Partial root-zone drying irrigation improves intrinsic water-use efficiency and maintains high photosynthesis by uncoupling stomatal and mesophyll conductance in cotton leaves

Partial root-zone drying irrigation (PRD) can improve water-use efficiency (WUE) without reductions in photosynthesis; however, the mechanism by which this is attained is unclear. To amend that, PRD conditions were simulated by polyethylene glycol 6000 in a root-splitting system and the effects of PRD on cotton growth were studied. Results showed that PRD decreased stomatal conductance (gs) but increased mesophyll conductance (gm). Due to the contrasting effects on gs and gm, net photosynthetic rate (AN) remained unaffected, while the enhanced gm/gs ratio facilitated a larger intrinsic WUE. Further analyses indicated that PRD-induced reduction of gs was related to decreased stomatal size and stomatal pore area in adaxial and abaxial surface which was ascribed to lower pore length and width. PRD-induced variation of gm was ascribed to the reduced liquid-phase resistance, due to increases in chloroplast area facing to intercellular airspaces and the ratio of chloroplast surface area to total mesophyll cell area exposed to intercellular airspaces, as well as to decreases in the distance between cell wall and chloroplast, and between adjacent chloroplasts. The above results demonstrate that PRD, through alterations to stomatal and mesophyll structures, decoupled gs and gm responses, which ultimately increased intrinsic WUE and maintained AN.

Carbon assimilation and distribution in cotton photosynthetic organs is a limiting factor affecting boll weight formation under drought

Previous studies have documented cotton boll weight reductions under drought, but the relative importance of the subtending leaf, bracts and capsule wall in driving drought-induced reductions in boll mass has received limited attention. To investigate the role of carbon metabolism in driving organ-specific differences in contribution to boll weight formation, under drought conditions. Controlled experiments were carried out under soil relative water content (SRWC) (75 ± 5)% (well-watered conditions, control), (60 ± 5)% (moderate drought) and (45 ± 5)% (severe drought) in 2018 and 2019 with two cultivars Yuzaomian 9110 and Dexiamian 1. Under severe drought, the decreases of photosynthetic rate (Pn) and carbon isotope composition (δ¹³C) were observed in the subtending leaf, bract and capsule wall, suggesting that carbon assimilation of three organs was restricted and the limitation was most pronounced in the subtending leaf. Changes in the activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy), invertases as well as the reduction in expression of sucrose transporter (GhSUT1) led to variabilities in the sucrose content of three organs. Moreover, photosynthate distribution from subtending leaf to seeds plus fibers (the components of boll weight) was significantly restricted and the photosynthetic contribution rate of subtending leaf to boll weight was decreased, while contributions of bracts and capsule wall were increased by drought. This, in conjunction with the observed decreases in boll weight, indicated that the subtending leaf was the most sensitive photosynthetic organ to drought and was a dominant driver of boll weight loss under drought. Therefore, the subtending leaf governs boll weight loss under drought due to limitations in carbon assimilation, perturbations in sucrose metabolism and inhibition of sucrose transport.

Exogenous melatonin improves cotton (Gossypium hirsutum L.) pollen fertility under drought by regulating carbohydrate metabolism in male tissues

Although exogenous melatonin can enhance the drought tolerance of plants, reports on the role of melatonin in drought tolerance in male reproductive organs are limited. To explore this, a pot experiment was conducted with cotton cultivar Yuzaomian 9110 to study the effects of exogenous melatonin (100, 200, and 1000 μM) on male fertility and related carbohydrate metabolism in anther under drought. Results showed that drought inhibited the translocation of carbon assimilates to anthers, however, melatonin application (100 and 200 μM) significantly improved the translocation of carbon assimilates to drought-stressed anthers. Drought reduced the deposition of starch, the hydrolysis of sucrose into hexoses, the generation of adenosine triphosphate (ATP) in anthers, restricting pollen viability and germination. Nevertheless, the appropriate melatonin concentrations (100 and 200 μM) increased the starch accumulation by enhancing ADP-glucose pyrophosphorylase and soluble starch synthases activities and accelerated the hydrolysis of sucrose by increasing sucrose synthase, acid and alkaline invertases activities in drought-stressed anthers. Appropriate melatonin concentrations (100 and 200 μM) also could help to generate more ATP for reproductive activities of drought-stressed anthers, finally increasing the pollen viability and germination of drought-stressed plants. These findings suggest that drought inhibited male fertility of cotton, but a precise melatonin application could regulate the carbohydrate balance of drought-stressed anthers to improve male fertility. This is the first report demonstrating the important role of exogenous melatonin in improving male fertility under drought conditions by regulating the carbohydrate metabolism in the male part of cotton.

