It is convenient to consider the growth of pulses crops in consecutive phases. The amount of growth during each stage is governed by environmental factors.
Legume yield depends on both vegetative and reproductive components as indicated below:
1. Number of nodes per plant (No) is governed by vegetative growth x duration of pre-flowering period.
2. Percentage of (No) which becomes reproductive is the phonological potential and is the product of 1 x 2.
3. Number of flowers per reproductive node (F).
4. Percentage of F, which set pods.
Items 3 and 4 govern the number of pods per reproductive node (P).
5. Percentage of P retained.
6. Number of seeds per pod (S) called reproductive efficiency governed by 3 x 4 x 5 x 6.
7. Percentage of (5) which attains maturity.
8. Mean seed weight governed by mean seed growth rate x duration of pod filling.
Yield per plant = (1 x 2) x (3 x 4 x 5 x 6) x (7 x 8). Yield per plant x population per unit area gives yield per unit area. In determinate types, number of the node at which flowing starts is more or less fixed genetically. In indeterminate types, flowing of nodes continues with increase in plant height. Pandey (1981) classified growth stages based on morphological and developmental changes during crop season (Table 8.9).
Germination and Seedling Growth:
Soil moisture content around field capacity is adequate for pulse seed germination. Germination will be poor when the soil moisture is excessive. Pulse crops seed germinate fast around 30°C. At 20°C it takes twice as long to attain the same percentage of germination. Optimum temperature for germination is 35°C and the maximum 44°C.
A good crop canopy in short time is necessary to intercept radiation more efficiently. Canopy development in pigeonpea and bengalgram is very slow. Early sowing in the season is desirable for obtaining adequate leaf area for efficient use of solar radiation.
Depth of roots in pulses is much more than in cereals. Deep root system may account for drought tolerance shown by some of the pulses. Deep seated root system is responsible for lack of apparent responsiveness of pulses to direct fertilisation.
Leaf Area Development:
Leaf area development in most pulses is very slow. Development of leaf area in cowpea, black and greengram is relatively very slow for the first 3-4 weeks after which it picks up very fast. In pigeonpea, bengalgram, peas and lentil, the period of slow growth could extend to 6 weeks or more. Temperature has profound effect on leaf area development in northern parts of the country.
Leaf area of pulses in the first 80 days of growth is hardly one-tenth of the maximum, which is achieved by 135 days. The phase of sharp increase in the leaf area coincidence with flowering and fruit setting. However, under south Indian conditions, the same verities attain higher rate of leaf area development much earlier due to shortening of life cycle because of higher temperature resulting in low dry matter protection and consequently low yield.
Soil type, temperature and water availability can have profound influence on leaf area development. Leaf area index of pigeonpea on black soil will be around 8 as against only 6 on red soils, largely due to higher water retentively of black soils. Application of nitrogen fertilisers increases the leaf area index. However, excessive application may lead to poor nodule development and influence the plant morphology to its disadvantage.
Dry Matter Accumulation:
Dry matter accumulation is the result of balance between photosynthetic activities and respiratory loss. In cowpea, blackgram and greengram, almost two-third to one-fourth period accounts for 60-80 per cent of the total dry matter. It is interesting to note that a major part of the dry matter is produced after flowering starts.
Dry matter production will be less (almost half) under south Indian conditions compared to north India due to higher temperature and shorter growth duration in south India. In general, determinate pulses will not accumulate much dry matter after flowering while indeterminate types continue to accumulate dry matter. This may be due to the fact that the indeterminate varieties have the capacity to revive and grow after a period of soil moisture stress.
The net assimilation rate (NAR) of pulses is maximum before flowering followed by gradual decline. The relative growth rate (RGR) is maximum in the initial stage of growth and continues to decline as the growth advances. In pulse crops, often it has been observed that they produce less amount of dry matter compared to other crops in the same duration.
In general, pulses are characterised by long vegetative period, photo and thermo sensitivity, indeterminancy and low yield. Photosynthetic rates of most legumes are low ranging from 20-30 mg CO2 dm-2 hr-1. Optimum temperature is 25°-30°C and is light saturated at 50000 lux. Being C3 plants, they have photorespiration with absences of Krantz syndrome.
Mesophyll cells in legumes have cytosol and large central vacuole which lowers the productivity and water use efficiency. In comparison to C4 plants, the CGRs are low and in long duration pigeonpea, it was 71 kg ha-1 day-1. But for medium duration cultivars, CGR was 8.45 t ha-1 including 2.23 t ha-1of fallen leaf material. Net assimilation rate (NAR) ranges from 10-90 g m-2 wk-1. Many pulse crops are characterised by flower and fruit drop with low harvest index ranging from 0.2 to 0.3.
Reproductive Development:
Floral initiation and flower development are affected by air temperature, photoperiod and their interaction. Summerfield and Wien (1980) reported a number of generalisations.
