Gas exchange by pods and subtending leaves and internal recycling of CO2 by pods of chickpea (Cicer arietinum L.) subjected to water deficits

Terminal drought markedly reduces leaf photosynthesis of chickpea (Cicer arietinum L.) during seed filling. A study was initiated to determine whether photosynthesis and internal recycling of CO2 by the pods can compensate for the low rate of photosynthesis in leaves under water deficits. The influence of water deficits on the rates of photosynthesis and transpiration of pods and subtending leaves in chickpea (cv. Sona) was investigated in two naturallylit, temperature-controlled glasshouses. At values of photosynthetically active radiation (PAR) of 900 mu mol m(-2) s(-1) and higher, the rate of net photosynthesis of subtending leaves of 10-d-old pods was 24 and 6 mu mol m(-2) s(-1) in the well-watered (WW) and water-stressed (WS) plants when the covered-leaf water potential (Vr) was -0.6 and -1.4 MPa, respectively. Leaf photosynthesis further decreased to 4.5 and 0.5 mu mol m(-2) s(-1) as Psi decreased to -2.3 and -3.3 MPa, respectively. At 900-1500 mu mol m(-2) s(1) PAR, the net photosynthetic rate of 10-d-old pods was 0.9-1.0 mu mol m(-2) s(-1) in the WW plants and was -0.1 to -0.8 mu mol m(-2) s(-1) in the WS plants. The photosynthetic rates of both pods and subtending leaves decreased with age, but the rate of transpiration of the pods increased with age. The rates of respiration and net photosynthesis inside the pods were estimated by measuring the changes in the internal concentration of CO2 of covered and uncovered pods during the day. Both the WW and WS pods had similar values of internal net photosynthesis, but the WS pods showed significantly higher rates of respiration suggesting that the WS pods had higher gross photosynthetic rates than the WW pods, particularly in the late afternoon. When (CO2)-C-13 was injected into the gas space inside the pod, nearly 80%, of the labelled carbon 24 h after injection was observed in the pod wall in both the WW and WS plants. After 144 h the proportion of C-13 in the seed had increased from 19% to 32% in both treatments. The results suggest that internal recycling of CO2 inside the pod may assist in maintaining seed filling in water-stressed chickpea.