| The effects of moderate low temperature (15℃/10℃, day/night) and severe low temperature (10℃/5℃, day/night)on photosynthesis and chlorophyll fluorescence characteristics were studied in watermelon cultivar ‘Zaojia8424’. The resultsshowed that: (1) The chlorophyll (a+b) content, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) were gradually decreased under low temperature stress, and the lower the temperature was, the greater the photosynthetic parametersdecreased. Severe low temperature stress reduced Pn to 1.56 and decreased by 85.9% compared to non -stress after 6 d of treatment, indicating that severe low temperature caused serious damage to photosynthetic apparatus of watermelon seedlings and greatly inhibited the photosynthesis. (2) Intercellular CO2 concentration (Ci) shown to decrease under moderate low temperature stress, consistent with the change trend of Pn and Gs, indicating that stomatal restriction was the main factor leading to reduction of photosynthetic rate. However, severe low temperature stress increased Ci , demonstrating a fact that non-stomatal restriction was the main factor for the decrease of Pn. The chlorophyll a/b increased under low temperature stress, which may be a self-protective mechanism of watermelon seedlings to low temperature with light stress, because the relatively small amount of chlorophyll b could avoid photoinhibition caused by excessive light absorbed by chlorophyll. (3) Low temperature stress led a significant decline in the maximum photochemical efficiency of photosystem (Fv/Fm) and photochemical quenching coefficient (qP), severe low temperature stress also caused a decrease in the non-photochemical quenching coefficient (qN), whereas qN showed a increase under moderate low temperature stress. This suggested that watermelon seedlings grown under moderate low temperature could consume excessive light energy absorbed by antenna pigment through thermal dissipation in xanthophylls cycle to alleviate the damage of PS, whereas under severe low temperature stress, the photo -protective mechanisms of watermelon seedlings destroyed and it also caused a irreversible damage to photosynthetic apparatus. |
