingsgebied, morfologie, verschillen in verwante geslachten, indeling in cultuurgroepen, belangrijkste in de herfst bloeiende soorten, ziekten, plagen, vermeerdering, cultuur en gebruik
Philadelphus: sortiments- en gebruikswaardeonderzoek
Hoffman, M.H.A. - \ 1994
Dendroflora 31 (1994). - ISSN 0374-7247 - p. 44 - 70.
plantkunde - cultivars - houtachtige planten als sierplanten - plantenanatomie - plantenmorfologie - rassen (taxonomisch) - taxonomie - rassen (planten) - rassenproeven - botany - ornamental woody plants - plant anatomy - plant morphology - races - taxonomy - varieties - variety trials
Morphology of germlings of urediniospores and its value for the identification and classification of grass rust fungi
Swertz, C.A. - \ 1994
Agricultural University. Promotor(en): J.E. Parlevliet, co-promotor(en): R.E. Niks. - Baarn etc. : Centraal Bureau voor Schimmelcultures - ISBN 9789070351243 - 157
pucciniales - roestziekten - graansoorten - voedselgewassen - plantenanatomie - plantenmorfologie - plantenziekteverwekkende schimmels - planten - identificatie - voedergrassen - pucciniales - rust diseases - cereals - food crops - plant anatomy - plant morphology - plant pathogenic fungi - plants - identification - fodder grasses

The identification and classification of grass rust fungi is often difficult since most traditionally used morphological characters are quantitative and subjective. Besides, when using the host range as a taxonomic criterion, it is important to realize that a rust fungus may have jumped to a new host species and that host range may also be affected by the variability and age of the host plant, and inoculation conditions. The present study was initiated to assess the taxonomic value of the germling morphology of the urediniospores of grass rust fungi.

The germling morphology of grass rust fungi was observed after inoculation on the barley line L94. Since the rate of development and the morphology of the germlings was similar in host and non-host plants until formation of the first haustorium, rust fungi collected from barleys were studied at about 20 h after inoculation and from other grasses at about 40 h.

Germling morphology was proven to be a reliable and useful criterion for identification and classification of grass rust fungi. It enabled an easy discrimination of species complexes that are very similar in traditionally used morphological characters, e.g. Puccinia hordei and P.recondita. Species complexes which are distinct on the basis of these traditionally used morphological characters were also distinct in germling morphology, viz. Puccinia coronata, P. graminis and P.brachypodii. Besides, germling morphology differed greatly between taxa subsumed under the P. recondita and P. brachypodii complexes. In other taxa the differences were mostly quantitative.

The differences and similarities in germling morphology observed within and between species complexes were in general correlated with differences in isozyme banding patterns, nuclear DNA contents, and literature data on several other molecular, biochemical, and hybridization experiments.

The results from the studies on germling morphology and isozyme banding patterns suggest to treat the species included in the P. brachypodii and P. recondita complexes as separate species, to recognize varieties within P. coronata and P.striiformis (including the newly described var. poae), to unite P.hordei and Uromyces rusts from barleys in one species, and not to assign any taxonomic rank yet to taxa subsumed under P.graminis.

The genus Lolium : taxonomy and genetic resources
Loos, B.P. - \ 1994
Agricultural University. Promotor(en): L.J.G. van der Maesen; R.G. van den Berg. - S.l. : Loos - ISBN 9789073771116 - 101
grassen - poaceae - lolium - genenbanken - genetische bronnen - germplasm - hulpbronnenbehoud - genetische bronnen van plantensoorten - taxonomie - plantkunde - iso-enyzmen - enzymologie - plantenanatomie - plantenmorfologie - grasses - poaceae - lolium - gene banks - genetic resources - germplasm - resource conservation - plant genetic resources - taxonomy - botany - isoenzymes - enzymology - plant anatomy - plant morphology

Several aspects of variation within the genus Lolium, and more in detail within Lolium perenne (perennial ryegrass) have been highlighted. As the results are extensively discussed in each chapter, the general discussion is focused on two aspects of the research.

