Photosynthesis and Water Vapor Exchange of Pigeonpea Leaves in Response to Water Deficit and Recovery

Pigeonpea [Cajanus cajan (L.) Millsp.; cv. UW-10], a crop being developed for use in low-rainfall environments, was studied during development of water stress and recovery after rewatering to determine the treatment effects on photosynthesis and leaf diffusive conductance. Plants were grown in a greenhouse and leaf gas exchange was measured at various photosynthetic photon flux densities during an 8-d period of water stress development and during an 18-d period of recovery following rewatering. Predawn total water potential (~w) declined by 1.0 MPa after 8 d of withholding water, whereas the pressure component of water potential (ψₚ) remained positive due a decrease in the solute component of water potential (ψₚ) from −1.2 to −1.7 MPa. During stress development, decreases in leaf diffusive conductance (gL) occurred earlier than decreases in photosynthetic CO₂ exchange rate (CER), indicating that a water conservation mechanism was induced. Differences in gL between unwatered and control plants were first detected at 5 d after withholding water. Differences in CER between control and unwatered plants were not detected until Day 8 of the treatment. As a result of these CER and gL changes, the CO₂ concentration in the intercellular air space (Cᵢ) decreased during the period from 5 to 8 d after withholding water. After rewatering, CER and gL slowly recovered over an 18-d period. It is concluded that pigeonpea responds to water stress by partially limiting the rate of water loss and maintaining a low CER, and it responds to rewatering by slowly recovering CER and gL. Joint contribution from the Dep. of Plant Science and Biochemistry, Univ. of the West Indies; and the Dep. of Agronomy, Cornell Univ. Supported in part by funds from National Science Foundation grant no. INT 8025 899 to T.L. Setter.