A study has been made concerning the effect of high temperatures on the symbiosis of Rhizobium leguminosarum
and pea plants (Pisum sativum).
At 30°C, no nodules were found on the roots of plants growing in nutrient solution after inoculation with the appropriate bacteria. This is in contrast to the ready nodulation at lower temperatures, and to the observation that at the high temperature both partners of the symbiosis, when growing separately, develop normally, provided that they dispose of adequate amounts of combined nitrogen.
From the results of experiments recorded in Chapter 3 it was concluded that the inhibition of nodulation by high temperatures occurs only when the sites of nodulation are exposed to such temperatures. Incubation of other parts of the plant at 30 °C (Table 1), or incubating the entire plant during several days before inoculation at that temperature, had no effect.
The unfolding of the first true leaf of pea plants was found to be correlated with the onset of the liability of the root system to infection by R.leguminosarum.
By blackening root systems with charcoal at different times, it was found that nodulation occurred usually on parts of the roots that were formed from one day before until 3-4 days after inoculation (Figs. 7, 8, 9. Table 2).
An inventarisation of morphological characters of pea roots, affected by high temperatures and by the presence of rhizobia, is given in Chapter 4. Numbers of nodules were found to be dependent not only on temperature, but also on unknown factors that could not be controlled (Table 4a). To a lesser extent, the same was true of the nodulation time (Table 4b). For this reason. the main part of the present investigation was restricted to a comparison of nodulation phenomena at two temperatures viz. 22 °C at which nodulation took place readily, and 30°C at which usually no nodules were formed. Root-growth rate and formation of lateral roots were found to be affected by temperature as well as by the presence of rhizobia (Tables 5 and 6, Fig. 10).
By transferring pea plants from 22 to 30°C at different times after inoculation, the temperature sensitivity of various stages of nodule formation was ascertained. Nodulation was found to be suppressed when the 22°C-incubation ended at 3 days after inoculation, but a number of nodules were formed when the 22°C-period was terminated at 4 days, demonstrating that between 3 and 4 days after inoculation, a highly temperature-sensitive stage of nodulation was terminated (Fig. 11). Although nodule initiations beyond this stage developed to nodules at 30°C, the size of the latter remained considerably smaller than those of control plants incubated continuously at 22°C (Fig. 14), whereas leghaemoglobin and acetylene-reducing activity did not develop (Table 8). This demonstrates that in addition to the main temperature-sensitive stage, later stages of nodulation are adversely affected by high temperatures.
Chapter 6 contains the results of experiments in which it was shown that enhancement of the photosynthetic activity of pea plants (by increasing the light intensity or by extending the illumination period) raised the probability of such plants of being nodulated when growing at moderately high temperatures (Fig. 18). The same effect was obtained by inserting a diurnal period at 22 °C, particularly during the dark period, which decreases the respiratory activity and consequently enhances the carbohydrate content of the plants (Table 9).
In Chapter 7 experiments on the first contact between R. leguminosarum
and pea roots are recorded. Attachment of the rhizobia to pea roots was found to be adversely affected by high temperatures (Table 14). Once attached to the root, rhizobial growth in the rhizosphere at 30°C, is faster than at 22°C (Table 16).
Chapter 8 contains the results of experiments with antibiotics added to R. leguminosarum
-pea plant associations at different periods after inoculation to find out at what time the bacteria were no more susceptible to the antibiotics. This was found to be the case at 3-4 days after inoculation (Table 15), i.e. the time at which the main temperature-sensitive period ended (Fig. 11).
The main effect of high temperatures during the first 3 days after inoculation is connected with root-hair formation and root-hair deformation (Chapter 9). Pea plants at 22°C form many root hairs but deformations do not occur (Fig. 22a). R. leguminosarum
and also other Rhizobium
species tested stimulate root-hair development and deformation of root hairs (curling, branching) (Figs. 23a to 27a, Table 17) which precede infection. This depends on the formation by the rhizobia of a dialysable compound (Fig. 42). Pea plants without rhizobia, incubated at 30°C, were found to form no root hairs (Fig. 22b, Table 17). In the presence of R. leguminosarum
or a number of other Rhizobium
species, root hairs at 30 °C were formed in relatively large numbers, but then they had an abnormal, spheriod shape (Figs. 23b to 27b) and did not give rise to nodule formation.
In conclusion, it can be stated that a number of processes are involved in causing the adverse effect of high temperatures on nodulation and nitrogen fixation of the Rhizobium
-pea association. Interference with the formation and deformation of root hairs in the presence of an established flora of R. leguminosarum
was shown to be responsible for the absence of nodules. Reduced growth of the nodules, absence of leghaemoglobin formation, absence of nitrogenase activity (both characters probably being linked) are further important processes, adversely affected by high temperatures.