Introduction:
Pigeonpeas are an important legume crop of rainfed agriculture in the semiarid tropics. The Indian subcontinent, eastern Africa and Central America are the world's three main pigeon pea-producing regions. Pigeonpeas are cultivated in more than 25 tropical and subtropical countries
Pigeonpea (Cajanus cajan L.) is an important legume crop of rainfed agriculture in the semi-arid tropics. The Indian subcontinent, eastern Africa and Central America are the world's three main pigeon pea-producing regions.
The growth and yield of Pigeonpea is affected by heavy metal Cadmium. The uptake of Cadmium by the roots of Pigeonpea is affected by numerous plant and soil factors like Cadmium concentration in the rooting medium, Cadmium availability, Plant species and Plant age.
Selection of the Variety and Raising of the Crop:
The seeds of UPAS-120 variety of Pigeonpea were raised in earthen pots in a naturally lit net house. The pots were lined with polyethylene bags and filled each with washed river sand. Four seeds of Pigeonpea per pot were sown approximately at a uniform depth and distance. Before sowing, the seeds were surface sterilised and inoculated with suitable rhizobium culture. Thirty days after sowing (DAS), the plants were thinned to two per pot. The pots were supplied with tap water as and when required. The nitrogen free solution was supplied to each pot at weekly intervals.
Sampling:
Sampling was done at 7-8 days intervals starting from 30 days after sowing (DAS) of the crop till its maturity. Eight plants from each treatment were used at each sampling. Two plants constituted one replicate.
Growth observations:
The plants were uprooted and separated into different parts, i.e. stem, leaves, roots, nodules, flowers, buds, pod walls and seeds. Following growth parameters were recorded -
The fresh and dry weight.
The dry weight of abscissed leaves.
Plant height.
Leaf area.
Results and Discussions:
In comparison with control treatment, the major morphological changes that occurred in cadmium treatment plants were, the yellowing of leaves and their subsequent absicission, browning and bending of stem, swelling at the base of stem, increase in the secondary branches, early flowering, more leaves per plant, smaller leaves, fewer pods, fewer seeds per pod and smaller seeds.
The plant height was reduced with both the cadmium levels i.e. 3mM and 6mM, but the reduction was significant only with 6mM Cd. With 3mM Cd, there was no significant reduction upto 53 days after sowing (DAS). In control and 6mM Cd treatments, the maximum height was attained at 97 DAS.
The total leaf area was reduced at all the stages of plant growth. In control, the leaf area was maximum at 97 DAS, whereas in Cd treated plants, it was maximum at 111 DAS. There after, the reduction was maximum with 6mM Cd due to leaf senescence and abscission.
Cadmium drastically reduced the fresh weight of all plant organs at all stages of plant growth. The fresh weight of leaves and stem increased upto 97 DAS in control and decreased there after.
The fresh weight of flowers, pod walls and seeds was also reduced by both the levels of cadmium, reduction being more at the heigher concentration. The flowering in cadmium treated plants was observed at 75 DAS whereas in control, at four days later i.e. at 79 DAS.
The dry weight of flowers, pod walls and seeds were also reduced in cadmium treated plants. The maximum dry weight of flowers in control was observed at 111 DAS, there after there was a slow decrease. In cadmium treated plants, the maximum dry weight of flowers was attained a little earlier, i.e. at 97 DAS, there after there was a decrease in the dry weight. The dry weight of seeds and pod walls increased in both the control and the cadmium treated plants, but at every stage, the dry weight of seeds of the cadmium treated plants was less than that of the control.
The reduction in plant growth could be due to the adverse effect of the heavy metal cadmium on the various physiological and metabolic processes such as photosynthesis, nitrogen fixation and partitioning of carbon and nitrogen.The deleterious effect of low concentration of cadmium was due to decrease in seed number but at higher doses, the reduction in yield was because of the formation of lesser number of seeds with smaller size.
Control plants accumulated 19.1 gm dry matter through its life span, whereas 3mM and 6mM Cd treated plants accumulated only 13.8 gm and 10.4 gm dry matter, respectively. The rate of dry matter deposition during vegetative stage was 135.8 mg per day for control, 63.9 mg per day for 3mM Cd and 40.9 mg per day for 6mM Cd. The highest rate of dry matter accumulation occurred during the 'flowering and pod setting' stage of growth being 282 mg per day, 274 mg per day and 209 mg per day for control, 3mM Cd and 6mM Cd respectively. Loss of dry matter due to leaf abscission started early in Cd treated plants and increased till harvest in both control and treated plants. This loss amounted to 7.8, 10.8 and 12.5 % fror control, 3mM and 6mM Cd respectively. Thus Cd affected both the rate of dry matter accumulation and dry matter loss from the plant.
Dry matter harvest index (HI) were reduced at higher Cd concentration (6mM Cd), however, plants with 3mM Cd exhibit a higher Harvest Index (HI) than the control.
