Exogenous spermidine enhances the photosynthetic and antioxidant capacity of citrus seedlings under high temperature

Abstract

Studies have not fully explained the underlying mechanism of spermidine-mediated heat tolerance. This study investigated the possible role of spermidine (Spd) in regulating citrus heat tolerance. The results showed that exogenous Spd effectively alleviated the limitation of high temperature (HT) on photosynthesis. Exogenous Spd increased the chlorophyll content, net photosynthetic rate, intercellular carbon dioxide concentration, stomatal conductance, maximum and effective quantum yield of PSII photochemistry, nonphotochemical quenching coefficient, and electron transport rate in citrus seedlings under HT stress, but declined the stomatal limitation value. In addition, Spd treatment promoted the dynamic balance of the citrus enzymatic and non-enzymatic antioxidants system. Spd application significantly increased the activity of superoxide dismutase, peroxidase, catalase, ascorbic acid, and glutathione and the expression level of corresponding genes at high temperature, while reducing the content of H₂O₂ and malondialdehyde. Therefore, our findings suggested exogenous Spd significantly ameliorated citrus physiological and photosynthetic adaptation under HT stress, thereby providing helpful guidance for citrus cultivation in HT events.

Keywords: Antioxidants; chlorophyll fluorescence; gas exchange; heat stress; photosynthesis.

Figure 1.

Effects of exogenous spermidine treatment on chlorophyll and carotenoid concentrations of citrus plants exposed to high temperature stress. Note: Different letters indicate statistically significant by Duncan multiple- range test at P < .05. Data were expressed as the mean ± standard error of three independent biological replicates. Key: Chl – chlorophyll, Car – carotenoid.

Figure 2.

Effects of exogenous spermidine treatment on light response curves of citrus plants exposed to high temperature stress. Note: Data were expressed as the mean ± standard error of three independent biological replicates. Key: PN – net photosynthetic rate, PAR – photosynthetically active radiation.

Figure 3.

Effects of exogenous spermidine treatment on gas exchange parameters of citrus plants exposed to high temperature stress. Note: Different letters indicate statistically significant by Duncan multiple- range test at P < .05. Data were expressed as the mean ± standard error of three independent biological replicates. Key: PN – net photosynthetic rate, gs – stomatal conductance, Ci – intercellular carbon dioxide concentration, Ls – stomatal limitation value.

Figure 4.

Effects of exogenous spermidine treatment on Chl fluorescence parameters of citrus plants exposed to high temperature stress. Note: Different letters indicate statistically significant by Duncan multiple- range test at P < .05. Data were expressed as the mean ± standard error of three independent biological replicates. Key: Fv/Fm – maximal quantum yield of PSII photochemistry, NPQ – nonphotochemical quenching, ΦPSII – effective quantum yield of PSII photochemistry, ETR – electron transport rate.

Figure 5.

Effects of exogenous spermidine treatment on the activity of SOD, POD and CAT in citrus plants exposed to high temperature stress. Note: Different letters indicate statistically significant by Duncan multiple- range test at P < .05. Data were expressed as the mean ± standard error of three independent biological replicates. Key: SOD – superoxide dismutase, POD – peroxidase, CAT – catalase.

Figure 6.

Effects of exogenous spermidine treatment on the content of ASA and GSH in citrus plants exposed to high temperature stress. Note: Different letters indicate statistically significant by Duncan multiple- range test at P < .05. Data were expressed as the mean ± standard error of three independent biological replicates. Key: ASA – Ascorbic acid, GSH – glutathione, CAT – catalase.