Single and combined effects of heat and water stress and recovery on cotton (Gossypium hirsutum L.) leaf physiology and sucrose metabolism

High temperatures and water-deficit stress limit cotton production around the world. Their individual effects on plant physiology and metabolism have been extensively studied, however, their combination has received considerably less attention. To that end, growth chamber experiments were conducted using cotton (Gossypium hirsutum L.) cultivar ST5288B2F and the objectives were to discern physiological and metabolic alterations after heat and water stress application (single or combined) and recovery, during cotton's vegetative stage. Under water stress conditions, leaf physiological parameters were suppressed and changes in carbohydrate levels, due to alterations in sucrose-metabolizing enzymes activities, were observed. Heat stress alone increased carbohydrate content, and activities of sucrose-degrading enzymes, while leaf physiology remained unaffected. The combined stress did exacerbate decreases in leaf water potential and soluble acid invertase activity, but the rest of the responses were similar to those of water stress. After stress alleviation, leaf physiological parameters of water-stressed plants did not manage to recover and substantial decreases were observed in leaf starch levels and activities of sucrose-cleaving enzymes, while the majority of parameters of heat-shocked plants returned to control levels. Recovery of the plants subjected to the combined stress was comparable to that of water-stressed plants, but significant differences were observed in carbohydrate levels and sucrose synthase activity. Our study demonstrated that under combined stress and post-stress conditions, water stress was the dominant factor affecting cotton leaf physiology and sucrose metabolism, highlighting however, the unique responses of some traits that could not be deduced from the additive effects of the single stresses.

Drought-induced disturbance of carbohydrate metabolism in anthers and male abortion of two Gossypium hirsutum cultivars differing in drought tolerance

Cotton pollen abortion, under drought stress, was closely associated with changes in anther carbohydrate metabolism, and pollen abortion rate due to drought was higher in drought-sensitive cultivars than drought-tolerant cultivars. Cotton reproductive failure under drought stress is intrinsically connected with altered male fertility, however, studies investigating the effect of drought stress on cotton male fertility are nonexistent. Thus, a drought stress experiment was conducted with two cotton cultivars, differing in drought tolerance, to study pollen fertility and anthers' physiology. Results indicated that drought stress reduced pollen fertility of both cultivars due to decreases in anther starch and adenosine triphosphate (ATP) synthesis. Lower assimilate supply capacity in conjunction with impaired activities of ADP-glucose pyrophosphorylase and soluble starch synthase were the main reasons for the decreased starch levels in drought-stressed anthers. The decreased activities of sucrose synthetase and acid invertase were responsible for the higher sucrose level in drought-stressed anthers than well-watered anthers and the changing trend of sucrose was intensified by the decreased expressions of sucrose synthase genes (GhSusA, GhSusB, GhSusD) and acid invertase genes (GhINV1, GhINV2). However, despite sucrose degradation being limited in drought-stressed anthers, glucose level was higher in droughted anthers than well-watered ones, and that might be attributed to the down-regulated respiration since decreased anther ATP levels were detected in drought-stressed plants. Furthermore, compared to the drought-tolerant cultivar, pollen fertility was more suppressed by drought stress for the drought-sensitive cultivar, and that was attributed to the larger decrease in starch and ATP contents.

