Non-stomatal Limitations Research Articles

Leaf Gas Exchange of Tomato Depends on Abscisic Acid and Jasmonic Acid in Response to Neighboring Plants under Different Soil Nitrogen Regimes.

High planting density and nitrogen shortage are two important limiting factors for crop yield. Phytohormones, abscisic acid (ABA), and jasmonic acid (JA), play important roles in plant growth. A pot experiment was conducted to reveal the role of ABA and JA in regulating leaf gas exchange and growth in response to the neighborhood of plants under different nitrogen regimes. The experiment included two factors: two planting densities per pot (a single plant or four competing plants) and two N application levels per pot (1 and 15 mmol·L−1). Compared to when a single plant was grown per pot, neighboring competition decreased stomatal conductance (gs), transpiration (Tr) and net photosynthesis (Pn). Shoot ABA and JA and the shoot-to-root ratio increased in response to neighbors. Both gs and Pn were negatively related to shoot ABA and JA. In addition, N shortage stimulated the accumulation of ABA in roots, especially for competing plants, whereas root JA in competing plants did not increase in N15. Pearson’s correlation coefficient (R2) of gs to ABA and gs to JA was higher in N1 than in N15. As compared to the absolute value of slope of gs to shoot ABA in N15, it increased in N1. Furthermore, the stomatal limitation and non-stomatal limitation of competing plants in N1 were much higher than in other treatments. It was concluded that the accumulations of ABA and JA in shoots play a coordinating role in regulating gs and Pn in response to neighbors; N shortage could intensify the impact of competition on limiting carbon fixation and plant growth directly.

Intrinsic non-stomatal resilience to drought of the photosynthetic apparatus in Coffea spp. is strengthened by elevated air [CO2

Growing water restrictions associated with climate changes constitute daunting challenges to crop performance. This study unveils the impacts of moderate (MWD) or severe (SWD) water deficit, and their interaction with air [CO2], on the photosynthetic apparatus of Coffea canephora Pierre ex A. Froehner cv. Conilon Clone 153 (CL153) and Coffea arabica L. cv. Icatu. Seven year-old potted plants grown under 380 (aCO2) or 700μll -1 (eCO2) [CO2] gradually reached predawn water potentials between -1.6 and -2.1MPa (MWD), and below -3.5MPa (SWD). Under drought, stomata closure was chiefly related to abscisic acid (ABA) rise. Increasing drought severity progressively affected gas exchange and fluorescence parameters in both genotypes, with non-stomatal limitations becoming gradually dominating, especially regarding the photochemical and biochemical components of CL153 SWD plants. In contrast, Icatu plants were highly tolerant to SWD, with minor, if any, negative impacts on the potential photosynthetic functioning and components (e.g., Amax, Fv/Fm, electron carriers, photosystems (PSs) and ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) activities). Besides, drought-stressed Icatu plants displayed increased abundance of a large set of proteins associated with the photosynthetic apparatus (PSs, light-harvesting complexes, cyclic electron flow, RuBisCO activase) regardless of [CO2]. Single eCO2 did not promote stomatal and photosynthetic down-regulation in both genotypes. Instead, eCO2 increased photosynthetic performance, moderately reinforced photochemical (PSs activity, electron carriers) and biochemical (RuBisCO, ribulose-5-phosphate kinase) components, whereas photoprotective mechanisms and protein abundance remained mostly unaffected. In both genotypes, under MWD, eCO2 superimposition delayed stress severity and promoted photosynthetic functioning with lower energy dissipation and PSII impacts, whereas stomatal closure was decoupled from increases in ABA. In SWD plants, most impacts on the photosynthetic performance were reduced by eCO2, especially in the moderately drought affected CL153 genotype, although maintaining RuBisCO as the most sensitive component, deserving special breeder's attention to improve coffee sustainability under future climate scenarios.

