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Aims: Determine if the use of novel chlorophyll a fluorescence parameters could be utilized to predict yield loss of cotton exposed to sublethal rates of 2,4-dichlorophenoxyacetic acid (2,4-D) at various growth stages. Study Design: All trials were arranged in a randomized complete block design with four replications. Treatment means were subjected to analysis of variance and linear regression was utilized to determine relationship between chlorophyll a parameters and yield. Place and Duration of Study: University of Georgia Gibbs Farm in Tifton, GA, USA and the Sunbelt Agricultural Exposition in Moultrie, GA, USA during the 2013 growing season. Methodology: Two sublethal rates of 2,4-D were applied to cotton at six distinct growth stages. The rates consisted of 2 g and 40 g ae ha-1 equivalent to 1/421 and 1/21 of the full rate (0.532 kg ae ha-1), respectively. The sublethal rates were applied to cotton at six growth stages, including the four leaf, nine leaf, first bloom, two, four and six weeks after first bloom growth stages. A fluorometer was used to obtain the fluorescence parameters Fv/Fm, ΦEO and PIABS from the uppermost fully expanded leaves at various intervals after 2,4-D exposure. Results: Despite yield losses ranging from 20 – 90% of the non-treated control, no consistent patterns resulted from utilizing fluorescence transients to detect 2,4-D injury and overall instances of significant difference were minimal and typically not biologically relevant. In many cases, treatments exposed to 2,4-D that exhibited yield loss showed evidence of greater photosynthetic efficiency than the non-treated control. In the majority of instances, many of fluorescence parameters measured fell within ranges observed in previous studies in cotton produced under typical or non-stressed conditions. Conclusion: While it has been proven as a valuable tool in other plant screening endeavors, chlorophyll a fluorescence were not able to detect the effects of sub-lethal rates of 2,4-D on cotton, even in instances that resulted in severe yield loss.

Sprouting by woody plants can increase species resilience to disturbance and foster regeneration during periods with little recruitment from seed. Though sprouting often plays a critical role in oak forest regeneration, there is little information available on sprouting capacity and sprout physiology at the seedling stage, particularly for new germinants. This study compared sprouting capacity and sprout photosynthesis of shoot-clipped germinants of two temperate oaks established in contrasting light environments. We studied the North American Quercus alba and the European Q. robur, both are in the section Quercus and appear to share similar biological and ecological requirements. Sprouting capacity for both species was enhanced under high light availability (29% more sprouts per plant), a response not previously noted for oak germinants. Seedling sprouts acclimated to high light with a 34% decrease in leaf area ratio, a 56% increase in leaf mass per area, and a 49% increase in the light-saturated maximum photosynthetic rate. Though both species appeared similarly adapted to shoot loss, a greater sprouting capacity (29% more sprouts per plant) and plant-level net photosynthesis (73% higher) was observed for Q. robur, regardless of light environment. As naturally regenerated oak seedlings in forest understories often experience disturbance or stress resulting in shoot loss or die-back, our results highlight the importance of the light environment during early plant development. Our comparison of temperate oaks from different continents should facilitate exchange of successful stand regeneration practices within the range of temperate oak forests.

Ambient temperature has a profound effect on agricultural production. Generally, sub- and supra-optimal temperatures have a negative effect on growth/yield, provided the deviation for optimal is large enough. On average, mean global temperatures are projected to increase by 0.3 to 0.6 °C per decade over the next century. Nighttime temperatures (TN) are projected to increase faster (20%–40% by 2100) than day maximum temperatures (TD). Therefore, an experiment was conducted to determine the effects of projected sub- and supra-optimal day/night temperatures on two diverse rice (Oryza sativa) cultivars, IR6 and Rex, during reproductive growth. At flowering, nine different day/night temperature treatments (TTs) were imposed and maintained until harvesting using sunlit growth chambers (SPAR units). Alterations to plant metabolism were evaluated using CO2 response functions, also, to survey gas exchange readings under ambient conditions. Net photosynthesis (AN) declined under both TD and TN effects. The decline in maximum carboxylation rate allowed by Rubisco (Vcmax) and maximal rate of electron transport allowed by the thylakoid reactions (Jmax), with increasing TN mainly contributed to the decline in AN. Dark respiration rates (RD) declined with increasing temperatures could be a cultivars acclimation mechanism to prevent excessive carbon losses. Cultivars varied mainly for primary photochemistry, with cultivar Rex exhibited significantly higher RD, and pigment contents but lower Jmax than IR6 across TTs. The information on genetic, biochemical and physiological processes in these two diverse cultivars under varied temperature regimes could be useful for developing breeding tools to enhance thermotolerance in US rice cultivars for future climatic conditions.

