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Temperature and Light
Critical investigations on the effect of root temperature on nodulation and infection processes carried out with temperate species of legumes such as clovers (Trifolium spp.) grown on agar slopes in environment controlled growth cabinets have shown that temperatures below 10°C retard root hair infection by Rhizobium, whereas temperatures higher than 24°C promote - the same.
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The most congenial range of temperature for bacteroid tissue formation in nodules appears to be 20-30°C, although nitrogen fixation remains unchanged in the range of 12-32°C. These results are subject to inherent variations between strains of Rhizobium and host cultivars.
Among the tropical legumes, effects of day temperature on root nodulation have been studied in soybean (Glycine max) and Bengal gram (Cicer arietinum) in pot trials using selected strains of both the symbionts in environment controlled and illuminated growth cabinets. One of the bacterial strains was most effective at 33°C on soybean, while others showed no difference in effectiveness at 21°C. In Bengal gram, not withstanding the variations noticed in bacterial strain effects with regard to temperature, it was noteworthy that none of the bacterial strains produced nodules at root temperatures beyond 32°C.
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Nitrogenase activity was higher in the temperature range of 24 to 33°C with little activity beyond the upper limit. These results appear significant for a crop such as Bengal gram which is grown in India mostly under rainfed conditions where day temperatures do not exceed 32°C.
Studies on the effects of combinations of photoperiods and environmental temperature on nodulation of cluster clover (Trifolium glomeratum) have revealed that initial nodulation was delayed as the day length was increased and that photoperiods influenced the formation, size and the number of nodules on the root system.
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Apart from products of photosynthesis, which obviously influence nodulation, experiments with soybean seedlings have shown that cotyledons supply some essential factor for nodulation in seedlings. Removal of cotyledons from clover seedlings soon after germination has been shown to delay, reduce or prevent infection of root hairs and subsequent nodulation.
The influence of different combinations of photoperiods and temperature on symbiosis of T. glomeratum with R. trifolii in test-tubes (from Subba Rao, 1971)
| Treatment |
Age at which first nodule appeared |
At 65 days (av. Of 10 tubes) |
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Number of leaves |
Number of nodules |
Height (cm) |
Weight (mg) |
Flowering at |
A. 8 hr L 15°C |
8.5 |
7.7 |
6.3 |
2.2 |
11 |
No flowering |
B. 8 hr 20°C |
8.0 |
7.0 |
3.8 |
3.2 |
9 |
“ |
C. 16 hr L 15°C |
8.5 |
6.8 |
3.7 |
3.4 |
14 |
52 days |
D. 16 hr 20°C |
8.9 |
6.1 |
2.8 |
2.7 |
11 |
43 days |
E. 16 hr L 15-20°C |
10.6 |
6.0 |
3.3 |
4.3 |
15 |
55 days |
F. 24 hr L 15-20°C |
10.0 |
5.0 |
3.0 |
2.0 |
11 |
38 days and seed setting 45 days |
S.Em |
0.228 |
0.11 |
0.44 |
0.21 |
0.78 |
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C.D at 1% level |
0.64 |
0.36 |
1.24 |
0.58 |
2.18 |
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Combined Nitrogen
Leguminous plants are unique among flowering plants by virtue of the fact that they can obtain their nitrogen requirements either through inorganic nitrogenous fertilizers and/or through symbiosis in root nodules of nodulating species. Both beneficial and detrimental effects of mineral nitrogen on growth and yield of nodulating legumes have been observed.
In spite of seemingly divergent results obtained under field conditions, work carried out under controlled environmental conditions has shown that mineral nitrogen beyond certain levels interfere with infection of root hairs, number of nodules, structure of nodules and the amount of nitrogen fixed.
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In ammonium nitrogen (HN4NO3)-treated clover plants, the bacteroids in the nodules become enlarged and synthesis of membrane envelopes gets affected.
Spraying of plants with urea prevents nodulation whereas spraying with sucrose enhances nodulation and N2 fixation. In experiments with excised roots, nitrate provided through the base (cut end) of the root, does not inhibit nodulation, whereas inhibition occurs when nitrate is supplied in the external medium in which roots are growing. Such an inhibition is offset by addition of sucrose, mannitol or L-arabinose to the external medium.
The mechanism behind inhibition of nodulation by nitrate is entirely speculative. It is not unlikely that there exists a critical balance between the available carbon and nitrogen in the root system which is offset by extraneous source of nitrate. It has also been hypothesized that nitrate is converted to nitrite in the root environment mediated by Rhizobium and the nitrite so formed destroys the auxin, indole acetic acid (IAA). The frequency of nodulation of lucerne is known to increase at an optimum level of IAA (10-8 M) in the root medium whereas KNO3 at the rate of 140 N (ppm), has just the opposite effect and also decreases the total number of nodules produced.
This nitrate-induced inhibition, however, is reversed by additions of 10-8M IAA into the root region which also partially reverses the nitrate induced inhibition of curling of root hairs and formation of infection threads in them. Such interlinked facts serve to demonstrate the interesting relationship among products of photosynthesis, mineral sources of nitrogen, soil reaction (pH) and growth promoting substances which are known to operate in the root region during different stages of symbiosis in nodulating legumes.
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