A PHYSIOLOGICAL DISORDER OF SALINE STRESS RELEASED GREENHOUSE TOMATO PLANTS

High electrical conductivity (EC) usually causes salinity stress for greenhouse crops. The relatively high EC is needed for fruit quality improvement and enough nutrient supplies. The main objective of the present work was to examine the aftereffect of high EC and substrate water deficit when the stresses were released. A hypotonic disorder was observed the second day after over-watering in the one-time re-watered plants in high EC-low substrate water combined plot. Some gray spots were apparent on the leaf blades and then dried. In some plants with severe symptoms, leaves died completely. This hypotonic disorder did not occur in step-by-step re-watered plants. This might be due to the swelling of cells in stress-released leaves caused by sudden rehydration. However, photosynthesis increased dramatically in step-by-step re-watered plants for all treatments, especially significant in the pre-stressed plants. The photosynthetic rate in stress-released plants was even higher than in constantly well-watered plants. The increase in photosynthesis by rehydration was caused mainly by an increase in stomatal conductance rather than mesophyll conductance. A modeling analysis showed that quantum yield also increased after over-watering. Before over-watered, leaf water potential and turgor potential were decreased by substrate water deficit, high EC and the combined stress. Osmotic potential was lower in stressed plants. After over-watered, turgor potential was high in pre-stressed plants, and even higher than that in the constantly well-watered plants because the osmotic potential was lower with a similar level of leaf water potential. This suggested that osmotic adjustment occurred during the period of stresses. A higher turgor potential accounted for a higher stomatal conductance and a higher photosynthesis as a consequence.