High nitrate supply promotes nitrate assimilation and alleviates photosynthetic acclimation of cucumber plants under elevated CO2

With respect to vegetables with high nitrate accumulation, very little information is available on how elevated CO2 affects their nitrate assimilation. In this study, cucumber (Cucumis sativus L) plants were hydroponically grown for two stages (the seedling stage and the initial fruit stage) under three levels of CO2 [400 (aCO(2)), 625 (subeCO(2)) and 1200 (eCO(2)), mu mol mol(-1)] with three NO3- concentrations [2 (low NO3-), 7 (moderate NO3-) and 14 (high NO3-), mmol L-1] in open top chambers. Our results showed that subeCO(2) had no significant effects on plant growth at either stage, whereas eCO(2) increased both plant photosynthesis and biomass, with the increase being greater at the seedling stage. The alleviation of photosynthetic acclimation was not only at the seedling stage, but also in high NO3- treatment indicated by higher net photosynthesis rates and plant biomass, higher C, fructose and glucose concentration in leaves, and lower starch concentration in leaves. The yield of cucumber increased by 73% under eCO(2) in high NO3- treatment, with no promotion in moderate NO3- treatment. Our results also also found that the alleviation of photosynthetic acclimation accompanied with higher N assimilation. Specifically, eCO(2) increased N content of entire seedlings in high NO3- supply and maintained N concentration in leaves. Elevated CO2 increased the ratio of NH4+-N to total N in roots more than that in leaves and decreased the ratio of NO3--N to total N in roots greater than that in leaves, which implied that eCO(2) probably promoted NO3- assimilation in roots more than that in leaves. Elevated CO2 is more likely to inhibit N assimilation, and then decreases the sink strength, thus limits photosynthate transportation from leaves. Therefore, we suggest more NO3- fertilizer is needed to match CO2 fertilization to enhance cucumber yield. (C) 2016 Published by Elsevier B.V.