Drought limits pollen tube growth rate by altering carbohydrate metabolism in cotton (Gossypium hirsutum) pistils

It has been reported that drought stress (DS) reduces cotton yield by negatively affecting reproductive activities. Some studies have investigated the effects of DS on pollen physiology and biochemistry, but studies exploring the impact of drought on pistil biochemistry and its relationship with pollen tube growth rates in vivo are scarce. In order to investigate these objectives, a greenhouse study was conducted with a drought sensitive cotton cultivar, Yuzaomian 9110. Two water treatments were imposed at flowering stage, 1. control, where plants were irrigated with optimum quantity of water and 2. DS treatment, where plants were irrigated with 50% of the optimum quantity of water. Results indicated that stored starch content at the early stage of pollen tube growth (12:00 h) was 31.6% lower in drought-stressed pistils than control pistils, and it was highly correlated with pollen tube growth rate. The decline in starch accumulation of drought-stressed pistils could be attributed to the impeded transport of photosynthetic carbon assimilates. Moreover, decreased ADP-glucose pyrophosphorylase and soluble starch synthase activities also resulted in curtailing starch accumulation in drought-stressed pistils. Furthermore, pistil sucrose concentration was significantly higher in droughted plants relative to control plants at 12:00 and 18:00 h (during the rapid growth period), which was due to lower activities of sucrose synthase and acid invertase, and the down-regulated expressions of sucrose synthase genes, GhSusA, GhSusB and GhSusD, and acid invertase genes, GhINV1 and GhINV2, in drought-stressed pistils, limiting as a result the hydrolysis of sucrose into hexose. Drought-stressed pistils sampled at 18:00 h had lower α-amylase activity compared to control pistils, resulting in decreased starch decomposition, which, in conjunction with the decreased hydrolysis of sucrose, led to lower glucose and fructose contents in drought-stressed pistils at 18:00 h. Finally, lower pyruvate level in drought-stressed pistils could not produce enough acetyl-CoA in the tricarboxylic acid cycle to yield sufficient energy (ATP) for pollen tube growth. We conclude that DS disrupts the carbohydrate balance of pistil, reducing as a consequence carbon and energy supply for pollen tube elongation in the style, which will ultimately result in reproductive failure.

Potassium deficiency affects the carbon-nitrogen balance in cotton leaves

Potassium (K) plays important roles in the metabolism of carbon (C) and nitrogen (N), but studies of K deficiency affecting C-N balance are lacking. This study explored the influence of K deficiency on C-N interaction in cotton leaves by conducting a field experiment with cotton cultivar DP0912 under two K rates (K0: 0 kg K₂O ha^-1 and K67: 67 kg K₂O ha^-1) and a controlled environment experiment with K-deficient solution (K1: 0 mM K⁺) and K-sufficient solution (K2: 6 mM K⁺). The results showed that leaf K content, leaf number, leaf area, boll number, reproductive dry weight and total dry weight were significant lower under K deficiency (K0 or K1). Lower total chlorophyll content and Chl a/b ratio, and decreased Pn along with lower Gs and higher Ci were measured under K deficiency, suggesting that the decrease in Pn was resulted from non-stomatal limitation. Leaf glucose, fructose, sucrose and starch contents were higher under K deficiency, because lower sucrose export was detected in phloem. Although leaf nitrate and ammonium contents significantly decreased, free amino acid content was increased by 40-63% under K deficiency, since lower amino acid export was also measured in phloem. K deficiency also induced lower soluble protein content in leaves. Leaf ATP level was significantly increased under K deficiency, indicating ATP utilization was lower, so that less energy was supplied to C and N metabolism. The ratio of soluble sugar to free amino acid and the C/N ratio markedly increased under K deficiency, and one reason was that the phloem export reduced more prominent for sucrose (54.6-78.0%) than amino acid (36.7-85.4%) under K deficiency. In addition, lower phosphoenolpyruvate carboxylase activity limited malate and citrate biosynthesis under K deficiency, causing a decrease of C flux into the amino acids, which was not beneficial for maintaining C-N balance. Sucrose phosphate synthase and nitrate reductase activities were lower under K deficiency, which would limit sucrose biosynthesis and nitrate assimilation. This was another factor altering soluble sugar to free amino acid ratio and C/N ratio in the K-deficient leaves.