Extreme temperatures are considered one of the main constraints that limit the growth and development of rice. We elucidated the root and shoot developmental plasticity of 64 rice genotypes during early seedling establishment, using the sunlit plant growth chambers at 22/14 (low), 30/22 (optimum), and 38/30 &deg, C (high) day/night temperatures. Low temperature severely inhibited 23 traits, such as shoot (68%), root (57%), and physiological (35%) attributes. On the contrary, the high temperature positively affected most of the shoot (48%) and root (31%) traits, except root diameter and root/shoot ratio, compared with the optimum. Alternatively, leaf chlorophyll fluorescence-associated parameters declined under low (34%) and high (8%) temperatures. A weak correlation between cumulative high-temperature response index (CHTRI) and cumulative low-temperature response index (CLTRI) indicates the operation of different low- and high-temperature tolerance mechanisms at the early seedling stage. Groups of distinct rice genotypes associated with low or high-temperature tolerance were selected based on CHTRI and CLTRI. The genotypes that commonly performed well under low and high temperatures (IR65600-81-5-2-3, CT18593-1-7-2-2-5, RU1504114, RU1504122, Bowman, and INIA Tacuari) will be valuable genetic resources for breeders in developing early-season high- and low-temperature-tolerant genotypes for a broad range of both tropical and temperate rice-growing environments.

Plants, being sessile, are subjected to multiple environmental stresses of varying intensity throughout their life cycle. Among the stresses, low fertility, drought, salinity, and nonoptimal temperatures limit crop productivity worldwide. Functional relationships between leaf N and crop growth are prerequisites to developing management tools that optimize productivity in the field. An outdoor pot‐culture experiment was conducted to determine the relationship between leaf N and sesame (Sesamum indicum L.) growth and development. Plants were grown in pots filled with fine sand and irrigated with full‐strength Hoagland's nutrient solution until 10 d after sowing (DAS). Five N treatments were then imposed: one treatment of full‐strength Hoagland's nutrient solution (control, 100% N), and four reformulations of Hoagland's nutrient solution containing reduced N at 60, 20, 10, and 0% of the control. Treatments were maintained until plants were harvested at 31 DAS. Growth, including several root traits, photosynthesis, and leaf N were measured at the end of the experiment. Maximum values were achieved at 5.3% leaf N in the leaves except for transpiration rate, stomatal conductance, and root/shoot ratio. Even though all growth rates declined with lower leaf N, the leaf area expansion among shoot and root volume among the root growth traits were most sensitive to leaf N. Among the root developmental traits, number of root tips was more sensitive to leaf N than other parameters. Among the plant dry components, leaf dry weight had the most considerable decrease. The root/shoot ratio increased under N deficiency. These N stress response indices and critical leaf N levels for various growth processes will help model and manage sesame crops in the field.

Soil fertility and reniform nematode (RN) directly affect earlyseason growth and physiology of cotton. The growth responses to soil fertility and RN may, however, vary across germplasm. A greenhouse study was conducted to gain information on the role that host plant resistance plays in influencing RN populations, and cotton growth and physiological response to a range of soil nitrogen (N) levels in the presence and absence of RN. RN-resistant cotton lines (08SS110-NE06.OP and 08SS100) along with susceptible cultivars (Deltapine 16 and PHY 490 W3FE) were subjected to four levels of N from planting until biomass harvesting, 60 days after planting(DAP), under the presence orabsence of RN. The linear and quadratic functions (r2 = 0.72 to 0.99) bestdescribed measured responses of cotton genotypes to soil N. However, the responses were not different among genotypes, except for plant height at 30 DAP. This study revealed significant increases in several morphological parameters with increasing rates of N. RN population in the pots grown with resistant lines was lower whencompared to susceptible cultivars at biomassharvest. Physiological responses indicated that 08SS110-NE06.OP was more resilient to RN stress than other genotypes. The information from this study could be useful in managing the early season growth of cotton.

