Stomatal and Cuticular Transpiration of Greenhouse Tomato Plants in Response to High Solution Electrical Conductivity and Low Soil Water Content

Abstract

`Capello' tomato plants (Lycopersicon esculentum Mill.) were grown in a greenhouse in peat-based substrate (70% sphagnum peat and 309'. perlite, by volume) and supplied with nutrient solutions of high (4.5 mS·cm-1) or low (2.3 mS·cm-1) electrical conductivity (EC) under high (95% ± 5%) or low (55% ± 8% of capillary capacity) soil water conditions. Three weeks after treatments started, stomatal transpiration (TRst) and cuticular transpiration (TRcu) rates were measured by three methods: 1) analyzing TRst and TRcu from a water retention curve obtained by drying excised leaves in air under a photosynthetic photon flux (PPF) of 400 μmol·m-1·s-1, 2) analyzing TRst and TRcu from a transpiration decline curve obtained by measuring transpiration rates after cutting the leaf from the stem of the dehydrated plant in the gas-exchange system, and 3) measuring transpiration rates under light and in dark respectively using the gas-exchange method. TRst and TRcu were decreased by high EC and/or low soil water content. For method 1, the transpiration decline curve shows two distinct phases: the initial steep slope that indicates TRst and the gently sloped section that indicates TRcu. Both slopes were lower for high EC and/or water-stressed plants compared to the control (low EC and high soil water content). The tangent lines of these two phases of the curve intersect at one point (t, w). The value oft that indicates the time for stomatal closure was longer and the value of w that indicates the critical tissue water level for stomatal closure was lower for high EC and/or water-stressed plants. In method 2, the initial rate of total transpiration was higher in high EC and/or water-stressed plants. Leaf wax content increased, especially under high EC stress. This suggests that increased deposition of wax prevents water loss from the cuticle. A delay in complete stomatal closure, complete closure at lower RWC, and reduced TRcu or an increase in wax deposit were adaptations to water and salinity stresses in tomato plants under our controlled environmental conditions.