Photosynthetic characteristics of Tamarix chinensis under different groundwater depths in freshwater habitats

Groundwater is the major source of water for Tamarix chinensis growth in the Yellow River Delta (YRD) region, and the groundwater depth (GWD) dramatically influences the physiological activities of T. chinensis. The quantitative response of the photosynthetic physiological process of T. chinensis to the GWD in freshwater habitats remains unclear. In this study, the response characteristics of gas exchange parameters in the leaves of three-year-old T. chinensis seedlings were measured and analyzed at a graded series of seven GWDs (0 m, 0.3 m, 0.6 m, 0.9 m, 1.2 m, 1.5 m and 1.8 m). The GWD thresholds corresponding to drastic changes in the photosynthetic efficiency and the GWDs of several levels of photosynthetic productivity and efficiency were also determined. In the freshwater habitats of the YRD, variations in GWD significantly altered the relative soil water content (RSWC) and thus influenced the photosynthetic efficiency of T. chinensis. RSWC at 0 ≤ GWD ≤ 0.9 m and GWD at 1.2 m ≤ GWD ≤ 1.8 m directly influenced the photosynthetic physiology of T. chinensis. When the GWD was 1.2 m, net photosynthetic rate (Pn), apparent quantum efficiency and water use efficiency (WUE) values all peaked. Thus, T. chinensis exhibited a high light and water use efficiency, wide ecological amplitude in terms of light, and high photosynthetic capacity. The optimum GWD for photosynthetic carbon assimilation and WUE in T. chinensis was determined to be 1.2 m. At a deep (≥1.64 m) or shallow (≤0.53 m) GWD, both Pn and WUE in T. chinensis clearly decreased below the corresponding mean values. The main causes for the reduction in Pn in these two GWD ranges (≤0.53 m, ≥1.64 m) were stomatal and nonstomatal limitations, respectively. Additionally, a moderate GWD of 1.09–1.25 m corresponded to the “high-productivity and high-efficiency GWD” range, in which T. chinensis displayed a high photosynthetic yield and WUE. Overall, the photosynthetic capacity of T. chinensis shows characteristics of high tolerance to moderate GWDs from 1.09 m to 1.25 m but intolerance at both shallow (≤0.53 m) and deep (≥1.64 m) GWDs in freshwater habitats.

Kaolin foliar spray improves olive tree performance and yield under sustained deficit irrigation

Regarding the foreseeing climate change is reasonable to expect harmful consequences to olive tree (Olea europaea L.), an iconic species of Mediterranean basin. In large areas of the Region, the sustained deficit irrigation (SDI) accounts with a reduced amount of water that might not be enough to keep the plant functioning at desirable levels. In this study the use of a reflective clay, kaolin, was evaluated in order to alleviate the negative summer stress effects on olive trees (cv. Cobrançosa) under an SDI system. Kaolin reduced the stomatal and the non-stomatal limitations to photosynthesis at high light intensity, improved plant water status, and decreased the oxidative damages. The amelioration of physiological and metabolic functions and the lower allocation of resources to summer stress tolerance mechanisms, allowed the KL-sprayed trees to increase crop yield, the oil yield and its oxidative stability, with the reduction of peroxide index and K232. Thus, the application of kaolin had a positive synergistic effect with SDI, especially important for semi-arid areas where water available for irrigation do not cover all the necessities.

Stomatal and non-stomatal limitations of Rosa hybrida ‘Pink Bell’ under drought stress

Stomatal and non-stomatal limitations of <i>Rosa hybrida</i> ‘Pink Bell’ under drought stress

Alterations in leaf photosynthetic electron transport in Welsh onion (Allium fistulosum L.) under different light intensity and soil water conditions.