Cotton ( Gossypium hirsutum ) is often planted under sub-optimal early season temperatures, yet it is unknown whether this increases susceptibility to acute high temperature exposure. To address this, cotton seedlings were grown under optimal (30/20 °C) and sub-optimal (20/15 °C) growth temperature regimes, and comprehensive fluorescence analysis was conducted for cotyledons from both temperature regimes in response to incubation temperatures ranging from 30 to 50 °C. Results indicated that low growth temperature did not alter reaction center density (RC/CS 0 ), but increased antenna size (ratio of antenna chlorophyll to reaction center chlorophyll; ABS/RC) and decreased the amplitude of the I to P phase of the fluorescence transient [ΔV IP ; associated with the photosystem (PS) I content]. Photosynthetic performance indices (PI total and PI ABS ), quantum yield of energy trapping (φ Po ), the contribution of the thylakoid reactions to PI ABS (F v /F 0 ), electron transport to intersystem electron acceptors (φ E0 ) and reduction of PSI end electron acceptors (φ Ro ) were all substantially lower in plants grown under sub-optimal temperature than in plants grown at optimal temperature. Furthermore, thermotolerance of all the aforementioned parameters was significantly reduced by low early season growth temperatures. Among different OJIP-derived parameters, PI ABS and PI total were more heat sensitive than other fluorescence-based parameters, and φ Po and φ Ro were the most heat tolerant. In addition, thermotolerance of PI total (overall photosynthetic thermotolerance) was significantly correlated with thermotolerance of all other OJIP-derived parameters. Regression analysis indicated that heat tolerance trends for φ E0 were more consistent with the changes in thermotolerance of PI total . We conclude that 1) low growth temperature in the seedling phase of cotton results in decreased photosynthetic thermotolerance and 2) overall photosynthetic thermotolerance is primarily constrained by the ability of intersystem electron transport to acclimate to growth environment.

Novel yield component traits and fiber quality indices have been recently developed by breeders to screen for desirable cotton (Gossypium hirsutum L.) lines, but assessing these components in response to water-induced yield variability has received little attention. We investigated the hypothesis that differential sensitivities of whole-crop and intra-boll yield components to drought will largely explain water-induced yield and fiber quality variation in cotton. To test this hypothesis, two cotton cultivars were grown in the field under five contrasting irrigation regimes during the 2013 and 2014 growing seasons near Camilla, GA. Measurements included predawn leaf water potential (ΨPD) throughout the growing season and extensive yield component and fiber quality assessments at the end of the season. Water-induced yield variability was primarily associated with ΨPD at the flowering and boll development phase of crop growth. Boll density (bolls ha−1) was the dominant driver of drought-induced yield loss, but reduced boll mass and seed number per boll also contributed somewhat to yield loss. By comparison, increased drought severity decreased fiber density but increased individual fiber mass, producing a peak in total fiber weight per seed at a −0.7 MPa ΨPD irrigation threshold and increasing lint percentage in stressed treatments. Thus, the negative impacts of drought on overall boll mass and seed number per boll are partially offset by increased dry matter partitioning toward fiber growth. Fiber length declined with increased drought severity, whereas fiber micronaire increased in the most severely stressed treatments. This indicates that increasing drought severity during flowering and boll development decreases the number of individual fibers per seed and their final length, but their thickness is increased. The end result is a decline in the overall fiber quality index (Q-score) under drought stress.

Recently published information on cotton agronomic and photosynthetic component responses to nitrogen (N) application rate is limited for the southeastern US. This experiment was conducted to 1) evaluate cotton yield, fiber quality, growth, and maturity responses to N application rate and 2) quantify associations between photosynthetic component processes and N-induced variation in net photosynthesis. A two-year field experiment was conducted during the 2015 and 2016 growing seasons at a field site in Tifton, GA and evaluated cotton agronomic and physiological response to six N fertilizer application rates ranging from 0 to 168 kg N ha−1. Cotton lint yield was 42–51% lower in 0 N treatments than all other fertilized treatments in both years of the study, and N rates that substantially limited yield decreased final plant height, reduced mainstem node development, and substantially hastened maturity. Date of cutout ranged from 73 to 91 DAP for all N rates and years of the study. Photosynthetic rates were also responsive to N rate on later sample dates (July and August) for both growing seasons. Although declines in chlorophyll content, specific leaf nitrogen, and stomatal conductance were observed for low N plants in some instances, none of these parameters were strongly associated with net photosynthesis overall. In contrast, light-dependent electron transport rate (ETR) was strongly correlated with N-induced variation in net photosynthesis. Estimates of ETR may need to be included along with SLN estimates in future efforts to scale from leaf to canopy level photosynthetic performance in cotton.

