GREENHOUSE TOMATO PHOTOSYNTHETIC ACCLIMATION TO WATER-DEFICIT AND RESPONSE TO SALT ACCUMULATION IN THE SUBSTRATE

Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) w ere grown in a peat-moss based substrate (70% sphag peat and 30% perli te, v/v) and treated with a salinity stress (4.5 mS cm(-1) of electric al conductivity, EC) and a low (55 +/- 8% on gravimetric basis) substr ate water content (SWC) to examine the effects of salt accumulation an d a prolonged substrate water deficit on photosynthesis and plant wate r relations. Net photosynthetic rate (Pn) decreased by 21% compared to the control one day after SWC was depleted to 55%. However, as SWC wa s maintained at the same level for several days, the effect of water s tress diminished, with the decreasing extent of 14%, 15%, and 14% comp ared to the control on the 11th, 16th, and 28th days, respectively, fr om the beginning of treatments. This demonstrated that tomato plants a cclimated to substrate water deficit. One day after SWC reached 55%, l eaf turgor potential (psi(p)) decreased substantially as leaf water po tential (psi(W)) declined. However, as SWC was maintained constant ove r a period, psi(p) recovered to a large extent even at the same psi(W) level. This turgor recovery was based on osmotic adjustment shown by the decrease in osmotic potential (psi(pi)) at fully hydrated status. The effect of salinity on Pn was not observed under both high and low SWC one day after the beginning of treatments, but that effect became larger and larger as the treatment was prolonged. Although psi(W) and psi(p), declined steadily in salinity stressed plants during the exper iment, osmotic adjustment also occurred, resulting in a partial turgor maintenance. The combined treatment of salinity and water deficit imp osed an additive effect on Pn, psi(W), and psi(p), which did not allow Pn to recover despite the osmotic adjustment.