Decrease In Stomatal Conductance Research Articles

Mn Toxicity Differentially Affects Physiological and Biochemical Features in Highbush Blueberry (Vaccinium corymbosum L.) Cultivars

Manganese (Mn) is an essential micronutrient for plants; however, in excess, it can have negative effects on their physiological and biochemical characteristics. We evaluate photosynthetic performance, antioxidant activity, and organic acid exudations to clarify the mechanisms involved in highbush blueberry (Vaccinium corymbosum L.) under Mn excess. One-year-old plants of V. corymbosum cultivars (Legacy, Brigitta, and Bluegold) were grown in hydroponic solution with 2, 10, 50, 250, 500, and 1000 μM of Mn treatments for 25 days. Plant growth as shoot and root biomass, Mn content, photosynthetic performance [photochemical efficiency of PSII, CO2 assimilation, and stomatal conductance (gs)], lipid peroxidation, radical scavenging activity, superoxide dismutase activity, and organic acid exudations were analyzed. Mn excess triggered detrimental effects in terms of plant growth, photochemical efficiency, and CO2 assimilation, as well as in biochemical features in V. corymbosum cultivars. Despite Legacy decreasing its photosynthesis, plant growth was maintained throughout the experiment; by contrast, Brigitta maintained photosynthesis and growth despite the decrease in stomatal conductance (gs). Meanwhile, Bluegold presented the lowest level in net photosynthesis and photochemical processes and an increase in lipid peroxidation. Oxalate and citrate were the most important organic acid anions in Legacy and Brigitta, gradually increasing their concentration with the enhancement of Mn doses. Our study demonstrated that Mn excess negatively and differently affects the physiological and biochemical features of V. corymbosum cultivars, with Legacy and Brigitta being Mn-resistant and Bluegold Mn-sensitive.

PHYSIOLOGICAL AND BIOCHEMICAL RESPONSES OF SUGARCANE VARIETIES TO SALT STRESS

ABSTRACT Salt stress is one of the most limiting environmental factors for agricultural yields in the Semiarid region of Brazil. Considering the expansion of areas with sugarcane crops in this region, the selection of more adapted plant varieties to this environment is an essential tool for the sustainability of this activity. Thus, the objective of the present work was to evaluate the physiological and biochemical responses of plants of ten sugarcane varieties to salt stress. The experiment was conducted in a greenhouse using a randomized block design, in a 6×10 factorial arrangement consisting of six salinity levels in the irrigation water (0, 1.0, 2.0, 4.0, 6.0, and 8.0 dS m-1) and ten sugarcane varieties (VAT 90212, RB 72454, RB 867515, Q 124, RB 961003, RB 957508, SP 791011, RB 835089, RB 92579, and SP 943206), with three replications. Salt waters were applied every two days to increase the soil moisture to field capacity and promote leaching. The gas exchange, chlorophyll content, and total soluble and reducing sugars of leaves were evaluated at 90 days after planting. The highest chlorophyll contents were found in plants of the varieties Q 124, RB 96103, RB 835089, and SP 943206. The effect of salinity on the sugarcane plants affected their leaf gas exchanges and total soluble and reducing sugar contents, denoting the adaptability of plants to the stress conditions evaluated. The decrease of stomatal conductance resulted in decreases in photosynthetic rates in plants of all sugarcane varieties evaluated, except for RB 867515.