Parental environments (PEs) affect seed quality and might alter the re-establishment of big bluestem grass due to impacts on seed germination. An in vitro study was conducted to quantify the temperature response of seed germination and its interaction with the PE in big bluestem. Seeds developed under eight PEs consisting of a combination of four day/night growth temperatures (GTs) (20/12, 25/17, 30/22, and 35/27 &deg, C) and two CO2 levels (360 and 720 &micro, L L&minus, 1) were germinated at eight temperatures (germination temperatures (GRTs)) ranging from 10 to 42.5 &deg, C. Quadratic and modified bilinear regressions best described the cardinal temperatures for the estimated maximum seed germination (MSG) and seed germination rate (SGR), respectively. The average MSG and SGR showed differential responses to the PEs and significantly declined above a 35 &deg, C GRT across the PEs. For the SGR, the minimum and optimum temperatures showed significant differences from other treatments but the opposite response to elevated CO2, while maximum temperatures significantly declined at high (35/27 &deg, C) and low GTs (20/12 &deg, C). Seed quality parameters, individual seed weight, and C and N contents showed a high correlation (R2 &gt, 60) with the average percentage of seed germination and the SGR. Thus, high temperatures for both the PEs (&gt, 30/22 &deg, C) and GRTs (&gt, 30 &deg, C) could significantly reduce germination, affecting the re-establishment of big bluestem.

Heat stress can reduce crop yield or even cause total crop failure. The ability to predict heat stress in advance would allow growers time to implement protective measures, helping to avoid such losses. This study presents a strategy for producing probabilistic heat stress forecasts for well-watered cotton (Gossypium hirsutum L.) in the Camilla, GA, region. Multiple linear regression was used to develop a cotton canopy temperature model based on predicted air temperature, humidity, solar radiation, and wind speed. The European Centre for Medium-Range Weather Forecasts Ensemble Prediction System was used to predict the meteorological variables used in the canopy temperature model, which produced 10-d probabilistic canopy temperature forecasts for each day of observations during 2014. A statistical mean bias correction was applied to improve on the raw model forecasts. The forecasts were found to be skillful, with relative operating characteristic areas greater than 0.5. The bias-corrected forecasts were found to increase skill. A heat stress warning system was then created using the forecasts. Additionally, an economic analysis was performed as an example of how probabilistic forecasts can be used to aid producers with financial decisions pertaining to weather-related risks.

Temperature and drought are major abiotic limitations to crop productivity worldwide. While abiotic stress physiology research has focused primarily on fully expanded leaves, no studies have investigated photosynthetic tolerance to concurrent drought and high temperature during leaf ontogeny. To address this, Gossypium hirsutum plants were exposed to five irrigation treatments, and two different leaf stages were sampled on three dates during an abnormally dry summer. Early in the growing season, ontogenic PSII heat tolerance differences were observed. Photosystem II was more thermotolerant in young leaves than mature leaves. Later in the growing season, no decline in young leaf net photosynthesis (PN) was observed as leaf temperature increased from 31 to 37°C, as average midday leaf water potential (ΨMD) declined from -1.25 to -2.03MPa. In contrast, mature leaf PN declined 66% under the same conditions. Stomatal conductance (gs) accounted for 84-98% of variability in leaf temperature, and gs was strongly associated with ΨMD in mature leaves but not in young leaves. We conclude that young leaves are more photosynthetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms causing these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.

Drought is known to limit carbon assimilation in plants. However, it has been debated whether photosynthesis is primarily inhibited by stomatal or non-stomatal factors. This research assessed the underlying limitations to photosynthesis in peanuts (Arachis hypogaea L.) grown under progressive drought. Specifically, field-grown peanut plants were exposed to either well-watered or drought-stressed conditions during flowering. Measurements included survey measurements of gas exchange, chlorophyll fluorescence, PSII thermotolerance, pigment content, and rapid A-Ci response (RACiR) assessments. Drought significantly decreased stomatal conductance with consequent declines in photosynthesis (AN), actual quantum yield of PSII, and electron transport rate (ETR). Pigment contents were variable and depended on stress severity. Stomatal closure on stressed plants resulted in higher leaf temperatures, but Fv/Fm and PSII thermotolerance were only slightly affected by drought. A strong, hyperbolic relationship was observed between stomatal conductance, AN, and ETR. However, when RACiR analysis was conducted, drought significantly decreased AN at Ci values comparable to drought-stressed plants, indicating non-stomatal limitations to AN. The maximum rate of carboxylation and maximum electron transport rate were severely limited by drought, and chloroplast CO2 concentration (CC) declined substantially under drought along with a comparable increase in partitioning of electron flow to photorespiration. Thus, while stomatal conductance may be a viable reference indicator of water deficit stress in peanut, we conclude that declines in AN were largely due to non-stomatal (diffusional and metabolic) limitations. Additionally, this is the first study to apply the rapid A-Ci response method to peanut, with comparable results to traditional A-Ci methods.

