Monitoring of Salinity, Temperature, and Drought Stress in Grafted Watermelon Seedlings Using Chlorophyll Fluorescence

Abstract

Chlorophyll fluorescence (<i>CF</i>) is used to measure the physiological status of plants affected by biotic and abiotic stresses. Therefore, we aimed to identify the changes in <i>CF</i> parameters in grafted watermelon seedlings exposed to salt, drought, and high and low temperatures. Grafted watermelon seedlings at the true three-leaf stage were subjected to salinity levels (0, 50, 100, 150, and 200 mM) and temperature [low (8°C), moderate (24°C), and high (40°C)] stresses for 12 days under controlled environmental conditions independently. Eight <i>CF</i> parameters were measured at 2-day intervals using the FluorCam machine quenching protocol of the FluorCam machine. The seedlings were also exposed to drought stress for 3 days independent of salinity and temperature stress; <i>CF</i> parameters were measured at 1-day intervals. In addition, growth parameters, proline, and chlorophyll content were evaluated in all three experiments. The <i>CF</i> parameters were differentially influenced depending on the type and extent of the stress conditions. The results showed a notable effect of salinity levels on <i>CF</i> parameters, predominantly in maximum quantum yield (Fv/Fm), non-photochemical quenching (NPQ), the ratio of the fluorescence decrease (Rfd), and quantum yield of non-regulated energy dissipation in PSII [Y(NO)]. High temperature had significant effects on Rfd and NPQ, whereas low temperature showed significant results in most <i>CF</i> parameters: Fv/Fm, Y(NO), NPQ, Rfd, the efficiency of excitation capture of open photosystem II (PSII) center (Fv'/Fm'), and effective quantum yield of photochemical energy conversion in PSII [Y(PSII)]. Only NPQ and Rfd were significantly influenced by severe drought stress. Approximately, all the growth parameters were significantly influenced by the stress level. Proline content increased with an increase in stress levels in all three experiments, whereas the chlorophyll (a and b) content either decreased or increased depending upon the stressor. The results provided here may be useful for understanding the effect of abiotic stresses on <i>CF</i> parameters and the selection of index <i>CF</i> parameters to detect abiotic stresses in grafted watermelon seedlings.