Greenhouse tomato photosynthetic acclimation to water deficit and response to salt accumulation in the substrate

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 gravimetric 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 of 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 (Ψ p) decreased substantially as leaf water potential (Ψ w) declined. However, as SWC was maintained constant over a period, Ψ p recovered to a large extent even at the same Ψ w level. This turgor recovery was based on osmotic adjustment shown by the decrease in osmotic potential (Ψ(π)) 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 steadily in salinity stressed plants during the experiment, osmotic adjustment also occured, resulting in partial turgor maintenance. The combined treatment of salinity and water deficit imposed and additive effect on Pn, Ψ w, and Ψ p, which did not allow Pn to recover despite the osmotic adjustment.