The purpose of this study was to explore the carbon and nitrogen metabolism mechanisms of sand-cultivated cucumbers under different deficit irrigation-nitrogen management strategies and provide a theoretical basis for their greenhouse management. This study set up two factors, the deficit irrigation level and the nitrogen application rate, and conducted an experiment on deficit irrigation-nitrogen coupling of sand-cultivated cucumbers using a quadratic saturation D-optimal design. Seven treatments were set up in the experiment, to measure the soluble sugar and protein contents, as well as the activity of key enzymes for carbon and nitrogen metabolism at five different growth stages. The results indicate that the 80% irrigation with 623 kg N hm-2 (IN4) treatment significantly improved the soluble sugar, protein, and actual leaf nitrogen contents of cucumber at the five different growth stages and, as a result, achieved higher sucrose synthase (SS) and sucrose phosphate synthase (SPS) activities in the cucumber leaves. Furthermore, such improvements were due to the reduction in oxidative damage of sand-cultivated cucumber at various growth stages. The IN4 and 89% irrigation with 1250 kg N hm-2 (IN5) treatments significantly increased the activities of RuBisCO, catalase (CAT), peroxidise (POD), and superoxide dismutase (SOD) at various growth stages of sand-cultivated cucumber. The higher activities of glutamate dehydrogenase (GLDH), glutamate synthase (GOGAT), nitrate reductase (NR), glutamine synthase (GS), acid invertase enzyme (AIE), neutral invertase enzyme (NIE), and better antioxidative enzyme activities were recorded under the IN4 treatments at various growth stages, which effectively improve (69.6%) cucumber yield. The soil properties, carbon and nitrogen metabolism, and antioxidant metabolism were positively correlated with sand-cultivated cucumber yield in a greenhouse. We concluded that the IN4 treatment was the better deficit irrigation-nitrogen management strategy because it considerably improves carbon and nitrogen metabolism, antioxidant enzyme activities, and sand-cultivated cucumber yield in a greenhouse.

Reproductive failure in cotton caused by drought has been reported to be closely associated with alterations in pistil fertility; however, the mechanism of the effect of drought on pistil fertility in cotton is less studied. We hypothesized that drought would inhibit the ovule formation to alter pistil potential fertility. To address this hypothesis, we conducted a water deficit induction experiment with a cotton cultivar, Dexiamian 1. Results showed that drought damaged the cytological structure of the developing ovules. This resulted in a lower ovule number, finally leading to lower cottonseed number and boll weight. And the decreased ovule number was closely related to the reactive oxygen species (ROS) accumulation in pistil during ovule development. Further analysis of antioxidant metabolism found that in the enzymatic antioxidant system, drought decreased the activities of superoxide dismutase (SOD) and catalase (CAT), resulting in the accumulation of superoxide anion (O2 center dot-$$ {{\mathrm{O}}_2}{\bullet -} $$) and hydrogen peroxide (H2O2). Regarding the non-enzymatic antioxidant system, the elevated glutathione reductase gene (GhGR) expression under drought promoted the glutathione (GSH) accumulation; however, the decreased dehydroascorbate reductase gene (GhDHAR2) expression under drought inhibited the conversion of GSH to ascorbic acid (AsA). Although the increased monodehydroascorbate reductase gene (GhMDHAR) expression under drought promoted AsA accumulation, drought-induced reduced ascorbate peroxidase gene (GhAPX) expression inhibited the reduction of H2O2 by AsA, which ultimately led to higher AsA content and H2O2 content. We conclude that drought impedes the ovule formation by disturbing pistil's antioxidant metabolic homeostasis to destruct the cytological structure of the developing ovules.