Welsh onions (Allium fistulosum L.) are often affected by stressful environments, such as high light and drought, during summer cultivation, which hinders their growth. We used CO2 assimilation, OJIP transient and MR curves to analyse the photosynthetic characteristics of Welsh onion. The results showed that single high light stress caused a decrease in the net photosynthesis rate through stomatal limitation, while the single drought treatment and the combined stress induced nonstomatal limitation. FO and FJ increased, Fm decreased, and a distinct K-phase was induced. High light and drought stress blocked MR transients, leading to a gradual decrease in VPSI and VPSII-PSI . In general, photosynthesis of Welsh onion was inhibited by high light and drought, which destroyed the receptor and donor side of PSII and reduced electron transport capacity of PSII and PSI.

Drought stress strengthens the link between chlorophyll fluorescence parameters and photosynthetic traits.

Chlorophyll fluorescence (ChlF) has been used to understand photosynthesis and its response to climate change, particularly with satellite-based data. However, it remains unclear how the ChlF ratio and photosynthesis are linked at the leaf level under drought stress. Here, we examined the link between ChlF ratio and photosynthesis at the leaf level by measuring photosynthetic traits, such as net CO2 assimilation rate (An), the maximum carboxylation rate of Rubisco (Vcmax), the maximum rate of electron transport (Jmax), stomatal conductance (gs) and total chlorophyll content (Chlt). The ChlF ratio of the leaf level such as maximum quantum efficiency of PSII (Fv/Fm) is based on fluorescence kinetics. ChlF intensity ratio (LD685/LD740) based on spectrum analysis was obtained. We found that a combination of the stomatal limitation, non-stomatal limitation, and Chlt regulated leaf photosynthesis under drought stress, while Jmax and Chlt governed the ChlF ratio. A significant link between the ChlF ratio and An was found under drought stress while no significant correlation in the control, which indicated that drought stress strengthens the link between the ChlF ratio and photosynthetic traits. These results suggest that the ChlF ratio can be a powerful tool to track photosynthetic traits of terrestrial ecosystems under drought stress.

Seasonal variations in photosynthetic functions of the urban landscape tree species Gingko biloba: photoperiod is a key trait

Seasonal variations in the leaf photosynthetic traits of an urban tree, Ginkgo biloba, were almost synchronized with the photoperiod. Non-stomatal limitations were a cue for photosynthesis in Ginkgo biloba. Photosynthetic functions, which are key traits in determining the carbon uptake of urban trees, exhibit significant seasonal variations in temperate zones. It is essential to clarify the seasonal dynamics of photosynthesis to evaluate the CO2 uptake in urban areas. We investigated seasonal variations in the photosynthetic traits of Ginkgo biloba, which is a major urban deciduous tall tree often planted in Japan. Seasonal variations in the leaf photosynthetic traits, including the maximum photosynthesis rate, maximum carboxylation rate, and mesophyll and stomatal conductance, were well fitted to quadratic models, in which they peaked around the summer solstice and then declined with time. Seasonal variations in the environmental variables, such as photoperiod, temperature, and solar radiation, were compared to those of the leaf photosynthetic traits, in which the photoperiod explained well variations in the leaf photosynthetic traits. Seasonal variations in photosynthesis were largely governed by non-stomatal limitations, i.e., mesophyll and biochemical limitations. The high synchrony of the photoperiod and photosynthetic traits during leaf maturation may cause an enhancement in the daily carbon uptake of G. biloba leaves around the summer solstice, which has the longest photoperiod, and thus, will lead to an increase in the annual carbon uptake.

Non-stomatal processes reduce gross primary productivity in temperate forest ecosystems during severe edaphic drought.