Relationship between potassium fertilization and nitrogen metabolism in the leaf subtending the cotton (Gossypium hirsutum L.) boll during the boll development stage

The nitrogen (N) metabolism of the leaf subtending the cotton boll (LSCB) was studied with two cotton (Gossypium hirsutum L.) cultivars (Simian 3, low-K tolerant; Siza 3, low-K sensitive) under three levels of potassium (K) fertilization (K0: 0 g K2O plant(-1), K1: 4.5 K2O plant(-1) and K2: 9.0 g K2O plant(-1)). The results showed that total dry matter increased by 13.1-27.4% and 11.2-18.5% under K supply for Simian 3 and Siza 3. Boll biomass and boll weight also increased significantly in K1 and K2 treatments. Leaf K content, leaf N content and nitrate (NO3(-)) content increased with increasing K rates, and leaf N content or NO3(-) content had a significant positive correlation with leaf K content. Free amino acid content increased in the K0 treatment for both cultivars, due to increased protein degradation caused by higher protease and peptidase activities, resulting in lower protein content in the K0 treatment. The critical leaf K content for free amino acid and soluble protein content were 14 mg g(-1) and 15 mg g(-1) in Simian 3, and 17 mg g(-1) and 18 mg g(-1) in Siza 3, respectively. Nitrate reductase (NR), glutamic-oxaloace transaminase (GOT) and glutamic-pyruvic transaminase (GPT) activities increased in the K1 and K2 treatments for both cultivars, while glutamine synthetase (GS) and glutamate synthase (GOGAT) activities increased under K supply treatments only for Siza 3, and were not affected in Simian 3, indicating that this was the primary difference in nitrogen-metabolizing enzymes activities for the two cultivars with different sensitivity to low-K.

High temperature limits in vivo pollen tube growth rates by altering diurnal carbohydrate balance in field-grown Gossypium hirsutum pistils

It has recently been reported that high temperature slows in vivo pollen tube growth rates in Gossypium hirsutum pistils under field conditions. Although numerous physical and biochemical pollen-pistil interactions are necessary for in vivo pollen tube growth to occur, studies investigating the influence of heat-induced changes in pistil biochemistry on in vivo pollen tube growth rates are lacking. We hypothesized that high temperature would alter diurnal pistil biochemistry and that pollen tube growth rates would be dependent upon the soluble carbohydrate content of the pistil during pollen tube growth. G. hirsutum seeds were sown on different dates to obtain flowers exposed to contrasting ambient temperatures but at the same developmental stage. Diurnal pistil measurements included carbohydrate balance, glutathione reductase (GR; EC 1.8.1.7), soluble protein, superoxide dismutase (SOD; EC 1.15.1.1), NADPH oxidase (NOX; EC 1.6.3.1), adenosine triphosphate (ATP), and water-soluble calcium. Soluble carbohydrate levels in cotton pistils were as much as 67.5% lower under high temperature conditions (34.6 °C maximum air temperature; August 4, 2009) than under cooler conditions (29.9 °C maximum air temperature; August 14, 2009). Regression analysis revealed that pollen tube growth rates were highly correlated with the soluble carbohydrate content of the pistil during pollen tube growth (r² = 0.932). Higher ambient temperature conditions on August 4 increased GR activity in the pistil only during periods not associated with in vivo pollen tube growth; pistil protein content declined earlier in the day under high temperatures; SOD and NOX were unaffected by either sample date or time of day; pistil ATP and water soluble calcium were unaffected by the warmer temperatures. We conclude that moderate heat stress significantly alters diurnal carbohydrate balance in the pistil and suggest that pollen tube growth rate through the style may be limited by soluble carbohydrate supply in the pistil.