It is clear that the genus Lolium is a very variable genus. The variation within the species reduces the clarity of separation of the species. Stebbins (1956) found the differences between Lolium and Festuca not sufficient to justify two separate genera. He also states that the family of Poaceae is a phylogenetically derived family, and therefore of comparatively recent origin. Differences between species and between genera are still developing. In older families, intermediate forms or species have become extinct, therefore genera and species delimitations are clearer in these families, e.g. the Papaveraceae (Stebbins, 1956). Producing additional information besides morphological data, such as cytogenetic studies and biochemical studies, is of little use for these families as genera and species delimitations are easily made with simple morphological characters. In families of more recent origin, such as the Poaceae, as many characteristics as possible should be used to establish the relationships between the species. In Chapters 2, 3 and 4 several types of characters have been analyzed for the genus Lolium. No phylogenetic analysis was performed with these data, only phenetic analysis due to the nature of the characters used. Results indicate that all species, as mentioned in the general introduction, can be recognized, although species delimitations are not unambiguous. Only for L. persicum and L. temulentum the results indicate that these species could possibly be two varieties of one species. All species show diagnostic characters for one ore more of the different type of characters. Although the overlap of species is often unsatisfactory, joining of species would be even more artificial. As speciation is a continuous process it cannot be predicted at which point in time species are going to be sufficiently delimited.

Man has had large influence on the speciation within Lolium. This is illustrated by the three weedy species within the genus. L. remotum is known as a weed in flax (Hjelmqvist, 1950), L. temulentum and L. persicum are known as weeds in cereals (e.g. Dore, 1950). All three are mimicry weeds, the morphology of the seeds andlor the habit of the plant is similar to the crop in which it grows. Until a few decades back, these three species had a significant impact as weeds, but due to enhanced seed cleaning techniques the distribution area of these species has largely decreased (Hubbard, 1954).

Other examples of the influence of man on the genus Lolium, are the species L. perenne and L. multiflorum. Their distribution area has largely increased due to sowing by man. Scholz (1975) stated that man has had an enormous influence on the development of both species. According to Scholz (1975), this influence started no more than a few thousand years back, with the cutting and burning of forest for replacement by grassland for cattle, and the discovery of hay making. This has made it almost impossible to determine in which parts of the world both species are indigenous. Not only the distribution area of both species is influenced by man but also the phenotype. Selection changes the phenotype in favour of character states desired by man, such as increased yield. Tyler (1979) observed that after a period in which the standard of management is relaxed, natural phenotypes reoccur. This is confirmed with the results from Chapter 5: Dutch perennial ryegrass populations, collected after a period of more relaxed management, have a distinct phenotype compared to cultivars. Tyler (1979) also indicated that the differences between wild and cultivated forms are extremely blurred for L. perenne. This statement is confirmed by the results from Chapter 6: for allozymic variation, cultivars show absolutely no reduction in variation compared to natural populations. Ellenberg (1963) calls the type of plant as L. perenne semi-domesticated, as the crop is not harvested each year but only kept at an acceptable production level using reseeding. During each phase of their lifecycle, populations are exposed to selection forces. Leading to the situation that in grasslands cultivars are often mixed with plants that have been exposed to enviromnental selection for a number of years. This makes the distinction between natural and cultivated grassland extremely vague.

Chapter 5 illustrates, as management is the factor that optimizes the amount of genetic variation found within a location, the enormous influence man has had and still has on the amount of variation in phenotypes of L. perenne. Reduction of the influence of man would probably lead to the existence few differing perennial ryegrass phenotypes, and could in some areas even mean extinction of perennial ryegrass. In the Netherlands, foreland and salt marshes are the only original habitats for grassland (Bink et al., 1984). Although L. perenne is a species with much competitive ability, it would suffer from a large reduction of distribution area in case management of grasslands was totally abandoned. Because mainly under man-made conditions, e.g. fertilizing, treading, intensive grazing, drainage, L. perenne expresses this competitive nature.