Severe drought events are known to cause important reductions of gross primary productivity (GPP) in forest ecosystems. However, it is still unclear whether this reduction originates from stomatal closure (Stomatal Origin Limitation) and/or non-stomatal limitations (Non-SOL). In this study, we investigated the impact of edaphic drought in 2018 on GPP and its origin (SOL, NSOL) using a dataset of 10 European forest ecosystem flux towers. In all stations where GPP reductions were observed during the drought, these were largely explained by declines in the maximum apparent canopy scale carboxylation rate VCMAX,APP (NSOL) when the soil relative extractable water content dropped below around 0.4. Concurrently, we found that the stomatal slope parameter (G1, related to SOL) of the Medlyn et al. unified optimization model linking vegetation conductance and GPP remained relatively constant. These results strengthen the increasing evidence that NSOL should be included in stomatal conductance/photosynthesis models to faithfully simulate both GPP and water fluxes in forest ecosystems during severe drought. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.

Maintenance of photosynthetic capacity in flooded tomato plants with reduced ethylene sensitivity.

Ethylene is considered one of the most important plant hormones orchestrating plant responses to flooding stress. However, ethylene may induce deleterious effects on plants, especially when produced at high rates in response to stress. In this paper, we explored the effect of attenuated ethylene sensitivity in the Never ripe (Nr) mutant on leaf photosynthetic capacity of flooded tomato plants. We found out that reduced ethylene perception in Nr plants was associated with a more efficient photochemical and non-photochemical radiative energy dissipation capability in response to flooding. The data correlated with the retention of chlorophyll and carotenoids content in flooded Nr leaves. Moreover, leaf area and specific leaf area were higher in Nr, indicating that ethylene would exert a negative role in leaf growth and expansion under flooded conditions. Although stomatal conductance was hampered in flooded Nr plants, carboxylation activity was not affected by flooding in the mutant, suggesting that ethylene is responsible for inducing non-stomatal limitations to photosynthetic CO2 uptake. Upregulation of several cysteine protease genes and high protease activity led to Rubisco protein loss in response to ethylene under flooding. Reduction of Rubisco content would, at least in part, account for the reduction of its carboxylation efficiency in response to ethylene in flooded plants. Therefore, besides its role as a trigger of many adaptive responses, perception of ethylene entails limitations in light and dark photosynthetic reactions by speeding up the senescence process that leads to a progressive disassembly of the photosynthetic machinery in leaves of flooded tomato plants.

Negative effects of long-term moderate salinity and short-term drought stress on the photosynthetic performance of Hybrid Pennisetum

Plants are always suffering periods of soil water deficit and sustained soil salinity during their life cycle. Unraveling the mechanisms underpinning the responses of plants, especially the photosynthesis, to drought, salinity, and co-occurring stresses is critical for both the protection of natural vegetation and the stabilization of crop production. To better understand the downregulation of photosynthetic capability induced by soil salinity and drought, gas exchange parameters, leaf pigment contents, and chlorophyll (Chl) a fluorescence transients were analyzed in leaves of Hybrid Pennisetum. Our results showed that long-term moderate salinity, short-term drought, and the combination of these stressors decreased leaf pigment content by 11.4–31.5% and net photosynthetic rate (Pn) by 14.6–67.6% compared to those in untreated plants. The reduction of Pn in Hybrid Pennisetum under long-term salinity stress mainly occurred by stomatal limitation, whereas non-stomatal limitation played a dominant role under short-term drought stress. The changes in Chl a fluorescence kinetics (especially the appearance of the L-band and K-band) in both stress treatments showed that salinity and drought stress damaged the structural stability of photosystem II (PSII) and disturbed the equilibrium between the electrons at the acceptor and donor sides of PSII. Furthermore, although the negative effect of drought stress on leaf photosynthesis was much greater than that of salinity stress, moderate salt stress alleviated the negative effect of drought stress on the photosynthetic performance of Hybrid Pennisetum after long acclimation times.

Resilient and Sensitive Key Points of the Photosynthetic Machinery of Coffea spp. to the Single and Superimposed Exposure to Severe Drought and Heat Stresses.