Photoresponse to different lighting strategies during red leaf lettuce growth

The objective of the study was to investigate the effects of growth-stage specific lighting for the physiological homeostasis of red leaf lettuce (Lactuca sativa L. cv. Red Cos), by measuring the productivity of photosynthesis and primary metabolism. In the experiments, the main photosynthetic photon flux consisted of red (R) and blue (B) light, supplemented with blue, green (G) or UV-A wavelengths. Decrease of fructose, accompanied by significant decrease of stomatal conductance (gs), the ratio of intracellular to ambient CO2 concentration (Ci/Ca), photosynthetic rate (Pr), light adapted actual quantum yield of PSII photochemistry (ΦPSII), biomass formation and significant increase of transpiration rate (Tr) suggest that supplemental UV-A during maturity stage, after supplemental green irradiation during seedling stage (BRG to BRUV) was the least favourable condition for red leaf lettuce. However, constant irradiation with supplemental green (BRG) or supplemental green irradiation after increased blue exposure (B↑R to BRG) resulted in significant increase of Pr, gs, Ci/Ca, and light use efficiency(LUE), and decrease of Tr and Water use efficiency (WUE). Significant increase of leaf area was observed under supplemental green in both seedlings (BR; BRG) and matured plants (B↑R to BRG). Significant increase of specific leaf area was found under supplemental green (BRG) for seedlings and under increased blue (B↑R) for matured plants. Accordingly, the most favourable growth-stage specific lighting spectrum strategy for red leaf lettuce, based on photosynthetic and primary metabolite response, is supplemental green irradiation after increased blue exposure (B↑R to BRG), whereas, the most favourable condition for seedlings is BRG. According to the PCA correlation matrix, associations among the measured data indicate that WUE negatively correlated with gs and Ci/Ca, while LUE positively correlated with gs and Pr. However, weak correlations between ФPSII, LUE and photochemical reflectance index (PRI) suggest that selected light conditions were not optimal for red leaf lettuce.

Morpho-physiological responses of chili peppers (Capsicum annuum) to short-term exposure of water-saturated rhizosphere

Chili pepper is frequently grown by local farmers at riparian wetland during dry season in Indonesia. However, during the last decade, unpredictable distribution and intensity of rainfall have increasingly threatened chili pepper production at the wetlands due to untimely water-saturated rhizosphere (WSR) occurrences. WSR is a condition when all pores within root zone were filled with water. This condition can be simulated by adding water into growing substrate until a thin layer of water was visible above substrate surface. Two Indonesian varieties (Laris and Romario) and one Japanese variety (Takanotsume) were used in this study. Aim of this study was to evaluate morpho-physiological effects of short-term (4 days) WSR exposure in chili pepper. Results of this study revealed that roots suffered more than aerial organs as indicated by the increase of shoot/root ratio from 4.56 at pre-exposure to 7.03 at end of the exposure. Total leaf area significantly reduced since larger older leaves were replaced by newly developed smaller leaves. Relative water content (RWC) in all organs was decreased, but did not reach a detrimental level. Leaf RWC was decreased from 83.6% at pre-exposure to 77.8% after the exposure; however, leaf RWC was able to rebound to 81.5% after 7 days of recovery. Photosynthetic and transpiration rates sharply decreased, associated with decrease in stomatal conductance during WSR exposure. Chlorophyll fluorescence also sharply declined. Gas exchange parameters did not significantly recover after 7 days of recovery in all varieties. Meanwhile, SPAD values were not affected by WSR exposure.

Physiological responses and aquaporin expression upon drought and osmotic stress in a conservative vs prodigal Fragaria x ananassa cultivar

In order to improve the understanding of plant water relations under drought stress, the water use behavior of two Fragaria x ananassa Duch. cultivars, contrasting in their drought stress phenotype, is identified. Under drought, stomatal closure is gradual in Figaro. Based on this, we associate Figaro with conservative water use behavior. Contrarily, drought stress causes a sudden and steep decrease in stomatal conductance in Flair, leading to the identification of Flair as a prodigal water use behavior cultivar. Responses to progressive drought on the one hand and an osmotic shock on the other hand are compared between these two cultivars. Tonoplast intrinsic protein mRNA levels are shown to be upregulated under progressive drought in the roots of Figaro only. Otherwise, aquaporin expression upon drought or osmotic stress is similar between both cultivars, i.e. plasma membrane intrinsic proteins are downregulated under progressive drought in leaves and under short term osmotic shock in roots. In response to osmotic shock, root hydraulic conductivity did not change significantly and stomatal closure is equal in both cultivars. De novo abscisic acid biosynthesis is upregulated in the roots of both cultivars under progressive drought.