For L. rigidum the influence of man is less strong. L. rigidum is used in some parts of the world (e.g. Australia) as a cultivated fodder crop, but in Europe this is not current. In Europe the fate of L. rigidum depends on the perspectives of L. rigidum as a fodder crop in dry areas or as a crossing parent in breeding programmes. Next to its presence in cultivation L. rigidum is well capable to maintain itself under less cultivated circumstances, this in contrast to, especially, L. perenne. Hartley (1956) states that L. rigidum originates from the Mediterranean region and that L. perenne and L. multiflorum originate from the Eurasian region. The ancestral species of the genus Lolium is supposed to have originated in the Mediterranean region (Malik, 1967). This would indicate that L. rigidum could be the wild form for both cultivated species. The relation between wild and cultivated is often confirmed by the reduction of genetic variation within the cultivated forms. Brown (1978) mentions two examples, based on allozyme variation, for which this assumption is valid. Lycopersicon pimpinellifolium has 61 % unique allelic forms compared to those in L. esculentum. Both species share 37% of the allelic variants and 2% is unique for L. esculentum. Oryza perennis has 47% unique peroxidase alleles, and 22% unique esterase alleles, compared to 0. sativa. Both species share 53 % and 78 % of the alleles respectively. The results from Chapter 3 do not indicate a reduction in allelic variation within L. perenne and L. multiflorum, compared to L. rigidum. This indicates that, if L. perenne and L. multiflorum indeed did arise from L. rigidum, this speciation is of recent origin. Phylogenetic relations between species cannot be determined on basis of these data.

L. loliaceum is not known as a weed nor as a crop plant, it mainly grows under poor and maritime conditions. The influence of man on populations of this species is not large, therefore it is not likely that this species becomes extinct nor that its distribution area will suddenly increase. Phenotypic developments are expected to be gradual and slow.
For L. canariense the same holds true as for L. loliaceum, it is not a crop nor a weed and grows under poor conditions, making it a stable and localized species.

The screening of the Lolium species for allozymic variation, added little to the species determination within the genus Lolium. The pattern of allozyme diversity could hardly be linked with taxonomic classification (Chapter 3); mainly because all allelic variants were common in each population screened. As pointed out in the discussion of Chapter 3, this maybe caused by the small number of enzyme systems screened. A question that can be asked is whether increase of the number of allozymes could lead to better results for genotypic screening. In Chapters 3 and 6 the calculated diversity statistics for the cross-breeding Lolium species were above the average for other wind- pollinated cross-breeding species (Hamrick & Godt, 1990). These statistics indicated that a larger proportion than average of the loci screened were polymorphic, and also that the average heterozygosity of the loci was far above the mean. Extension of the number of loci screened would therefore most likely result in finding monomorphic loci or less variable polymorphic loci and would not enhance the results. In literature, analyses of L. perenne populations for several other allozymes are reported. These allozymes are Glutamate- oxaloacetate-transaminase (GOT, Hayward & McAdam, 1977; Arcioni et al., 1988; Charmet et al., 1993), Isocitrate dehydrogenase (IDH: Lallemand et al., 1991; Charmet et al., 1993), Peroxidase (PRX: Charmet et al., 1993) and Superoxide dismutase (SOD: Charmet et al., 1993). All authors report results that confirm the expectation that higher number of allozymes screened do not improve the elucidation of speciation. Again, the within- population variation is too large compared to the betweenpopulation variation.

