Growth, morphogenesis and carbohydrate content of the onion plant (Allium cepa L., cv. 'Wijbo' as influenced by light and temperature, during the entire growth cycle, were studied under field conditions and controlled conditions (phytotron).
A. LIGHT INTENSITY EFFECTS
Plants were grown at various light intensities in the field and in the phytotron.
1. Fresh and dry weight of the entire plant and its various organs, i.e., root, blade, neck, and bulb increased with increasing light intensity. The time trend shows a rise in entire plant fresh and dry weight up to a maximum at the end of the growing season. Bulb weight progressively increased with time to a maximum at the end of the growth period, whereas root, neck, and leaf weight usually increased with time up to a certain moment and then decreased or flattened off; however, in some cases, leaf and neck continued to increase up to the end of experiment.
2. The average daily growth rate over the entire season (dry weight gain per plant per day) increased with increasing light intensity; the increase was exponential in 1964, linear in 1965 and curvilinear in the phytotron experiment. The average NAR over 126 and 135 days after sowing in the 1964 and 1965 field experiments respectively was linearly related to relative light intensity, while in the phytotron a curvilinear relationship was found. Although NAR at the highest light intensity in the phytotron only was about one-half of that in the field experiments, the daily growth rate in the phytotron was about two times that in the field experiments, owing to more vigorous growth in leaf area under controlled conditions.
3. Leaf number per plant was positively correlated with light intensity; at all light intensities, leaf number increased with time to a maximum, and thereafter decreased. Leaf thickness and diameter decreased with reduction of light intensity; this also holds true for leaf length in the early stages of growth, while later on leaf length at 37 % in the 1965 field experiment and at 35 % in the phytotron experiments, surpassed that at all other light intensities. Leaf length/diameter ratio increased with reduction of light intensity.
4. In the early stages of growth, total green leaf area per plant in all experiments increased with increasing light intensity; later on total green leaf area at 35 % light intensity the phytotron experiment exceeded that in all other treatments. In all cases, total green leaf area increased with time to a maximum, and then decreased.
5. There was a positive effect of light intensity on neck diameter; it increased with time to a maximum, and then fell off in most cases. In the 1965 field experiment and in the phytotron experiment, neck length in the 37 and 35 % treatments at a certain moment surpassed that at all light intensities.
6. Leaf area ratio, top/bulb ratio and top/root ratio were negatively correlated with light intensity. With the exceptional rise between the first and the second harvest, leaf area ratio decreased with time to a minimum at the end of the experiment.
7. The reduction of light intensity to as low as 12 % in the field experiments and to 11 % = 10 000 ergs/cm 2
/sec in the phytotron experiment delayed bulb development, but did not prevent it.
8. With reduction in light intensity, the total soluble sugar content in various plant organs decreased. The sugar level in different plant parts increased with time to a maximum and then, in some cases, tended to decline especially in leaf and neck.In general, growth of the onion plant was reduced by the reduction of light intensity and, moreover, distribution of dry weight over the various organs changed with change in light intensity in such a way that with reduction of light intensity, leaves accumulated relatively more weight, roots and bulbs relatively less. Along with energetic effects, light intensity also induced some morphogenetic changes, e.g., relative increase in length and decrease in diameter of leaves with the reduction in light intensity. As long as the light intensity is sufficient for the survival of the plants, bulb development will ultimately occur.
B. DAYLENGTH, INTENSITY AND QUALITY OF SUPPLEMENTARY LIGHT EFFECTS
1. Under short day conditions (8hrs. in the field, or 12 hrs. of a mixture of fluorescent and incandescent light in the phytotron), the plants failed to develop bulbs. This also holds true when fluorescent light of 120 W/33 or 40 W/55 Philips tubes was used to extend a short photoperiod or even under continuous fluorescent light alone (24 hrs.).
2. In the early stages of growth (up to 120 days after sowing), total plant fresh and dry weight was little affected by the quality of the supplementary light (fluorescent or incandescent); being somewhat higher under supplementary incandescent light than under short day (8 hrs.) or short day with supplementary fluorescent light. Later on, the position was reversed. Similarly, root, neck and swollen neck base early in the season tended to have slightly higher weights under incandescent light supplementation than under short day or short day with fluorescent light extension. At later stages the reverse was true, however, bulb weight under incandescent supplementary light exceeded that of swollen neck base under short day or short day extended by fluorescent supplementary light. Leaf fresh and dry weight and total green leaf area per plant were more or less the same in all treatments from the date of sowing up to 120 days old; at more advanced age, leaf fresh and dry weight and total green leaf area were lower in plants grown under long day with incandescent light supplementation.
3. In general, longer leaves were produced under long day conditions, and particularly with incandescent light supplementation. Leaf diameter under short day and long day with supplementary incandescent light, while close together, exceeded that under long day with supplementary fluorescent light. Specific leaf weight (weight per unit area) under long day with supplementary incandescent light was lower than under short day and under long day with supplementary fluorescent light.
4. Neck length was greater under long day conditions, especially with in candescent light extension. Up to 120 days after sowing, neck length, while equal under short day and long day with supplementary incandescent light, surpassed that under long day with supplementary fluorescent light. At more advanced age, neck diameter under long day with supplementary incandescent light lagged behind those at the other treatments.
