Osmotic adjustment, water relations and carbohydrate remobilization in pigeonpea under water deficits

Osmotic adjustment (OA) is considered an important mechanism for drought adaptation. The objective of this study was to evaluate the contribution of OA to growth and productivity of extra-short-duration pigeonpea (Cajanus cajan (L.) Millsp.) during soil moisture deficits. Using two automated rain-out shelters, six pigeonpea genotypes were grown under well-watered conditions throughout the growth period or under water deficits imposed from flowering until physiological maturity. Soil moisture content was monitored using neutron probes. Osmotic adjustment in leaves increased with the depletion of soil moisture and reached close to 0.5 MPa at physiological maturity. Genotypic variation in OA (ranging from 0.1 to 0.5 MPa) was significant. Total non-structural carbohydrates mobilized from stem for reproductive growth varied substantially among genotypes. Contribution of stem-re serves to the grain yield, however, was less than 5 %. Below 30 cm depth, 60 to 80 % of the plant extractable soil water was not utilized at physiological maturity in the drought treatment. Genotypic variation in leaf relative water content was correlated with OA (r(2) = 0.66*, n = 6). Leaf OA was correlated with the amount of stem sucrose mobilized (r(2) = 0.67*; n = 6). The relationship between OA and leaf area duration was significant (r(2) = 0.94*; n = 5). Genotypic variation in leaf relative water content was correlated with crop growth rate (r(2) = 0.74*; n = 6) and radiation-use efficiency (r(2) = 0.84**; n = 6) under moisture deficits. The results indicate that OA could influence radiation use efficiency and crop growth rate of extra-short duration pigeonpea indirectly by increasing leaf relative water content during soil moisture deficits.