Another option would be to make use of molecular markers, e.g. restriction fragment length polymorphism (RFLP). Few reports are known for Lolium species, using these techniques. Darbyshire & Warwick (1992) report on the results for one L. perenne population, which was compared with several other grass populations classified in 26 Festuca species and the genera Vulpia, Poa and Puccinella. Eleven restriction endonucleases and twelve restriction fragments from chloroplast DNA of Petunia hybrida Vilm. were used in this analysis. In total 341 bands were observed of which 108 (31.7%) were polymorphic. Of these 108 bands, 34 were detected in the L. perenne population. Only one plant was analyzed from each population. Chloroplast DNA variation in other Lolium species (Lehväslaiho et al., 1987 ; Soreng et al., 1990) is only reported for one L. multiflorum population, using five restrictionenzymes and direct end labelling. Again only one plant has been analyzed and compared with a large set of populations and genera mainly from the family Poaceae. L. multiflorum differs in 11 bands on a total of 144 shared bands with Festuca pratensis. Only one report is known (Wu et al., 1992) on the between-population variation within L. perenne for RFLP's. Five cultivars of perennial ryegrass were screened, using 2 restriction enzymes and 37 probes from Festuca pratensis. Twenty-four (65 %) of these probes hybridized, resulting in on average 69% polymorphism between the five cultivars . On average 3.2 different banding patterns were observed for each restriction enzyme-probe combination. Again only one plant was analyzed for each population.

No reports on the between-species and the within-population variation are known for any of the Lolium species.

The results from Chapter 3 and Chapter 6 indicate that substantial variation is found within populations of the cross-breeding Lolium species, which makes results based on only one plant per population unreliable (Wu et al, 1992). It remains necessary to analyse a minimum number of plants for the crossbreeding Lolium species, unless an acceptable bulk sample can be taken. This would be desirable as otherwise the cost and time needed to analyse a population using molecular markers could be limiting. The danger of using a bulk sample would be that no differences between populations and even between species can be observed (as would be the case for a bulk sample when screening for allozyme variation). Screening of five enzyme systems resulted in a maximum of 10 bands observed ( 13 , if the heterozygous bands were also counted), in case a plant was heterozygous (maximum variation) for each enzyme. This is a much better result as reported by Darbyshire & Warwick (1992), 34 bands out of 132 restriction enzyme-probe combinations. It is equal to the theoretical maximum number of bands reported by Wu et al. (1992), 96 bands in case of heterozygosity at all 48 restriction enzyme-probe combinations. The preliminary conclusion would therefore be that RFLP analysis will not greatly enhance the distinction of crossbreeding Lolium species and populations.

For the inbreeding Lolium species the analysis of few plants is sufficient. The observed variation would probably increase compared to the observed allozyme diversity (Chapter 3: fixation for four of the five enzymes), as the number of possible markers would greatly increase using RFLP's. The use of molecular markers for the screening of inbreeding Lolium populations would therefore be a valuable extension of the knowledge on these species.

Genetic resources: in situ conservation
In the general introduction three research questions were mentioned, concerning the genetic resources of L. perenne. What are the answers to these questions after analyzing the results from four years of research? Firstly, the Dutch populations do form, morphologically, a distinguishable group of forms within the genetic variation for perennial ryegrass. This conclusion is based on the observation of morphological characters only, as the heritability of these characters is better determined than for agronomically important traits as winter hardiness, spring growth etc. The date of ear emergence is one of the most important characters, both agronomically and for the recognition of breeders rights. Results indicate that genetic variation for this character is substantial in the Netherlands: in natural surroundings, populations varying from very early till very late heading could be collected. It is expected that if the Dutch populations show this amount of genetic variation for morphological characters, results can be analogous for agronomical characters. The morphological variation found within the Dutch populations is not comparable with the variation found in the cultivars used in these trials. Although date of ear emergence indicates that some Dutch populations are heading as late as the cultivars, morphologically they are distinct. The Dutch populations are a.o. more prostrate growing and shorter at ear emergence, this could indicate that this phenotype is natural for L. perenne in the Netherlands. The fact that Dutch populations are morphologically clearly distinct from the cultivars and that there was also substantial variation observed between Dutch populations, indicates that in situ conservation is a realistic option for L. perenne. Weibull (1989) gives several advantages and disadvantages for the in situ conservation of forages. The advantages are: continued co-evolution of populations and the possibility to study the ecology of the species. It is also possible to make successive collections, and it avoids space and time consuming activities for storage en regeneration for genebanks. A combination of the genetic resources conservation objective with other objectives like nature conservation could be an option. This possibility is clearly illustrated with the present results, as all Dutch populations were collected in areas managed by nature conservation organisations.