5. Whether admixed to fluorescent light in a long photoperiod or used to extend a short photoperiod, incandescent light proved essential to bring about the photoperiodic reactions. The superiority of incandescent light over other light sources is due to the ratio of red: far-red it contains; neither red nor far red alone induced bulbing.
6. The daily duration of light appears more important than the intensity of supplementary light. Eight hours of incandescent supplementary light, used to extend a main photoperiod of 12 hrs. supplied by fluorescent light tubes induced bulbing, whereas 4 hrs. failed to do so within the range of intensities of the supplementary light used. However, under 720 ergs/cm 2
/sec supplementary incandescent light for 8 hrs., bulb development was not homogeneous; increasing the intensity of supplementary light supplied for 8 hrs. tended to speed up bulb development.
7. Young (up to 45 days old) as well as very old plants (189 days) are less sensitive to photoperiodic treatments than those of intermediate ages. In 45 days old plants, total green leaf area per plant continued to increase during 8 weeks in long day; in plants ranging from 74 to 130 days old it increased during the first 4 weeks in long day only; in still older plants total green leaf area did not show any increase following transfer to inductive cycles.
8. A positive correlation was observed between the size of the plant at the time of exposure to long day, and the final bulb weight.Generally speaking, bulb development begins only under long day conditions provided the light is of the proper light quality. Incandescent light which contains a reasonable ratio of red: far-red energy proved essential in this respect. Increase of the intensity of supplementary light speeds up bulb development, the daily duration of light, however, is more important than the intensity of supplementary light. The quality of supplementary light induces some formative changes aside of induction of bulbing, e.g., in leaf shape. The plants do not respond to the photoperiodic treatment until after they had attained a certain physiological age.
C. TEMPERATURE EFFECTS (EXPERIMENTS UNDER CONTROLLED CONDITIONS)
1. In the early stages of growth (up to 73 days after sowing), differences in entire plant fresh and dry weight were not marked at a temperature range of 15 to 25°C, though there was a tendency to be slightly higher at 15 and 20°C than at 25°C, with temperatures beyond this range, total plant fresh and dry weight markedly decreased. The decrease, however, was more pronounced at 10 than at 30°C. At all temperatures, the entire plant fresh and dry weight increased with time till a maximum was obtained at the end of the experiment; the highest values recorded by then (186 days after sowing) were those at 20 and 25°C, Growth in weight of various plant organs was differently affected by temperature; leaf and bulb weight in contrast to root weight was favoured by relatively high temperature. The time trend shows an increase in root, leaf and neck weight up to a certain moment which varied with temperature, thereafter, usually decreased or levelled off. The higher the temperature, the earlier this tended to be. By contrast, bulb fresh and dry weight progressively increased with time, up to the end of the growth period; the highest bulb weight was found at 25°C,
2. Early in the growth cycle, leaf number per plant increased with rise in temperature up to 30°C. however, in the range from 20 to 30°C. differences in leaf number were not appreciable. Later on, leaf number at 25°C, exceeded those at all other temperatures. At all temperatures, leaf number increased with time to a maximum, and then decreased. The predominance of bulbing on new leaf emergence was clearer at 25 than at 30°C.
3. Temperature influences leaf shape. Increase in temperature up to 25°C, resulted in longer leaves; higher temperature (30°C) reduced leaf length. Leaf diameter was less influenced by temperature. Up to 127 days old, growth in leaf diameter tended to be favoured by relatively low temperature (15 -20 °C); leaf diameter at 10 and 30°C. while close together, lagged behind those at the other temperatures.
4. The largest total green leaf area per plant was found at 20 or 25°C higher or lower temperatures reduced green leaf area per plant. At all temperatures, green leaf area increased with time up to a certain moment, different according to treatment, and then declined.
5. Leaf area ratio and top/root ratio increased with temperature up to 25°C, and then slightly decreased with further increase in temperature; these ratio's, however, at 30 °C were still higher than in the temperature range from 10 to 20°C In general, leaf area ratio at all temperatures decreased with time.
6. Under long day conditions (15.5 hrs.) high temperatures speeded up bulb development; low temperatures (10 and 15°C) markedly delayed it and all plants bolted. Under the experimental conditions applied, 25°C, appeared optimal for bulb growth.
7. Throughout the growth cycle, the total soluble sugar content in various plant organs was highest at 15°C, except in the latest stage where the concentration at 10°C, generally, exceeded those at other temperatures. The time trend shows an increase in sugar level till a maximum was reached at a certain moment which differed according to treatment.On the whole, temperature influences growth and development of the onion plant, induces some morphogenetic changes, alters the duration of the growth cycle, affects dry weight distribution over the various plant parts and leads to changes in the total soluble sugar content of the different plant organs.Carbohydrate by itself does not appear to be a causal factor for bulb development.
It should be observed that the reported temperature effects are found under the values for the other experimental conditions as applied. It is likely that the effect of temperature on various growth and developmentphenomenadifrers, e.g., in different light intensity, as far as magnitude or optimal temperatures for the various effects are concerned.
It might be remarked that the same may hold for, e.g., light intensity effects with respect to temperature, but this is not the same, as light intensity as the main source of energy strongly predominates other effects under not specifically extreme conditions.