To determine if cultivar differences in thermotolerance plasticity of photosystem II promote yield or photosynthetic stability when variability in both parameters is water-induced, the temperature response of maximum quantum yield of photosystem II ( F v / F m ) was evaluated for two cotton cultivars (FM 1944 GLB2 and PHY 499 WRF), at three times (squaring, first flower, and peak bloom) during the 2014 growing season, under five different irrigation regimes. The temperature inducing a 15% decline in F v / F m from the optimum ( T 15 ) served as a measure of thermotolerance. Furthermore, during 2014 net photosynthesis ( A N ) measurements coincided with T 15 measurements; predawn water potential ( Ψ PD ) was measured three days per week; and canopy temperature ( T C ) was measured continuously. Yield was determined for both cultivars and all five irrigation regimes in 2013 and 2014 although significant irrigation effects were only observed during 2014. Thermotolerance, photosynthetic, and yield stability were determined using a regression approach to assess genotype × environment interaction. Irrigation treatments significantly affected Ψ PD and T C , and large variability in thermotolerance (although all T 15 values were well-above the observed air and canopy temperatures), A N , and lint yield were observed. The most thermotolerance–stable cultivar was also the most photosynthetically stable and yield stable, suggesting that greater thermotolerance plasticity of PSII does not necessarily promote yield stability when yield variability is water-induced. Also, the methods described here provide an approach to rapidly assess thermotolerance plasticity of field grown cotton that could be adopted on a large scale and that does not require extensive knowledge of site-specific conditions.

Respiratory carbon evolution by leaves under abiotic stress is implicated as a major limitation to crop productivity; however, respiration rates of fully expanded leaves are positively associated with plant growth rates. Given the substantial sensitivity of plant growth to drought, it was hypothesized that predawn respiration rates (RPD) would be (1) more sensitive to drought than photosynthetic processes and (2) highly predictive of water-induced yield variability in Gossypium hirsutum. Two studies (at Tifton and Camilla Georgia) addressed these hypotheses. At Tifton, drought was imposed beginning at the onset of flowering (first flower) and continuing for three weeks (peak bloom) followed by a recovery period, and predawn water potential (ΨPD), RPD, net photosynthesis (AN) and maximum quantum yield of photosystem II (Fv/Fm) were measured throughout the study period. At Camilla, plants were exposed to five different irrigation regimes throughout the growing season, and average ΨPD and RPD were determined between first flower and peak bloom for all treatments. For both sites, fiber yield was assessed at crop maturity. The relationships between ΨPD, RPD and yield were assessed via non-linear regression. It was concluded for field-grown G. hirsutum that (1) RPD is exceptionally sensitive to progressive drought (more so than AN or Fv/Fm) and (2) average RPD from first flower to peak bloom is highly predictive of water-induced yield variability.

Thermotolerance acclimation of photosystem II to heat and drought is well documented, but studies demonstrating developmental impacts on heat tolerance in field-grown plants are limited. Consequently, climatic variables, estimated canopy temperature, predawn leaf water potential (ΨPD), and the temperature responses of maximum quantum yield of photosystem II (Fv/Fm), variable fluorescence (Fv/F0), quantum yield of electron transport (φEο) and efficiency of PSI electron acceptor reduction (REο/ABS) were characterized for Gossypium hirsutum at three sample times during the growing season (21 June, 2 July and 18 July 2013) under well-watered conditions. The temperature decreasing a given photosynthetic parameter 15% from the optimum is referred to as T15 and served as a standardized measure of heat tolerance. Ambient and estimated canopy temperatures were well within the optimal range for cotton throughout the sample period, and leaves were verified well watered using ΨPD measurements. However, T15 varied with sample date (highest on July 2 for all parameters), being 2 °C (Fv/F0) to 5.5 °C (φEο) higher on July 2 relative to June 21, despite optimal temperature conditions and predawn leaf water potential on all sample dates. These findings suggest that even under optimum temperature conditions and water availability, heat tolerance could be influenced by plant developmental stage.