This study unveils the single and combined drought and heat impacts on the photosynthetic performance of Coffea arabica cv. Icatu and C. canephora cv. Conilon Clone 153 (CL153). Well-watered (WW) potted plants were gradually submitted to severe water deficit (SWD) along 20 days under adequate temperature (25/20°C, day/night), and thereafter exposed to a gradual temperature rise up to 42/30°C, followed by a 14-day water and temperature recovery. Single drought affected all gas exchanges (including Amax) and most fluorescence parameters in both genotypes. However, Icatu maintained Fv/Fm and RuBisCO activity, and reinforced electron transport rates, carrier contents, and proton gradient regulation (PGR5) and chloroplast NADH dehydrogenase-like (NDH) complex proteins abundance. This suggested negligible non-stomatal limitations of photosynthesis that were accompanied by a triggering of protective cyclic electron transport (CEF) involving both photosystems (PSs). These findings contrasted with declines in RuBisCO and PSs activities, and cytochromes (b559, f, b563) contents in CL153. Remarkable heat tolerance in potential photosynthetic functioning was detected in WW plants of both genotypes (up to 37/28°C or 39/30°C), likely associated with CEF in Icatu. Yet, at 42/30°C the tolerance limit was exceeded. Reduced Amax and increased Ci values reflected non-stomatal limitations of photosynthesis, agreeing with impairments in energy capture (F0 rise), PSII photochemical efficiency, and RuBisCO and Ru5PK activities. In contrast to PSs activities and electron carrier contents, enzyme activities were highly heat sensitive. Until 37/28°C, stresses interaction was largely absent, and drought played the major role in constraining photosynthesis functioning. Harsher conditions (SWD, 42/30°C) exacerbated impairments to PSs, enzymes, and electron carriers, but uncontrolled energy dissipation was mitigated by photoprotective mechanisms. Most parameters recovered fully between 4 and 14 days after stress relief in both genotypes, although some aftereffects persisted in SWD plants. Icatu was more drought tolerant, with WW and SWD plants usually showing a faster and/or greater recovery than CL153. Heat affected both genotypes mostly at 42/30°C, especially in SWD and Icatu plants. Overall, photochemical components were highly tolerant to heat and to stress interaction in contrast to enzymes that deserve special attention by breeding programs to increase coffee sustainability in climate change scenarios.

Stomatal limitations to photosynthesis and their critical Water conditions in different growth stages of maize under water stress

Stomata are the channels by which plants exchange water vapor and carbon dioxide with the environment. Clarifying the change from stomatal limitations (SL) to non-stomatal limitations (NSL) of photosynthesis and their critical conditions is vital for accurately recognizing the degree of crop drought and formulating countermeasures. A field experiment was carried out from 2013 to 2015 to study the critical water conditions when maize photosynthesis changed from being limited by SL to NSL in different leaf positions under different degrees of water stress in different growth stages (3rd leaf stage, 7th leaf stage and jointing stage). Our results indicated that photosynthesis of maize leaves at different positions changed from being determined by SL to NSL under different water stress levels at different growth stages; moreover, maize photosynthesis changed from being directed by SL to NSL in the first fully expanded leaf at the top before the changes occurred in the third leaf. The effect of water stress during different growth stages on the maize leaf water content (LWC) at which photosynthesis changed from being limited by SL to NSL was not distinct. The changing point of SL at different leaf positions was closely related to the LWC, and the LWC at the changing point of SL was different at different leaf positions, which indicated that the change in SL is mainly determined by the leaf position and LWC, and its occurrence showed a decreasing trend from plant top to bottom. The LWC at which the SL transformation point occurred in the first fully expanded leaf at the top (75.5 % ± 1.5 %–75.7 % ± 1.3 %) was higher than that at which the change occurred in the third leaf at the top (73.2 % ± 1.1 %–73.4 % ± 1.6 %). The phenomenon of photosynthesis changing from being limited by SL to NSL occurred first in the first fully expanded leaf at the top; additionally, the LWC of the first fully expanded leaf at the top was the best indicator in maize under water stress and could be used as the critical condition marking the transformation of maize damage from water stress to damage from plant physiological and ecological stress. These results could provide a basis for the identification of crop drought disasters and their classification and provide a methodological reference for the identification and monitoring of drought in other crops.