1. Warm conditions, particularly at night, can compensate for longer photoperiod in some short day species.
2. Cool conditions, particularly at night, can substitute for longer photoperiods in long day species.
3. Indifference to day length (neutrality) with respect to the onset of flowering has been reported for many legumes.
4. The requirement for shorter or longer photoperiods becomes progressively more stringent after first flowers have been initiated.
5. Successive stages of reproductive ontogeny may have narrower temperature limits.
In general, warm (>30°C) and cool (<15°C) temperature, dry weather, water stress and for short day species, long photoperiods are unfavourable for floral parts development and seed set timing. Relative success of pod set in grain legumes reported to be 12 per cent in pigeonpea, 5-45 per cent in bengalgram, 11-32 per cent in blackgram, 25-31 per cent in greengram and 20-30 per cent in cowpea.
There is a direct relationship between relative humidity and pod setting percentage in bengalgram. A relative humidity of 35-53 per cent during middle of February can improve pod setting. Flowers that bloom in February only produce fruits under conditions of West Bengal. In a fruiting branch, flowers at the basal region and at the end of branch do not, generally, from seeds.
Most pulses produce a large number of flower buds and flowers but have relatively poor fruit setting. Several causes such as hormonal imbalance, limitations on photosynthesis, limited nitrogen availability, lowered gaseous exchange, reduced light intensity and available soil moisture have been ascribed for flower and fruit drop.
Yield:
Yield is a function of number of pods per plant, seeds per pod and seed weight. Pod number per unit area is the major yield determinant. To ensure optimum yield, sufficient pod development period is necessary. It ranges from 17 to 24 days in most pulses. Estimates show that nodulating roots consume 30-50 per cent of netphotosynthates.
It is, therefore, reasonable to assume that pulse yields are lower than most cereals. Further, energy conversion efficiency of pulses is much lower than cereals. It has been shown that 12.2 per cent netphotosynthates (1.49 g photosynthate g-1 of seed dry matter) are necessary to produce one gram of seed dry matter.
In general, pulses and oilseeds store more energy g-1 of seed but energy consumed is also much higher. Nevertheless, the energy conversion efficiency (ratio of energy stored to energy consumed) of maize, rice or fingermillet is much higher than in cowpea, pigeonpea or groundnut. The energy expended g-1 of seed yield ranges from 0.74 to 0.96 calories in cereals, while the range is 2.03 to 2.09 calories in pulses and oilseeds.
Nodulation and Nitrogen Fixation:
The success of leguminous crops in nitrogen deficient soils results from nodules containing symbiotic Rhizobium bacteria that reduce N2 to NH3. Bradyrhizobium and Azorhizobium are the other two genera of bacteria capable of fixing atmospheric nitrogen in root nodules of legumes. Chemical synthesis of nitrogen fertilisers requires fossil fuel but Rhizobium uses solar energy collected by plant photosynthesis.
Nodulation and nitrogen fixation are observed under wide range of temperatures with optima between 20° and 30°C. Edaphic and climate factors influence nitrogen fixation. Most legumes are either sensitive or moderately resistant to salinity and growth depression is attributed to toxic Na and CI ions. Low levels of Ca, P and Mo and excessive quantities of Al and Mn adversely affect nodulation.
Nitrogenage is the enzyme, which mediates the reduction of N2 to NH3. This system comprises two water-soluble proteins, the larger molecule known as dinitrogenase in Mo-P, while a smaller partner is an iron protein called as dinitrogenase reductase, which is most sensitive to oxygen.
The major energy cost is nodule respiration and 16 per cent of the carbohydrates synthesised by host plant is used up by the bacteriods. A total of 2.3 to 6.1 g carbon per gram of N2 fixed is utilised in the process.
Reasonable targets for N2 fixation for grain legumes is 250 kg N ha-1, while for grasses it is 500 kg N ha-1. Wide variations reported are mainly due to different methods of estimation. Factors like excess water, soil moisture stress, toxic ions like Na and CI, salinity, acidity etc. adversely affect nodulation and N2 fixation. It is reasonable to claim that 10 to 17 per cent nitrogen is transferred to the succeeding crop.
Importance of Rhizobium inoculation is realised when a legume is introduced in a new area where that particular legume was never grown. Some legume crops such as soybean have got specific requirement for Rhizobium. For successful growth of such crops, it is essential to inoculate the seed or soil with specific Rhizobium.
Most popular method of inoculating the seed is to mix the dry inoculant with seed (dry method), sprinkling water on seed and then mixing dry inoculum with seed (sprinkling method), sugar or molasses (slurry method) and pelleting the seed with several forms of calcium carbonate, gypsum, rock phosphate, superphosphate, peat soil etc. Thiram is not toxic to soybean bacteria.
Captan and Carboxin inhibit the activity of Rhizobium for a short time. The PCNB is toxic to the bacteria. Seeds treated with fungicides have no adverse effect on Rhizobia when placed in the seed furrow with granular or liquid inoculants. A mixture of systemic insecticide and inoculant may inhibit the activity of inoculant for a few days. The adverse effect on nodulation disappears within a week.