Much effort has been expended to improve irrigation efficiency and drought tolerance of agronomic crops; however, a clear understanding of the physiological mechanisms that interact to decrease source strength and drive yield loss has not been attained. To elucidate the underlying mechanisms contributing to inhibition of net carbon assimilation under drought stress, three cultivars of Gossypium hirsutum were grown in the field under contrasting irrigation regimes during the 2012 and 2013 growing season near Camilla, Georgia, USA. Physiological measurements were conducted on three sample dates during each growing season (providing a broad range of plant water status) and included, predawn and midday leaf water potential (ΨPD and ΨMD), gross and net photosynthesis, dark respiration, photorespiration, and chlorophyll a fluorescence. End-of-season lint yield was also determined. ΨPD ranged from -0.31 to -0.95MPa, and ΨMD ranged from -1.02 to -2.67MPa, depending upon irrigation regime and sample date. G. hirsutum responded to water deficit by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis, thereby limiting PN and decreasing lint yield (lint yield declines observed during the 2012 growing season only). Conversely, even extreme water deficit, causing a 54% decline in PN, did not negatively affect actual quantum yield, maximum quantum yield, or photosynthetic electron transport. It is concluded that PN is primarily limited in drought-stressed G. hirsutum by decreased stomatal conductance, along with increases in respiratory and photorespiratory carbon losses, not inhibition or down-regulation of electron transport through photosystem II. It is further concluded that ΨPD is a reliable indicator of drought stress and the need for irrigation in field-grown cotton.

Summary In this project, we hypothesize that cotton ( Gossypium hirsutum ) leaf temperature and the responses of leaf photosynthesis to temperature will change as the leaves expand and that differences between young and mature leaves will be associated with the proportion of saturated fatty acids in thylakoid and other membrane lipids. To that end, we studied main stem leaves obtained from plants growing in a temperature controlled greenhouse and at different times in the field season. We found that young leaves (∼5 d old) had higher mid day temperatures, lower stomatal conductance and higher thermal optima as measured by Φ PSII temperature curves than did more mature leaves (∼13 d old). Young leaves also had significant differences in fatty acid saturation with the warmer, young leaves having a higher proportion of palmitic acid (16:0) and lower linoleic acid (18:3) in total lipid extracts and higher 16:0 and lower palmitoleic acid (16:1) in the chloroplast membrane phosphoglycerides, digalactosyldiacylglycerol (in the greenhouse) and phosphatidylglycerol when compared with cooler, more mature leaves. Later in the growing season, leaf temperature, stomatal conductance and Φ PSII temperature curves for young and more mature leaves were similar and the proportion of 16:0 fatty acids decreased and 16:1 increased in phosphatidylglycerol. We conclude that changes in temperature as cotton leaves expand leads to alterations in the fatty acid composition of thylakoid and other membranes and, consequently, influence photosynthesis/temperature responses.

Despite exhaustive literature describing drought stress effects on photosynthesis in Gossypium hirsutum, the sensitivity of photosynthetic electron flow to water deficit is heavily debated. To address this, G. hirsutum plants were grown at a field site near Camilla, GA under contrasting irrigation regimes, and pre-dawn water potential (ΨPD), stomatal conductance (gs), net photosynthesis (PN), actual quantum yield of photosystem II (ΦPSII) and electron transport rate (ETR) were measured at multiple times during the 2012 growing season. ΨPD values ranged from −0.3 to −1.1 MPa. Stomatal conductance exhibited a strong (r2 = 0.697), sigmoidal response to ΨPD, where gs was ≤0.1 mol m−2 s−1 at ΨPD values ≤ −0.86 MPa. Neither ΦPSII (r2 = 0.015) nor ETR (r2 = 0.010) was affected by ΨPD, despite exceptionally low ΨPD values (−1.1 MPa) causing a 71.7 % decline in PN relative to values predicted for well-watered G. hirsutum leaves at ΨPD = −0.3 MPa. Further, PN was strongly influenced by gs, whereas ETR and ΦPSII were not. We conclude that photosynthetic electron flow through photosystem II is insensitive to water deficit in field-grown G. hirsutum.