Effects of Humidity and Soil Moisture Content on Chlorophyll Fluorescence of Cucumber Seedlings Exposed to High Air Temperature

Effects of atmospheric humidity and soil moisture content on the relative intensity of chlorophyll fluorescence of cucumber (Cucumis sativus L.) exposed to high air temperatures were assessed as a measure of heat stress or injury. 1. After exposure to 25 •Ž (control) and 42•‹, 44•‹ and 46•Ž for two to three hours, the intensity of fluorescence decreased a little at 42•‹ and 44•Ž but markedly at 46•Ž. However there was no significant difference in seedling growth among those grown for 12 days at the three high temperature treatments and those at 25•Ž. 2. Chlorophyll fluorescence decreased significantly in plant kept at a humidity of 0.3 kPa of vapor pressure deficit (VPD) and 46•Ž, compared with those held at the same temperature but at 4.8 kPa VPD. The intensity decreased to 5% of that at 25js_•Ž immediately after a high humidity and 46•Ž treatment. It recovered to 55% of the control two days after the treatment was terminated but never attained more than 70% of control even after 5 days at 25•Ž. The reduction in chlorophyll fluorescence is attributed to the high leaf temperature and humidity which depressed transpiration. 3. Chlorophyll fluorescence intensity of seedlings exposed to 46•Ž and 55% soil moisture content for one to three hours decreased significantly, whereas it decreased just slightly when cucumbers were grown at the same high atmospheric humidity but in soils with 97% moisture content. The leaf temperature of plants grown at the low soil moisture level gradually increased and surpassed that of plants grown at high soil moisture content ; the transpiration rate was apparently higher in moister soil than in drier soil. These results indicate that determinig chlorophyll fluorescence may be a method of detecting heat stress or injury to the photosyntetic apparatus. They support the idea that heat injury may be avoided by maintaining low atmospheric humidity and high soil moisture content which hasten the transpiration and thereby tend to keep the leaf temperahire low