Photosynthesis And PA/H₂O₂ Mechanism Of Momordica Rootstock Grafting To Alleviate Heat Stress In Cucumber

In recent years,with the change of global climate,extreme high temperature events have become more and more popular,which has brought great challenges to the control of facility horticultural environment.Cucumber(Cucumis sativus L.)is a vegetable crop that is widely respected in facility horticultural cultivation.In China’s facility horticulture cultivation,the summer and autumn seasons often suffer from the suppression of crop growth due to high temperature and strong light stress,which seriously affects the production and supply of vegetables.Grafting is widely concerned as a technique that can significantly improve the heat resistance of cucumber plants.Studies have shown that using heat-resistant rootstocks for grafting can significantly improve the heat resistance of cucumbers.However,at present,the physiological mechanism of graft replacement for relieving heat stress is still unclear.In this paper,the cucumber(Cucumis sativus L.)variety’Jinyou 35’ was used as the scion,and the heat-resistant Momordica(Momordica charantia L.)variety’Changlv’ was used as the rootstock.Cleft grafting was used in this study to study the effects of Momordica rootstock on fluorescence parameters,photosynthesis,polyamine-hydrogen peroxide signal molecules and antioxidant enzyme activities of cucumber seedlings under long-term heat stress.Below are key research findings:1.Heat stress significantly inhibited the biomass accumulation of Momordica-grafted seedlings and self-grafted seedlings,resulting in plant height,fresh and aboveground dry weight significantly lower than the control level,plants showed significant growth inhibition,while Momordica rootstock significantly alleviated the decline of biomass accumulation caused by heat stress,it can maintain the aboveground fresh and dry weight significantly higher than the level of self-grafted seedlings.Heat stress significantly reduced the maximum photochemical efficiency of the leaves of grafted cucumbers(FV/FM),actual photochemical efficiency Y(Ⅱ),and photochemical quenching coefficient qP,and the decline is more significant with the extension of high temperature stress time.Grafting of Momordica rootstock can significantly alleviate the decline in fluorescence parameters caused by high temperature stress and maintain a high level.The levels of FV/FM,Y(Ⅱ)and qP indicate that the grafting of Momordica rootstock can maintain high photosynthetic activity and photochemical efficiency under heat stress,thereby improving the plant’s heat resistance.2.Under heat stress,the photosynthetic parameters of the two grafted seedlings with different light intensities decreased significantly,including net photosynthetic rate(Pn),stomatal conductance(Cn)and transpiration rate(Tr),and with the duration of stress increased,the degree of decrease increased,but Momordica rootstock can significantly alleviate the decline of photosynthetic parameters.With the prolongation of heat stress,the decline degree of Momordica-grafted plants is significantly lower than that of self-grafted seedlings,and the light saturation point of self-rooted grafted seedlings is significantly higher shift.This shows that under long-term high temperature stress,the photosynthetic system of the self-grafted seedlings was irreversibly damaged,and Momordica rootstock grafting can gradually maintain a more stable photosynthetic system and maintain a higher level of photosynthesis in the leaves.3.High temperature stress significantly induced the transient increase of endogenous free polyamine content and hydrogen peroxide content in the leaves of grafted cucumber seedlings,but the induction effect of Momordica-grafted plants was more significant.The content of hydrogen peroxide in Momordica-grafted plants reached its peak 4h earlier than the self-grafted seedlings..Experiments with polyamine synthesis inhibitors(D-Arg,MGBG)showed that the increase of endogenous polyamines may have a mediating effect on the production of hydrogen peroxide under high temperature stress.Under high temperature stress,the antioxidant enzyme activity of the leaves of the grafted cucumber seedlings gradually increased from 8h.Except catalase(CAT),the antioxidant enzyme activities of the Momordica-grafted plants were significantly higher than the self-grafted seedlings within 72 hours of the test period,and the antioxidant enzyme activity of self-grafted seedlings began to decrease significantly after 24h;treatment with polyamine synthesis inhibitors(D-Arg,MGBG)and hydrogen peroxide scavenger(DMTU)could significantly reduce the antioxidant enzyme activity under high temperature stress,but Momordica-grafted Plant decline was significantly lower than that of self-grafted seedlings.In summary,the results show that Momordica rootstock can produce and accumulate more endogenous polyamines in plants under high temperature stress,and then quickly mediate the production of hydrogen peroxide,regulate the increase of antioxidant enzyme activity in the plant,and protect against the effects of high temperature stress to alleviate the damage caused by high temperature stress on plants.