Area under vegetable cultivation is expanding in arid and semi-arid regions of the world to meet the nutritional requirements of an ever-growing population. However, water scarcity in these areas is limiting vegetable productivity. New water-conserving irrigation management practices are being implemented in these areas. Under these irrigation management practices, crops are frequently exposed to some extent of water stress. Vegetables are highly sensitive to water stress. For the successful implementation of new irrigation practices in vegetable crops, it is of immense importance to determine the threshold water deficit level which will not have a detrimental effect on plant growth and yield. Along with this, plant response and adaptation mechanisms to new irrigation practices need to be understood for the successful implementation of new irrigation practices. To understand this, water stress indicators that are highly responsive to water stress; and that can help in early detection of water stress need to be identified for vegetable crops. Plant-based water stress indicators are quite effective in determining the water stress level in plants because they take into account the cumulative effect of water stress due to declining soil moisture status and increased evaporative demand of the atmosphere while determining the water stress level in plant. Water stress quantification using plant-based approaches involves direct measurements of several aspects of plant water status and indirect measurements of plant processes which are highly sensitive to water deficit. In this article, a number of plant-based water stress indicators were critically reviewed for (1) their efficacy to determine the level of water stress, (2) their potential to predict the yield of a crop as affected by different water-deficit levels and (3) their suitability for irrigation scheduling in vegetable crops.

Cotton leafroll dwarf disease (CLRDD) caused by cotton leafroll dwarf virus (CLRDV) is an emerging threat to cotton production in the United States. The disease was first reported in Alabama in 2017 and subsequently has been reported in 10 other cotton producing states in the United States, including Georgia. A field study was conducted at field sites near Tifton, Georgia in 2019 and 2020 to evaluate leaf gas exchange, chlorophyll fluorescence, and leaf temperature responses for a symptomatic cultivar (diseased plants observed at regular frequency) at multiple stages of disease progression and for asymptomatic cultivars (0% disease incidence observed). Disease-induced reductions in net photosynthetic rate (A_n, decreased by 63–101%), stomatal conductance (g_s, decreased by 65–99%), and efficiency of the thylakoid reactions (32–92% decline in primary photochemistry) were observed, whereas leaf temperature significantly increased by 0.5–3.8°C at advanced stages of the disease. Net photosynthesis was substantially more sensitive to disease-induced declines in g_s than the thylakoid reactions. Symptomatic plants with more advanced disease stages remained stunted throughout the growing season, and yield was reduced by 99% by CLRDD due to reductions in boll number per plant and declines in boll mass resulting from fewer seeds per boll. Asymptomatic cultivars exhibited more conservative gas exchange responses than apparently healthy plants of the symptomatic cultivar but were less productive. Overall, it is concluded that CLRDV limits stomatal conductance and photosynthetic activity of individual leaves, causing substantial declines in productivity for individual plants. Future studies should evaluate the physiological contributors to genotypic variation in disease tolerance under controlled conditions.

Nitrogen (N) deficiency limits net carbon assimilation rates (A_N), but the relative N sensitivities of photosynthetic component processes and carbon loss mechanisms remain relatively unexplored for field-grown cotton. Therefore, the objective of the current study was to define the relative sensitivity of individual physiological processes driving N deficiency-induced declines in A_N for field-grown cotton. Among the potential diffusional limitations evaluated, mesophyll conductance was the only parameter substantially reduced by N deficiency, but this did not affect CO₂ availability in the chloroplast. A number of metabolic processes were negatively impacted by N deficiency and these effects were more pronounced at lower leaf positions in the cotton canopy. RuBP regeneration and carboxylation, A_N, and gross photosynthesis were the most sensitive metabolic processes to N deficiency, whereas photosynthetic electron transport processes, electron flux to photorespiration, and dark respiration exhibited intermediate sensitivity to N deficiency. Among thylakoid specific processes, the quantum yield of PSI end electron acceptor reduction was the most sensitive process to N deficiency. It was concluded that A_N is primarily limited by Rubisco carboxylation and RuBP regeneration under N deficiency in field grown cotton and the differential N-sensitivities of the photosynthetic process and carbon loss mechanisms contributed significantly to photosynthetic declines.

Water deficit stress limits net photosynthetic rate (A_N), but the relative sensitivities of underlying processes such as thylakoid reactions, ATP production, carbon fixation reactions, and carbon loss processes to water deficit stress in field‐grown upland cotton require further exploration. Therefore, the objective of the present study was to assess (1) the diffusional and biochemical mechanisms associated with water deficit‐induced declines in A_N and (2) associations between water deficit‐induced variation in oxidative stress and energy dissipation for field‐grown cotton. Water deficit stress was imposed for three weeks during the peak bloom stage of cotton development, causing significant reductions in leaf water potential and A_N. Among diffusional limitations, mesophyll conductance was the major contributor to the A_N decline. Several biochemical processes were adversely impacted by water deficit. Among these, electron transport rate and RuBP regeneration were most sensitive to A_N‐limiting water deficit. Carbon loss processes (photorespiration and dark respiration) were less sensitive than carbon assimilation, contributing to the water deficit‐induced declines in A_N. Increased energy dissipation via non‐photochemical quenching or maintenance of electron flux to photorespiration prevented oxidative stress. Declines in A_N were not associated with water deficit‐induced variation in ATP production. It was concluded that diffusional limitations followed by biochemical limitations (ETR and RuBP regeneration) contributed to declines in A_N, carbon loss processes partially contributed to the decline in A_N, and increased energy dissipation prevented oxidative stress under water deficit in field‐grown cotton.