Leaf photosynthesis and CO2 assimilation in response to low nitrogen supply in bermudagrass (Cynodon dactylon (L.) Pers.)

Photosynthesis plays a vital role in maintaining vigorous, healthy turf under nitrogen (N)-limiting conditions. The effects of low N stress on the aboveground growth of bermudagrass (Cynodon dactylon (L.) Pers.) as well as its photosynthesis, chlorophyll fluorescence, CO2 assimilation, and carbohydrate pools were studied using hydroponic experiments. Two bermudagrass accessions, Yangjiang (low N tolerant) and C716 (low N sensitive), were used. Low N supply decreased the aboveground dry weight of both accessions. In comparison to C716, Yangjiang maintained better aboveground growth, which was attributed to its smaller reductions in leaf area and the net photosynthetic rate (Pn) at low N conditions. The Pn reduction in both accessions was on account of a reduction in stomatal conductance (Gs) and was partially due to nonstomatal limitations. Furthermore, the smaller Pn reduction in Yangjiang than in C716 was associated with the smaller decrease in CO2 assimilatory capacity in the former, and the ribulose-1,5-bisphosphate carboxylase (RuBisCO) and phosphoenolpyruvate carboxylase (PEPC) activities decreased based on leaf protein content. Although the leaf sucrose synthesis capacity and sucrose phosphate synthase (SPS) and sucrose synthase (SS) activities in Yangjiang increased more than those in C716, carbohydrates did not accumulate in the leaves of either accession to repress photosynthesis. This work provides new insights into CO2 assimilatory capacity rather than carbohydrate status under conditions of reduced leaf photosynthesis and decreasing aboveground growth in bermudagrass under low N conditions. Hence, bermudagrass accessions with low N tolerance are excellent sources of turf requiring reduced N fertilizer inputs.

Selenite Foliar Application Alleviates Arsenic Uptake, Accumulation, Migration and Increases Photosynthesis of Different Upland Rice Varieties.

This study investigates how arsenic (As) uptake, accumulation, and migration responds to selenium (Se) foliar application (0–5.0 mg × kg−1). Rice varieties known to accumulate low (DOURADOAGULHA) and high (SINALOAA68) concentrations of arsenic were chosen to grow on soil with different As concentrations (20.1, 65.2, 83.9 mg × kg−1). The results showed that Se of 1.0 mg × L−1 significantly alleviated As stress on upland rice grown on the As-contaminated soil. Under light (65.2 mg × kg−1) and moderate (83.9 mg × kg−1) As concentration treatments, the biomass of upland rice was increased by 23.15% and 36.46% for DOURADOAGULHA, and 46.3% and 54.9% for SINALOAA68. However, the high Se dose (5.0 mg × kg−1) had no significant effect on biomass and heights of upland rice compared to plants where no Se was added. Se significantly decreased As contents in stems and leaves and had different effects on As transfer coefficients for the two rice varieties: when grown on soil with low and moderate As concentrations, Se could reduce the transfer coefficient from stems to leaves, but when grown on the high As soils, this was not the case. The chlorophyll content in plants grown in soil with the moderate concentration of As could be improved by 27.4%–55.3% compared with no Se treatment. Under different As stress, the Se foliar application increased the net photosynthesis, stomatal conductance, and transpiration rate, which meant that Se could enhance the photosynthesis of rice. The intercellular CO2 concentration variation implied that the stomatal or non-stomatal limitations could both occur for different rice varieties under different Se application doses. In conclusion, under moderate As stress, foliar application of Se (1.0 mg × L−1) is recommend to overcome plant damage and As accumulation.

Chronic effects of microcystin-LR at environmental relevant concentrations on photosynthesis of Typha angustifolia Linn.

