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Aims: Studies dealing with plants' mineral nutrient status under elevated atmospheric CO2 concentration (eCO2) are usually conducted in closed hydroponic systems, in which nutrient solutions are entirely renewed every several days. Here, we investigated the contribution of the fluctuation of concentrations of N ([N]), P ([P]), and K ([K]) in nutrient solutions in this short period on their concentrations in cucumber plants exposed to different [CO2] and N levels. Methods: Cucumber (Cucumis sativus L.) plants were hydroponically grown under two [CO2] and three N levels. [N], [P], and [K] in nutrient solutions and cucumber plants were analyzed. Results: The transpiration rate (Tr) was significantly inhibited by eCO2, whereas Tr per plant was increased due to the larger leaf area. Elevated [CO2] significantly decreased [N] in low N nutrient solutions, which imposed an additional decrease in [N] in plants. [P] in nutrient solutions fluctuated slightly, so the change of [P] in plants might be attributed to the dilution effect and the demand change under eCO2. [K] in moderate and high N nutrient solutions were significantly decreased, which exacerbated the [K] decrease in plants under eCO2. Conclusions: The short-term fluctuation of [N] and [K] in nutrient solutions is caused by the asynchronous uptakes of N, K, and water under eCO2, which has an appreciable influence on [N] and [K] in plants besides the dilution effect. This defect of the closed hydroponic system may let us exaggerate the negative impact of eCO2 itself on [N] and [K] in plants. [ABSTRACT FROM AUTHOR]

With respect to vegetables with high nitrate accumulation, very little information is available on how elevated CO 2 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 CO 2 [400 (aCO 2 ), 625 (subeCO 2 ) and 1200 (eCO 2 ), μmol mol −1 ] with three NO 3 − concentrations [2 (low NO 3 − ), 7 (moderate NO 3 − ) and 14 (high NO 3 − ), 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 NO 3 − 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 NO 3 − treatment, with no promotion in moderate NO 3 − treatment. Our results also found that the alleviation of photosynthetic acclimation accompanied with higher N assimilation. Specifically, eCO 2 increased N content of entire seedlings in high NO 3 − supply and maintained N concentration in leaves. Elevated CO 2 increased the ratio of NH 4 + -N to total N in roots more than that in leaves and decreased the ratio of NO 3 − -N to total N in roots greater than that in leaves, which implied that eCO 2 probably promoted NO 3 − assimilation in roots more than that in leaves. Elevated CO 2 is more likely to inhibit N assimilation, and then decreases the sink strength, thus limits photosynthate transportation from leaves. Therefore, we suggest more NO 3 − fertilizer is needed to match CO 2 fertilization to enhance cucumber yield. [ABSTRACT FROM AUTHOR]

Greenhouse cultivation forms a relatively closed environment for heat preservation in winter, which inevitably leads to a lack of atmospheric CO 2 concentration ([CO 2 ]), and thus low [CO 2 ] 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 [CO 2 ], 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 [CO 2 ]. We also observed long-term elevated [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ], which indicated an alleviation of the photosynthetic acclimation of cucumber plants exposed to long-term elevated [CO 2 ]. [Display omitted] • Root-zone temperature (RZT) had a greater impact on cucumber dry weight than CO 2 concentration ([CO 2 ]). • Long-term elevated [CO 2 ] caused photosynthetic acclimation at ambient RZT. • Elevated RZT was favorable for transporting photosynthate from leaves to roots. • Elevated RZT promoted the nitrogen absorption by roots and upward transportation. • Elevated RZT alleviated the photosynthetic acclimation of cucumber plants. [ABSTRACT FROM AUTHOR]

The concentration changes of mineral elements in plants at different CO2 concentrations ([CO2]) and nitrogen (N) supplies and the mechanisms which control such changes are not clear. Hydroponic trials on cucumber plants with three [CO2] (400, 625, and 1200 μmol mol−1) and five N supply levels (2, 4, 7, 14, and 21 mmol L−1) were conducted. When plants were in high N supply, the increase in total biomass by elevated [CO2] was 51.7% and 70.1% at the seedling and initial fruiting stages, respectively. An increase in net photosynthetic rate (Pn) by more than 60%, a decrease in stomatal conductance (Gs) by 21.2–27.7%, and a decrease in transpiration rate (Tr) by 22.9–31.9% under elevated [CO2] were also observed. High N supplies could further improve the Pn and offset the decrease of Gs and Tr by elevated [CO2]. According to the mineral concentrations and the correlation results, we concluded the main factors affecting these changes. The dilution effect was the main factor driving the reduction of all mineral elements, whereas Tr also had a great impact on the decrease of [N], [K], [Ca], and [Mg] except [P]. In addition, the demand changes of N, Ca, and Mg influenced the corresponding element concentrations in cucumber plants. [ABSTRACT FROM AUTHOR]