Effects of elevated CO2 on foliar soluble nutrients and functional components of tea, and population dynamics of tea aphid, Toxoptera aurantii

The rising atmospheric CO2 concentration has shown to affect plant physiology and chemistry by altering plant primary and secondary metabolisms. Nevertheless, the impacts of elevated CO2 on plant nutrients and functional components of tea remain largely unknown, which will likely affect tea quality and taste under climate change scenario. Being sources of nutrients and secondary chemicals/metabolites for herbivorous insects, the variation in foliar soluble nutrients and functional components of tea plants resulting from CO2 enrichment will further affect the herbivorous insects' occurrence and feeding ecology. In this study, the tea aphid, Toxoptera aurantii was selected as the phloem-feeding herbivore to study the effects of elevated CO2 on foliar soluble nutrients and functional components of tea seedlings, and the population dynamics of T. aurantii. The results indicated that elevated CO2 enhanced the photosynthetic ability and improved the plant growth of tea seedlings compared with ambient CO2, with significant increases in net photosynthetic rate (+20%), intercellular CO2 concentration (+15.74%), leaf biomass (+15.04%) and root-to-shoot ratio (+8.08%), and significant decreases in stomatal conductance (−5.52%) and transpiration rate (−9.40%) of tea seedlings. Moreover, elevated CO2 significantly increased the foliar content of soluble sugars (+4.74%), theanine (+3.66%) and polyphenols (+12.01%) and reduced the foliar content of free amino acids (−9.09%) and caffeine (−3.38%) of tea seedlings compared with ambient CO2. Furthermore, the relative transcript levels of the genes of theanine synthetase (+18.64%), phenylalanine ammonia lyase (+49.50%), s’-adenosine methionine synthetase (+143.03%) and chalcone synthase (+61.86%) were up-regulated, and that of caffeine synthase (−56.91%) was down-regulated for the tea seedlings grown under elevated CO2 relative to ambient CO2. In addition, the foliar contents of jasmonic acid (+98.6%) and salicylic acid (+155.6%) also increased for the tea seedlings grown under elevated CO2 in contrast to ambient CO2. Also, significant increases in the population abundance of T. aurantii (+4.24%–41.17%) were observed when they fed on tea seedlings grown under elevated CO2 compared to ambient CO2. It is presumed that the tea quality and taste will be improved owing to the enhanced foliar soluble nutrients and functional components of tea seedlings under the climate change scenario, especially on account of the rising atmospheric CO2 concentration, while the climate change may exacerbate the occurrence of tea aphid, T. aurantii, despite the enhanced secondary defensive chemicals manifested by the CO2 enrichment.

Light stress-induced chloroplast movement and midday depression of photosynthesis in sorghum leaves

ABSTRACTPlants are exposed to high light intensity, high leaf temperatures and high air-to-leaf water vapor pressure deficit (ALVPD) during the day. These environmental stresses cause stomatal closure and photoinhibitory damage, leading to midday depression of photosynthesis. Chloroplast positioning is essential for the efficient operation of photosynthesis. However, chloroplast behavior before, during, and even after the midday depression of photosynthesis remains unknown. We investigated changes in the intracellular positioning of chloroplasts and photosynthetic traits under a diurnal pattern of light. Sorghum leaves were exposed to a 12-h regime of light mimicking the natural light environment, with constant leaf temperature and ALVPD. Net photosynthetic rate (Pn) showed a diurnal pattern, and midday depression in Pn was observed at 3.8 h of irradiation. Depression in Pn was attributed to stomatal limitation because the decrease in Pn was in accordance with the decrease in stomatal conductance. The maximum efficiency of photosystem II decreased with the increase in light intensity and remained low after 12 h of irradiation. Bundle sheath chloroplasts swelled after 8 h of irradiation, representing the accumulation of starch. Conversely, mesophyll chloroplasts exhibited avoidance response after 4 h of irradiation, and the avoidance position was maintained during the remainder of the daytime. These data suggest that chloroplasts are subject to light stress during and after the midday depression of photosynthesis. The intensity of natural light is excessive for most of the day and this light stress induces chloroplast avoidance response and depression of photosynthesis.