Disadvantages are that it is difficult to determine how many sites, and which sites should be preserved to optimize genetic variation. Natural populations are vulnerable to external factors, such as human influences and extreme weather conditions. Also the costs of the maintenance of conservation sites maybe high, and access of breeders can be a problem in case of a combination with nature conservation objectives.

For the allozyme variation no differences between the Dutch populations and the cultivars were found. Allelic variants were very common in all populations, the cultivars showed much larger differences in allelic frequencies than the Dutch populations. In situ conservation would be very successful in retaining genetic diversity at the allozyme level. The data were not useful for selection of accessions for genebanks. Phillips et al. (1993) reported for Avena sterilis L. (inbreeder), the wild progenitor of A. sativa L., the possibility to separate populations in six different groups based on 23 loci. Selection of genebank accessions can be facilitated using these six groups, combined with morphological data.

Francisco-Ortega et al. (1992) observed for Chamaecytisus proliferus (L. fil.) Link a totally different pattern. Morphologically this species can be separated into seven subspecies, which are morphologically distinct and ecologically each occupy a distinct niche. Allozyme diversity shows no differentiation between these seven subspecies.

Just like in the genus Lolium, almost all allelic variants are common and widespread, and the within-population variation is very large. Also in this case allozyme data were considered of no use for the selection of genebank accessions.

Generally, the usefulness of screening for allozyme variation varies substantially. Compatibility behaviour and age of the genus/species are the major factors, explaining the value of this kind of data.

Morphological study of the formation and development of basal shoots in roses.
Marcelis-van Acker, C.A.M. - \ 1993
Scientia Horticulturae 54 (1993)2. - ISSN 0304-4238 - p. 143 - 152.
gewassen, groeifasen - snijbloemen - groei - groeistadia - plantenanatomie - plantenmorfologie - crop growth stage - cut flowers - growth - growth stages - plant anatomy - plant morphology
Basal shoots are the vigorous shoots at the base of the plant. In roses, basal shoots determine the potential flower production of the plant. Although many attempts have been made to promote the formation of basal shoots for commercial production, little attention has been paid to the origin and development of these shoots. The present study addresses this by following the development of a rose plant, raised from a cutting. Basal shoots only originated from basal axillary buds and not from adventitious buds. The first basal shoot of a plant emerged from one of the two most basal axillary buds of the primary shoot. The second basal shoot also emerged from an axillary bud of the primary shoot or, sometimes, from an axillary bud of the first basal shoot. If a third basal shoot occurred, it originated from an axillary bud of a basal shoot. The buds, which became the first and second basal shoot, were already present as secondary buds in the axils of the scales of the axillary bud when used for propagation. During the development of this primary bud into the primary shoot the secondary buds continued to initiate new leaf primordia, but did not sprout until the growth of the primary shoot slowed down. Removal of these two secondary axillary buds in the primary bud resulted in less basal shoots per plant and the basal shoots developed from buds number 3, 4 or 7.
Proeven met planten : van opzet tot analyse
Hoveyn, C.A. - \ 1991
Wageningen : CABO-DLO - ISBN 9789073384118 - 240
plantenveredeling - methodologie - testplanten - plantenanatomie - plantenmorfologie - onderzoek - experimenten - wetenschap - plant breeding - methodology - test plants - plant anatomy - plant morphology - research - experiments - science
Elongation and contraction of the plant axis and development of spongy tissues in the radish tuber (Raphanus sativus L. cv. Saxa Nova).
Magendans, J.F.C. - \ 1991
Unknown Publisher (Wageningen Agricultural University papers 91-1) - ISBN 9789067541848 - 57
raphanus sativus - radijsjes - histologie - planten - plantenanatomie - plantenmorfologie - onderzoek - methodologie - radishes - histology - plants - plant anatomy - plant morphology - research - methodology
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