Precise irrigation management of grapevines in greenhouses requires a reliable method to easily quantify and monitor the grapevine water status to enable effective manipulation of the water stress of the plants. This study evaluated the applicability of crop water stress index (CWSI) based on the leaf temperature for diagnosing the grapevine water status. The experiment was conducted at Yuhe Farm (northwest China), with drip-irrigated grapevines under three irrigation treatments. Meteorological factors, soil moisture contents, leaf temperature, growth indicators including canopy coverage and fruit diameter, and physiological indicators including SPAD (relative chlorophyll content), stem water potential (${\phi }_s$), stomatal conductance (g_s), and transpiration rate (E) were studied during the growing season. The results show that the relationship between the leaf-air temperature difference (T_c-T_a) and the plant water status indicators (${\phi }_s$, g_s, E) were significant (P < 0.05), and the relationship between g_s, E and T_c-T_a was the closest, with R² values ranging from 0.530–0.604 and from 0.545–0.623, respectively. CWSI values are more easily observed on sunny days, and it was determined that 14:00 BJS is the best observation time for the CWSI value under different non-water-stressed baselines. There is a reliable linear correlation between the CWSI value and the soil moisture at 0–40 cm (P < 0.05), which could provide a reference when using the CWSI to diagnose the water status of plants. Compared with the T_c-T_a value, the CWSI could more accurately monitor the plant water status, and above the considered indictors, g_s has the greatest correlation with the CWSI.

More frequent and more intense heat waves and greater drought stress will occur in the future climate environment. Short-term extreme heat and drought stress often occur simultaneously after winter wheat anthesis, which has become the major constraint threatening future wheat yield. In this study, short-term heat, drought and their combination stress were applied to wheat plants after anthesis, and all wheat plants were restored to the outdoor normal temperature and full watering after stress treatment. The aim of the current study was to evaluate the role of nitrogen (N) in modulating the effects of post-anthesis short-term heat, drought and their combination stress on photosynthesis, N metabolism-related enzymes, the accumulation of N and protein and growth, as well as on the yield and water (WUE) and N use efficiency (NUE) of wheat after stress treatment. The results showed that compared with low N application (N1), medium application (N2) enhanced the activities of nitrate reductase (NR) and glutamine synthase (GS) in grains under post-anthesis heat and drought stress alone, which provided a basis for the accumulation of N and protein in grains at the later stage of growth. Under post-anthesis individual stresses, N2 or high application (N3) increased the leaf photosynthetic rate (A_n), PSII photochemical efficiency and instantaneous WUE compared with N1, whereas these parameters were usually significantly improved by N1 application under post-anthesis combined stress. The positive effect of increased A_n by N application on growth was well represented in a higher green leaf area, aboveground dry mass and plant height, and the variation in A_n can be explained more accurately by the N content per unit leaf area. Short-term heat, drought and combined stress after anthesis resulted in a pronounced decrease in yield by reducing grain number per spike and thousand kernel weight. The reduction in NUE under combined stress was higher than that under individual heat and drought stress. Compared with N1, N2 or N3 application significantly prevented the decrease in yield and NUE caused by post-anthesis heat and drought stress alone. However, N1 application was conducive to improving the productivity, WUE and NUE of wheat when exposed to post-anthesis combined stress. The current data indicated that under short-term individual heat and drought stress after anthesis, appropriately increasing N application effectively improved the growth and physiological activity of wheat compared with N1, alleviating the reduction in yield, WUE and NUE. However, under combined stress conditions, reducing N application (N1) may be a suitable strategy to compensate for the decrease in yield, WUE and NUE.

Early heat and drought priming may increase the plant’s ability to resist later drought and heat stress. However, it remains unclear whether combined heat and drought priming can enhance the acclimation of plants to later combined stress by improving physiological activities. In this study, maize seedlings were first pre-exposed twice to heat, drought, and a combination of stresses followed by recovery, and then subjected to six days of more severe stresses. A considerable reduction in photosynthetic pigment content, stomatal size, and photosynthesis was observed under heat and drought conditions, and the changes in the above indicators were amplified under combined stress conditions. Stress priming improves antioxidant defense and cellular osmoregulation, as indicated by improved superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione reductase (GR), and ascorbate peroxidase activities, as well as elevated soluble sugar (SS) and proline (Pro) contents. Lower superoxide anion and malondialdehyde contents and injury index in the primed seedlings demonstrated the mitigation of oxidative stress. ROC analysis revealed that SOD and POD had considerable reliability in determining that maize seedlings were experiencing heat stress (AUC = 0.941–0.971); GR and SS were capable of accurately monitoring drought stress that was being experienced by plants (AUC = 0.919–0.958); and SOD, GR, and Pro had more capability for detecting the combination of heat and drought stress (AUC = 0.907–0.958). Collectively, the primed seedlings exhibited better performance than the non-primed seedlings, exhibiting stronger stress acclimation supported by an effective antioxidant defense system and osmoregulatory function.

Precise irrigation management of grapevines in greenhouses requires a reliable method to easily quantify and monitor the grapevine water status to enable effective manipulation of the water stress of the plants. This study describes a study on stem diameter variations of grapevine planted in a greenhouse in the semi-arid area of Northwest China. In order to determine the applicability of signal intensity of stem diameter variation to evaluate the water status of grapevine and soil. The results showed that the relative variation curve of the grapevine stem diameter from the vegetative stage to the fruit expansion stage showed an overall increasing trend. The correlations of MDS (maximum daily shrinkage) and DI (daily increase) with meteorological factors were significant (p < 0.05), and the correlations with SWP, RWC and soil moisture were weak. Although MDS and DI can diagnose grapevine water status in time, SI_MDS and SI_DI have the advantages of sensitivity and signal intensity compared with other indicators. Compared with MDS and DI, the R² values of the regression equations of SI_MDS and SI_DI with SWP and RWC were high, and the correlation reached a very significant level (p < 0.01). Thus, SI_MDS and SI_DI are more suitable for the diagnosis of grapevine water status. The SI_MDS peaked at the fruit expansion stage, reaching 0.957–1.384. The signal-to-noise ratio of SI_DI was higher than that of MDS across the three treatments at the vegetative stage. The value and signal-to-noise ratio of SI_DI at the flowering stage were similar to those of SI_MDS, while the correlation between SI_DI and the soil moisture content was higher than that of SI_MDS. It can be concluded that that SI_DI is suitable as an indicator of water status of grapevine and soil during the vegetative and flowering stages. In addition, the signal-to-noise ratio of SI_MDS during the fruit expansion and mature stages was significantly higher than that of SI_DI. Therefore, SI_MDS is suitable as an indicator of the moisture status of grapevine and soil during the fruit expansion and mature stages. In general, SI_MDS and SI_DI were very good predictors of the plant water status during the growth stage and their continuous recording offers the promising possibility of their use in automatic irrigation scheduling in grapevine.