Drought stress in plants: An overview on implications, tolerance mechanisms and agronomic mitigation strategies

Drought is considered as one of the major limiting factors affecting growth and productivity of crop plants. It severely affects the morphological and physiological activities of the plants and hampers the seed germination, root proliferation, biomass accumulation and final yield of field crops. Drought stress disrupts the biosynthesis of chlorophyll contents, carotene and decreases photosynthesis in plants. It gradually reduces CO2 assimilation rates owing to decrease in stomatal conductance. In addition, drought affects cell membrane stability and disrupts water relations of a plant by reducing water use efficiency. To cope with these situations, plants adopt different mechanisms such as drought tolerance, avoidance and escape. In this review, we discussed about the effects of drought on morphological and physiological characteristics of plants and suggested the different agronomic practices to overcome the deleterious effects of drought stress.

Photosynthesis and biomass accumulation in young sugarcane plants grown under increasing ammonium supply in nutrient solution

The aim of this study was to evaluate the sugarcane responses to varying ammonium:nitrate (NH4+:NO3−) ratio and to reveal how much NH4+ plants can tolerate before showing impairment of photosynthesis and growth. Sugarcane plants were grown in nutrient solution with the following NH4+:NO3− ratios (%): 20:80; 30:70; 40:60; 60:40; 70:30; and 80:20. The lowest photosynthetic rates, stomatal conductance, instantaneous carboxylation efficiency and leaf chlorophyll a content were found in plants supplied with higher than 60% NH4+. The leaf content of chlorophyll b proved to be more sensitive than chlorophyll a and decreases were found from 40% NH4+. We did not observe significant differences in leaf NO3− concentration under varying NH4+:NO3− ratio. However, plants that received 80% NH4+ showed the highest leaf NH4+ concentration and lowest leaf [NO3−]:[NH4+] ratio. The estimated leaf nitrogen content was higher in plants supplied with 20% and 30% NH4+. Taken together, our data revealed that sugarcane plants are sensitive to NH4+, with photosynthesis and plant growth being impaired when NH4+ supply was higher than 30% in nutrient solution. Root biomass was significantly reduced under high NH4+ supply, which explains decreases in stomatal conductance. Besides stomatal limitation, photosynthesis was also limited by low carboxylation efficiency under high NH4+ supply. Apparently, leaf NH4+ concentrations higher than 1.0 µmol g−1 were enough to impair photosynthesis. The balance between [NO3−] and [NH4+] in leaves was more correlated to photosynthesis than either [NO3−] or [NH4+] alone.

Improving drought tolerance by altering the photosynthetic rate and stomatal aperture via green light in tomato (Solanum lycopersicum L.) seedlings under drought conditions

The regulation of stomatal aperture is one of the most important strategies for plants to tolerate drought. Green light has been shown to reverse some effects of red light and/or blue light on plant growth and development and can enhance plant defense against biotic and/or abiotic stress by triggering the expression of specific genes. However, the effects of green light on plant drought tolerance are still unknown. To elucidate the effects of green light on plant drought tolerance, tomato (Solanum lycopersicum L.) seedlings were treated with short-term drought stress and were concomitantly exposed to red and blue light-emitting diodes (LEDs) supplemented with or without green light in an environment-controlled growth chamber. The results show that adding green light induced significant decreases in stomatal conductance (gs), which increased the intrinsic and instantaneous water-use efficiency, concomitantly enhanced mesophyll conductance (gm) and maintained relatively high photosynthetic capability under short-term drought stress. Moreover, green light supplementation alleviated stomatal opening and not only involved the downregulation of the SlHA1, 2 and 4 during stomatal opening but also resulted from a SlAREB1-activated signaling pathway, which led to drought tolerance.

