Tuber sink modifies photosynthetic response in potato under water stress

Source-sink relations were studied by tuber detachment in potato (Solanum tuberosum cv. Kufri Sindhuri) under irrigated and drought conditions. Within 6 h of tuber detachment from irrigated plants (leaf water potential, psi(w), -0.2 MPa), light saturated rates of net photosynthesis (P-max) declined from 22 mu mol CO2 m(-2) s(-1) to a value close to zero along with a parallel decline in the stomatal conductance (g(s)). After 5 days of tuber excision, P-max partially recovered to 7 mu mol CO2 m(-2) s(-1) whereas g(s) recovered fully to pre-excision level. At this stage, plants with excised tubers were subjected to water stress along with plants having tubers. After PO days of stress (psi(w) - 0.8 MPa), P-max declined severely from 7 mu mol CO2 m(-2) s(-1) to almost zero in plants without tubers, whereas only 30% reduction in P-max was observed in plants with tubers. Irrespective of the presence or absence of drought, plants with excised tubers showed a high accumulation of leaf carbohydrates (sucrose and hexose), increased acid-invertase activity and low sucrose-phosphate synthase activity compared with plants having tubers. Under low sink activity (tuber removed), high levels of leaf carbohydrates with decreased sink demand caused a down-regulation of P-max or complete inhibition under water stress. This could be due to feedback inhibition of photosynthesis. On the other hand, the presence of a strong tuber sink led to a lower level of leaf carbohydrates, thus avoiding feedback inhibition of photosynthesis under drought. Stressed plants without tubers showed an irreversible decrease in the quantum yield of photosystem II (F-v/F-m), indicating photoinhibition when exposed to high irradiance. Interaction between the presence or absence of tubers and drought stress was highly significant (P < 0.01). The results suggest a dominant role of the tuber sink in avoiding feedback inhibition of photosynthesis. (C) 1999 Elsevier Science B.V. All rights reserved.