Understanding the growth and development of aquatic plants in eutrophic water is of great significance for the selection of potential candidate plant for use in the phytoremediation of eutrophic aquatic ecosystems. The present study aimed to investigate the chronic effects of microcystin-LR (MC-LR) on photosynthesis in the leaves of Typha angustifolia Linn. Photosynthetic activity was stimulated in the leaves following exposure to 4.6 μg L-1 MC-LR for six weeks based on the enhancement of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) activity and net photosynthetic rate (PN). However, PN decreased significantly after exposure to 49.1 or 98.3 μg L-1 MC-LR, via non-stomatal limitation by reducing the chlorophyll a and b contents and Rubisco activity. In addition, glycolate oxidase (GO) and serine:glyoxylate aminotransferase (SGAT) activities decreased significantly, indicating that the photorespiration pathway was affected adversely. The intercellular carbon dioxide (Ci) concentration decreased significantly following exposure to 98.3 μg L-1 MC-LR, accompanied with decreases in PN and stomatal conductivity (gs), indicating that stomatal limitation on the photosynthesis system in T. angustifolia L. was observed after exposure to 98.3 μg L-1 MC-LR. Under long-term exposure to MC-LR (49.1-98.3 μg L-1), oxidation stress was severe in the aquatic plant, and non-stomatal limitation or stomatal limitation effects on the photosynthesis system were obvious, resulting in decreases in photosynthesis rates.

Photochemistry and proteomics of ginger (Zingiber officinale Roscoe) under drought and shading

Drought has become an increasingly serious ecological problem that limits crop production. However, little is known about the response of ginger (Zingiber officinale Roscoe) to drought and shading, especially with respect to photosynthetic electron transport. Here, differential proteomics was used to study the response of ginger to four experimental treatments: control, drought, 50% shading, and the combination of 50% shading and drought. Proteomic analysis suggested that ginger increased cyclic electron flow under drought stress by enhancing the expression of proteins related to photosystem I and cytochrome b6f. Shading significantly increased the expression of proteins related to the light harvesting complex, even under drought stress. In addition, shading increased the expression of proteins related to the oxygen evolution complex, plastocyanin, and ferredoxin–NADP+ reductase (FNR), thereby enhancing the efficiency of photosynthetic electron utilization. The shading and drought combination treatment appeared to enhance ginger's drought tolerance by reducing the expression of FNR and enhancing cyclic electron flow. Photosynthetic and fluorescence parameters showed that drought stress caused non-stomatal limitation of photosynthesis in ginger leaves. Drought stress also significantly reduced the quantum efficiency of photosystem II (Fv/Fm), the non-cyclic electron transfer efficiency of photosystem II (ϕPSII), and photochemical quenching (qP), while simultaneously increasing nonphotochemical quenching (NPQ). The addition of shading improved photosynthetic efficiency under drought. These results provide important baseline information on the photosynthetic mechanisms by which ginger responds to drought and shading. In addition, they provide a theoretical basis for the study of shade cultivation during the arid season.

Non-stomatal limitation of photosynthesis by soil salinity

Soil salinity is a major threat to agricultural sustainability and a global food security. Until now, most research has concentrated around stomatal limitation to photosynthesis, while non-stomatal limitations receiving much less attention. This work summarizes the current knowledge of impact of salinity on chloroplast metabolism and operation and finding viable solutions to minimize it. The major topics covered are: (1) the key targets of the photosynthetic apparatus under salt stress; (2) a tolerance of PSII to salt stress and its repair; (3) salinity effects on biochemistry of CO2 fixation and its regulation; (4) ionic requirements for optimal operation of chloroplasts; and (5) ion transport systems in chloroplasts that optimize chloroplast performance under hostile saline conditions. We show that enhancing plant capacity for protection by modifying PSI cyclic electron transport, redistribution of electron transport between photosystems, thylakoid membrane composition and photosynthetic antioxidant enzymes activity may be a promising way to improve tolerance to salt stress under real-field condition. It is concluded that revealing the molecular nature of chloroplast ion transporters and understanding the modes of their operation will ensure the future sustainability of the world agriculture and the prospects of biological phytoremediation of salinized land via using salt-tolerant crop germplasm.

