Effects of electrical conductivity (EC) and substrate water content on photosynthetic response to irradiance were examined to understand the constraints in photosynthesis caused by these stresses. Tomato plants were grown under high (4.5 mS cm'1) or low (2.3 mS cm'1) EC and high (95 %) or low (55 % of capillary capacity) soil water content. Photosynthetic photon flux (PPF) was changed (I) from low to high and then in reverse from high to low, and (2) starting from high to low and then reversing from low to high. In both cases, photosynthetic rates (PN) at most levels of PPF were higher during the 2nd cycle than during the first one. The extent of this hysteresis was larger for high EC-treated and/or water-stressed plants. In addition, /’N was inhibited under veiy high PPF (1800 pmol nr2 s'1) in high EC and/or water-stressed plants (stress-induced photosynthetic depression under high PPF). After gradual declining for about 40 min under high PPF, /'N started to recover. When recovered to some extent, it went down again, showing oscillation cycles. Oscillation was clearly observed for the treatment of high EC combined with high soil water content, but not for the control. Thus the abnormal photosynthetic responses to irradiance, such as hysteresis, photosynthetic depression under high PPF and oscillations may be induced or promoted by some constraints in substrate water availability, which cause abnormal stomatal response and inactivity of mesophyll photosynthesis.