Elevated root-zone temperature promotes the growth and alleviates the photosynthetic acclimation of cucumber plants exposed to elevated [CO2]

Greenhouse cultivation forms a relatively closed environment for heat preservation in winter, which inevitably leads to a lack of atmospheric CO2 concentration ([CO2]), and thus low [CO2] level has become a limiting factor for the photosynthesis and growth of greenhouse vegetables. Most cucurbits and solanaceous vegetables are sensitive to low temperature, but it is difficult to keep the optimum root-zone temperature (RZT) for them in greenhouse soil cultivation in winter. Therefore, low RZT is another limiting factor for greenhouse vegetables growth in winter. This study investigated the effects of [CO2], RZT, and their interactions on cucumber yield, growth, photosynthesis, and photosynthate allocation in four open-top chambers for a growth period of 73 days after transplanting. We found elevated RZT increased yield and total dry weight of cucumber plants to a greater extent than elevated [CO2]. We also observed long-term elevated [CO2] limited the increase of net photosynthetic rate (Pn) and decreased the stomatal conductance (Gs) and transpiration rate (Tr) of cucumber plants at ambient RZT, resulted in the photosynthetic acclimation, as illustrated by the significant reduction of nitrogen concentration and the increased concentrations of soluble sugar and starch in leaves. In comparison, the combination of elevated [CO2] and RZT decreased the concentrations of soluble sugar and starch and maintained the nitrogen concentration in leaves, which caused a higher Pn, Gs and Tr. Meanwhile, more soluble sugar and starch were allocated from leaves to roots due to the higher root respiratory under elevated [CO2] and RZT treatment. Moreover, elevated RZT could facilitate the mineral nutrients absorption by roots and the upward transportation because of the larger root system and higher Tr. Therefore, elevated RZT could offset the decrease in Pn and improve Gs and Tr under elevated [CO2], which indicated an alleviation of the photosynthetic acclimation of cucumber plants exposed to long-term elevated [CO2].