| Tomato is one of the most popular vegetable in the world, all species of which are native to South America, and Mexico and Peru are believed to be the original sites. Due to the acceptable flavor, nutrient-rich, short grown cycle and high field, the crop gets the popularity. Tomato is also the main type of vegetable cultivated in protected condition in china. However, temperature has become the most important factor to limit vegetable production, as it displays negative effects on crop growth, survival, and yield. Polyamines are low molecular weight ubiquitous nitrogenous compounds and mainly concluding putrescine (Put), spermidine (Spd), and spermine (Spm). The hormone have been involved in a wide range of growth and developmental processes such as cell division, breaking of tubers dormancy and seed germination, stimulation, support and development of flower buds, embryogenesis, fruit set and growth, fruit ripening, plant morphogenesis and response to environmental stresses. Recently, growing interest has focused on the subject that the regulated and mediated effects of polyamines on various stress tolerance of plant. Whereas little information is currently available in literature regarding the effects of exogenous Spd on the photosynthetic mechanism of plants exposed to high temperature.Taking a relatively heat-sensitive tomato (Lycopersicon esculentum Mill.) cultivar ’Puhong 968’as test material, a substrate culture experiment was conducted in growth chamber to investigate the effects of foliar spraying spermidine (Spd) on the structure and function of photosynthetic apparatus in tomato seedling leaves under high temperature stress(38℃/28℃, day/night). We investigated the Spd-medicated photosynthetic mechanism of tomato seedlings adapt to high temperature stress including net photosynthesis (Pn), chlorophyll fluorescence, ultrastructure of the chloroplast, fatty acid composition of chloroplast membranes. The main results are as follows:1. Spayed application of 0.1-5 mM Spd could affect the inhibition of photochemistry efficiency in tomato. Among of them,1 mM Spd displayed the best performance, which significantly increased photosynthetic pigment content, enhanced the net photosynthesis (Pn) and improved the tomato seedlings growth.2. High temperature stress significantly increased the values of ABS/RC, TR0/RC, DI0/RC, but decreased ETo/RC and PI(abs) in leaves. However, Spd treatment reverted these changes induced by high temperature stress. Spd resulted in a better utilization of excitation energy and a higher stability of the photosystem II, causing much enhancement of the oxygen evolving complex capacity, connectivity among PSII units and electron transport activity.3. The result showed that high temperature stress resulted in a significant inhibition of the maximal photochemical efficiency of photosystem Ⅱ (Fv/Fm) and the actual efficiency of photosystem Ⅱ (OPSⅡ). Application of exogenous 1mM Spd alleviated the reduction of Fv/Fm and OPSⅡ under high temperature conditions, promoted the electron transport chain, relieved the inhibition of PSⅡ, thereby enhancing the photochemical efficiency of tomato seedlings under high temperature stress.4. The membrane lipids analysis showed that high temperature decreased levels of lipid unsaturation in chloroplasts, with the change being predominantly due to a decrease in linolenic acid (18:3) and an increase in palmitic acids (16:0). The microscopic analysis revealed that high temperature stress induced destruction of chloroplasts with ruptured thylakoid membrane. However, Spd increased the level of lipid unsaturation in the chloroplast membranes and prevented the destruction of photosynthetic apparatus induced by high temperature stress. These results demonstrate that Spd plays an important role in maintaining the stability and integrity of photosynthetic apparatus of tomato seedlings, thus enhancing the actual efficiency of photosystem Ⅱ(OPSⅡ) when exposed to high temperature stress. |
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