Abstract
Greenhouse tomato plants (Lycopersicon esculentum Mill. cv. Capello) were grown in a peat-moss based substrate (70% sphag peat and 30% perlite, v/v) and treated with a salinity stress (4.5 mS cm-1 of electrical conductivity, EC) and a low (55 ± 8% on gra-vimetric basis) substrate water content (SWC) to examine the effects of salt accumulation and a prolonged substrate water deficit on photosynthesis and plant water relations. Net photosynthetic rate (Pn) decreased by 24% compared to the control one day after SWC was depleted to 55%. However, as SWC was maintained at the same level for several days, the effect of water stress diminished, with the decreasing extent of 14%, 15%, and 14% compared to the control on the 11th, 16th, and 28th days, respectively, from the beginning of treatments. This demonstrated that tomato plants acclimated to substrate water deficit. One day after SWC reached 55%, leaf turgor potential (ΨP) decreased sub-stantially as leaf water potential (ΨW) declined. However, as SWC was maintained con-stant over a period, ΨP recovered to a large extent even at the same Ψw level. This tur-gor recovery was based on osmotic adjustment shown by the decrease in osmotic poten-tial (Ψπ) 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 ΨW and Ψp declined steadi-ly in salinity stressed plants during the experiment, osmotic adjustment also occurred, re-sulting in a partial turgor maintenance. The combined treatment of salinity and water de-ficit imposed an additive effect on Pn, ΨW, and Ψp, which did not allow Pn to recover despite the osmotic adjustment.
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