Photosynthesis And Salicylic Acid Regulation Mechanism Of Pumpkin Rootstock Grafting To Alleviate Salt Stress Injury In Cucumber

Due to unreasonable fertilization and environmental closure of protected horticulture,the secondary salinization of protected soils has become increasingly serious,which has become one of the main factors hindering the health and sustainable development of protected horticutural products in China.Studies have shown that the soil salinization in protected horticulture is characterized by excessive accumulation of Ca2+and NO3-.Cucumber(Cucumis sativus L.)is one of the main vegetable crops cultivated in the protected horticulture.Because of its weak roots and shallow distribution,its root system has relatively low absorption capacity and is very sensitive to the rhizosphere environment.It is often salt-stressed during production.The grafting of cucumber with salt-tolerant pumpkin is an effective and environmentally friendly measure to alleviate the salt stress damage of cucumber plants,which has been popularized all over the country.However,there are few studies on the grafting with pumpkin as rootstock to alleviate the physiological mechanism of Ca(NO3)2 stress damage in cucumber plants.Therefore,in this study,the salt-tolerant white-seed squash(Cucurbita maxima × Cucurbita moschata)variety ’Qing zhen No.1’ was used as the rootstock,and the ’Jinyou No.4’ cucumber was used as the scion,grafted by plugging and 80 mM Ca(NO3)2 was used for salt application.The physiological mechanism of relieved effect of pumpkin rootstock grafted on cucumber under salt stress was studied in three aspects:photosynthesis,chlorophyll fluorescence parameters and salicylic acid(SA)-hydrogen peroxide(H2O2)signal molecule content.The main results are as follows:1.The results showed that salt stress significantly inhibited the growth of cucumber plants,mainly showd by the sharp decline of fresh weight in the shoots.Salt stress significantly reduced the net photosynthetic rate of cucumber plants under different light intensity conditions,but the rootstock-grafted cucumber could maintained higher net photosynthetic rate during larger range of light intensity wiith the prolongation of salt stress.In the meantime,the change model of stomatal conductance and transpiration rate were similar to net photosynthetic rate,which indicating that grafting can make cucumber plants adapt to salt stress quickly.These results implied that rootstock-grafted cucumber could a relatively stable photosynthetic and accumulate more biomass.2.Salt stress reduced the actual photochemical efficiency(Y(Ⅱ))of self-rooted grafted seedlings,but had no significant effect on grafted seedlings of rootstock;the electron transport rate(ETR)of grafted seedlings of cucumber rootstock under salt stress(ETR)increased steadily with the increase of light intensity,while the self-rooted grafted seedlings showed a short-term decline when the light intensity increased,indicating that the self-rooted grafted seedlings had weak adaptability to light intensity changes;The energy consumption scattered quantum yields Y(NPQ)and Y(NO)of the rootstock grafted seedlings can quickly restore the control level under dark conditions while the self-rooted grafted seedlings still show large differences;and in illumination conditions,Y(NPQ)of rootstock-grafted cucumber is larger when Y(NO)is smaller,indicating that the damage of the photosynthetic system of rootstock-grafted seedlings was smaller than that of self-grafted plants.3.Salt stress induced the transient of conjugated SA and H2O2 in the roots of grafted rootstocks,and the self-grafted seedlings showed a sustained response of conjugated SA,which was stable at a higher level during stress;The transient increase of conjugated SA in leaves was only observed in rootstock-grafted cucumber,indicating that the response of SA and H2O2 in roots was significantly higher than that in leaves under salt stress;and the response to salt stress in rootstock grafted seedlings was more significant than that of self-grafted ones.The effect of AOPP on H2O2 was not significant,but the co-application of SA and AOPP significantly down-regulated the H2O2 level.The results showed that,on the one hand,the increase of SA level has a negative regulation effect on H2O2;on the other hand,the SA under salt stress is mainly synthesized by the ICS pathway rather than the PAL pathway.