Two spring wheat varieties were used to study alleviating effects of exogenous spermine (Spm) and spermidine (Spd) on agronomic traits and photosynthesis under high temperature treatment (HT). Our results showed that HT significantly decreased grain mass per panicle of heat-resistant variety (XC 6) by 25% and heat-sensitive variety (XC 31) by 32%. After HT, i.e., at 13 d after flowering, the decrease in net photosynthetic rate of XC 6 (38%) was lower than that of XC 31 (53%); the reason for this was related to XC 6, which could maintain correspondingly normal chlorophyll content (Chl), stomatal conductance (gs), transpiration rate (E), and intercellular CO2 concentration. Exogenous Spm and Spd could increase relative water content, Chl, gs, E, the maximal quantum yield of PSII photochemistry, effective quantum yield of PSII photochemistry, antenna conversion efficiency, and photochemical quenching coefficient of flag leaves under HT. Our results indicated that the heat resistance of XC 6 is better than that of XC 31 and exogenous Spm and Spd could alleviate the heat injury of photosynthesis of wheat flag leaves. (normal growth conditions outside the greenhouse); DAF -days after flowering; DM -dry mass; E -transpiration rate; FM -fresh mass; Fv/Fm -maximal quantum yield of PSII photochemistry; Fv'/Fm' -antenna conversion efficiency; GMP -grain mass per panicle; GNP -grain number per panicle; gs -stomatal conductance; HT -artificially simulated high temperature; HT + Spd -artificially simulated high temperature + exogenous application spermidine (1 mM L -1 ); HT + Spm -artificially simulated high temperature + exogenous application of spermine (1 mM L -1 ); MDA -malondialdehyde; PAs -polyamines; PN -net photosynthetic rate; qp -photochemical quenching coefficient; RWC -relative water content; Spd -spermidine; Spm -spermine; TKM -thousand kernel mass; TM -turgid mass; TW -test mass; ФPSII -effective quantum yield of PSII photochemistry.

Pumpkin rootstock affects watermelon scion growth, fruit yield, and quality, but the mechanisms of related key enzymes and photosynthesis remain unclear. In this study, net photosynthetic rate (PN), chlorophyll (Chl) fluorescence parameters, fruit mass and sugar accumulation, and related key enzyme activities were determined during the fruit development stage in diploid and triploid watermelon lines and corresponding pumpkin rootstock-grafted lines. The results showed that pumpkin rootstock increased PN and Chl fluorescence parameters of diploid and triploid watermelon, indicating that pumpkin rootstock could increase photosynthesis, the utilization efficiency of light energy of diploid and triploid watermelon lines. Pumpkin-grafted lines had higher alkaline α-galactosidase activity and lower activities of insoluble acid invertase, sucrose phosphate synthase, and sucrose synthase than corresponding own-root lines in diploid and triploid watermelon. It indicates that pumpkin rootstock could increase the unloading of photoassimilates and reduce the conversion of photoassimilates into sucrose in diploid and triploid watermelon fruits.