Growth and photosynthetic responses in Brassica napus differ during stress and recovery periods when exposed to combined heat, drought and elevated CO2

This study was intended to investigate how an agronomically important crop Brassica napus will be able to cope with the combined impact of a heatwave (21/14 °C vs. 33/26 °C day/night) and drought under ambient or elevated CO2 (800 vs. 400 μmol mol−1) and to what degree their recovery will be ensured after the stress, when additional CO2 is also removed. The obtained results revealed that, in the presence of an adequate water supply, B. napus performed well under heatwave conditions. However, drought fully negated all the advantages gained from hotter climate and led to a slower and incomplete recovery of gas exchange and retarded growth after the stress, regardless mitigating the effect of elevated CO2 during the stress. The mechanism by which the elevated CO2 diminished the adverse effect of a combined heat and drought stress on photosynthetic rate at saturating light (Asat) was attributed to the improved plant water relations. However, it had little effect on the recovery of Asat. In contrast, the mechanism by which photosynthesis was more impaired under the combination of heatwave and drought, compared to single drought treatment, was attributed mainly to the faster soil drying as well as faster and sharper decrease in stomatal conductance and subsequent in Ci/Ca. Keeping in mind that photosynthesis can acclimatize by downregulation to higher CO2, the results of this study, showing a weak memory of mitigating the effect of elevated CO2, highlight a potential risk of more intense and frequent heatwaves and droughts on B. napus.

Potassium uptake efficiency of two pear cultivars and leaf concentration at deficiency symptoms appears

Two pear cultivars (Meski Ahrech and Alexandrine) grown in pot culture under normal daylight conditions in a greenhouse were submitted to potassium deficiency treatments. The potassium-free treatment decreased leaf K concentration, relative water content, SPAD, and gas exchange comparatively to the control. “Alexandrine” appeared more sensitive to potassium deficiency due to the main decrease in stomatal conductance (gs), photosynthesis (A) and transpiration (E) compared to “Meski Ahrech” that reduced only carbon assimilation (A). After 42 days of treatments, leaf K concentration in “Alexandrine” was 0.75% DM compared to 1.15% DM in “Meski Ahrech”, which appeared to be more efficient in potassium uptake.

Anatomical and physiological adjustments of pubescent oak (Quercus pubescens Willd.) from two adjacent sub-Mediterranean ecosites

Patterns of drought severity in natural ecosystems depend on ecosite characteristics, particularly on local soil water availability and microclimate (evaporative demand). At the sites with less favourable conditions, the plasticity of fundamental drought adaptive traits can be exceeded under severe drought, even in well adapted species.In our study, anatomical and physiological adjustments to drought were studied in pubescent oak (Quercus pubescens Willd.) growing on adjacent sub-Mediterranean plots (ecosites) differing in bedrock and soil type (limestone with rendzic leptosol, low water retention - L; transition from limestone to flysch with eutric cambisol, high water retention - F).In both years of the study, 2015 and 2016, in the part of the season with sufficient water supply, sap flow density (SFD) was higher in oaks growing on L than those growing on F. With decreasing soil water content (SWC) in July, SFD was reduced on both plots, most in trees growing on limestone. In the period of severe drought, SFD in trees growing on L was decoupled from the changes of VPD, due to increased employment of water conservation strategies, such as decreases in stomatal conductance. This was further translated into stomatal limitation of photosynthesis and contributed to reduced radial stem growth.The seasonal dynamics of traits indicate that Q. pubescens trees use a predisposed anatomical strategy (average size of vessels in different years), adjusted to average ecosite conditions. At the same time, trees respond to local conditions and interanual variability of drought by shortening/prolonging the production and growth of vessels and by adjusting leaf functioning (stomatal conductance, sap flow) in accordance with hydraulic properties.

