Ninety percent of the variation in yield in semiarid topics is due to the availability of water. Groundnut developmental stages close to flowering and the post-flowering stages are especially sensitive to drought stress. Effect on vegetative growth - Drought stress inhibits leaf expansion and stem elongation through a reduction of relative turgidity thus altering both leaf and stem morphology.
Reductions in leaf number and individual leaf size contribute to decreases in leaf area. The extent of the reduction is determined by the intensity and duration of drought stress and stage of crop at which the stress is imposed. A 30 days drought stress would reduce the leaf area by 23%. Leaf longevity and leaf area duration are reduced by decreasing soil water potential. Main axis and cotyledonary branches are shorter and generally decrease more drastic in internode length than the node numbers. Exposure to drought stress decreases the overall vegetative growth of the plant.
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The first 50 days after emergence are important for root growth, and a decrease in water availability during this period severely reduces root growth. Root growth in groundnut is influenced by drought. Moderate drought stress stimulates root growth into deeper soils; under drought roots in the lower depths continue to grow deeper even though vegetative growth appears to stop. Groundnut roots can effectively extract soil water from depths of 180cm in fine sandy soils. Cultivars with drought tolerance usually explore a larger root volume and have a higher root density at deeper depths when grown under stress conditions. However, under severe drought conditions root growth is decreased.
Root parameters such as root volume, root dry weight, root length and number respond to water availability and can be used to screen for improved water use efficiency. Furthermore, the plant’s ability to maintain a viable root system during drought stress contributes to drought tolerance. Greater carbon partitioning to the root system before pod set and a root system that maintains itself for a long period for water extraction may have advantage over plants whose roots continue dying and re-growing during reproductive development.
Such a habit will help decrease the effects of
drought stress during critical stages of reproductive development. Drought
stress can also influence biological nitrogen fixation of the crop. This is
particularly critical as most groundnut crops in the semiarid tropics are dependent
upon biological nitrogen, while application of inorganic fertilizers is
limited.
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