The seasonal regulation of gas exchange and water relations of field grown macadamia

Macadamia F. Muell is a recently domesticated nut crop characterized by a certain amount of drought tolerance. Whilst stomatal regulation of gas exchange and water relations has been documented in macadamia, there are no reports on non-stomatal limitations to photosynthesis and if changes occur over a season in relation to different phenological stages. It was hypothesised that in order for macadamias to be adapted to a seasonally dry native environment the trees would be characterised by strong stomatal control over gas exchange, indicating an isohydric water use strategy that is related to a hydraulic limtation within the tree. However, due to high assimilate demand during nut filling, the level of stomatal control would vary between fruiting and non-fruiting phenological stages. Gas exchange and water relations measurements were made for 18 months on irrigated mature macadamia trees (cv. HAES 695, ‘Beaumont’) in a subtropical region of South Africa. Results confirmed that macadamias had relatively low light-saturated net CO2 assimilation rates (Amax) (8.34 ± 1.21 μmol CO2 m−2 s-1). The low Amax values resulted from relatively high stomatal and non-stomatal limitations and decreasing stomatal conductance (gs) in response to increasing leaf-to-air vapour pressure deficit (VPDleaf) above ca. 2 kPa. Strict stomatal control and nearly constant midday leaf water potential (ψleaf = -1.16 ± 0.43 MPa) confirmed the predominantly isohydric nature of the crop, which seemed to be a result of low hydraulic conductance in the stem to leaf pathway. Significant differences in leaf gas exchange capacity were, however, observed between the fruiting and non-fruiting phenological stages. The presence of fruit resulted in significantly higher Amax (10.27 ± 2.23 μmol CO2 m−2 s−1) compared to non-fruiting periods (Amax = 6.58 ± 2.00 μmol CO2 m−2 s−1). Increases in Amax were mediated by increased rates of electron transport (Jmax) and triose phosphate use (TPU). Fruiting stages were also characterized by variable responses of gs to increases in VPDleaf, which indicated varying degrees of isohydricity. This study reaffirms that macadamias are inherently adapted to seasonally dry environments, characterized by strict stomatal control, yet under humid conditions or in the presence of developing fruit, macadamias are proposed to deviate from a purely isohydric water management strategy.

Foliar Pre-Treatment with Abscisic Acid Enhances Olive Tree Drought Adaptability.

Water is the most widely limiting factor for plants distribution, survival and agricultural productivity, their responses to drought and recovery being critical for their success and productivity. Olea europaea L. is a well-adapted species to cyclic drought events, still at considerable expense of carbon reserves and CO2 supply. To study the role of abscisic acid (ABA) as a promoter of drought adaptability, young potted olive trees subjected to three drought-recovery cycles were pre-treated with ABA. The results demonstrated that ABA pre-treatment allowed the delay of the drought effects on stomatal conductance (gs) and net photosynthesis (An), and under severe drought, permitted the reduction of the non-stomatal limitations to An and the relative water content decline, the accumulation of compatible solutes and avoid the decline of photosynthetic pigments, soluble proteins and total thiols concentrations and the accumulation of ROS. Upon rewatering, ABA-sprayed plants showed an early recovery of An. The plant ionome was also changed by the addition of ABA, with special influence on root K, N and B concentrations. The improved physiological and biochemical functions of the ABA-treated plants attenuated the drought-induced decline in biomass accumulation and potentiated root growth and whole-plant water use efficiency after successive drought-rewatering cycles. These changes are likely to be of real adaptive significance, with important implications for olive tree growth and productivity.