Recent decadal drought reverts warming‐triggered growth enhancement in contrasting climates in the southern Andes tree line

AbstractAimsRising temperature and declining summer precipitation due to the 1970s‐climate shift in southern South America have reduced forest productivity at dry sites. Here, we worked with the most widespread Southern Hemisphere tree line species, Nothofagus pumilio, across contrasting climatic conditions and determined whether rising atmospheric CO2 concentrations as well as warmer and drier climatic conditions provoked by the 70s‐climatic shift have been causing systematic changes in tree line growth rates and intrinsic water‐use efficiency (iWUE).Location36–54°S, southern Andes.Time period1950–2010.Major taxa studiedNothofagus pumilio.MethodsWe worked at five disparate climatic tree line locations, spanning 18 degrees of latitude; at each location, we sampled trees at four different elevations, including tree line elevation. We quantified the variation in annual tree‐ring width (TRW) as a function of climate, elevation, tree age, size, annual CO2 concentrations and location, using linear mixed‐effects models and interpreted TRW trends in relation to iWUE and isotope (δ13C and δ18O) signalling.ResultsAcross locations, the patterns of tree line growth occurring in the 1980–2010 period exhibited a clear and significant negative trend, in contrast to the previous 1950–1980 period. We found an increase in iWUE and δ18O across time and locations. Given that an increase in δ18O indicates a decrease in stomatal conductance, we assert that drought‐induced stomatal closure appears to be causing the reduction in growth.Main conclusionsWe show unequivocal evidence that warmer and drier summer conditions translated into a decrease in growth rates along the elevational tree line of the southern Andes, reverting previous growth improvements linked to climate warming. An improvement in iWUE at all locations is most likely explained by decreased stomatal conductance given the rising δ18O signal. An iWUE–growth decoupling may act as an ecological strategy to respond to drought.

Exogenous salicylic acid alleviates the oxidative damageof Arabidopsis thaliana by enhancing antioxidant defense systemsunder high light

To understand the protective roles of salicylic acid (SA) under high light, we investigated oxidative damage of Arabidopsis thaliana under high light in the presence or absence of SA. The results indicate that the high light led to an increase in the levels of proline, soluble sugars, reactive oxygen species, malondialdehyde, and electrolyte leakage, and a decrease in stomatal conductance (gs). Activities of six antioxidant enzymes increased significantly under the high light for 1 h. However, the high light for 3 h decreased the activities of peroxidase, superoxide dismutase, and catalase. In addition, we found that exogenous SA effectively improved antioxidant enzyme activities and significantly alleviated ROS accumulation and cell death in A. thaliana under the high light. Therefore, our results show that the high light caused a severe oxidative damage, and SA effectively alleviated the adverse effects of the high light on the plants by regulating the antioxidative defense system.

The effect of potassium on photosynthetic acclimation in cucumber during CO<sub>2</sub> enrichment

Long-term CO2 enrichment (1,000 μmol mol-1) leads to photosynthetic acclimation in cucumber. Here, through hydroponic experiments in an open-top climate chamber system, we investigated key photosynthetic parameters of cucumbers using potassium stimulation (120 or 240 mg L-1). Short-term CO2 enrichment (less than 25 days) significantly increased the net photosynthetic rate in cucumber. However, long-term CO2 enrichment (43 d) led to photosynthetic acclimation and decrease in stomatal conductance. The increase in potassium alleviated the decrease in photosynthetic rate and stomatal conductance, which reduced photosynthetic acclimation. In addition, 13C isotope tracing showed that under CO2 enrichment, plants with higher potassium concentrations showed higher sink/source and flow/source ratios of photosynthetic assimilative C (δ13C) abundance. Moreover, the abnormal accumulation of soluble carbohydrates and starch resulted in photosynthetic acclimation in cucumber. Increasing potassium significantly reduced the accumulation of soluble carbohydrates and promoted the transport of soluble carbohydrates to the sink, which alleviated photosynthetic acclimation.

Leaf Functional Traits and Biomass of Wetland Plants in Forest and Steppe Zones

Leaf traits, the rate of gas exchange, and biomass were examined in five plurizonal species (Alisma plantago-aquatica L., Carex acuta L., Eleocharis palustris (L.) Roem. et Schult., Phalaris arundinacea L., and Typha angustifolia L.) of emergent plants growing in wetlands of forest (Middle Urals) and steppe (South Urals) zones. It was found that changes in the leaf thickness, leaf mass per area (LMA), the transpiration rate, and characteristics of the pigment complex associated with a rise in aridity were species-specific. Structural reorganization of the leaves of wetland plants growing in conditions of steppe wetland showed a general pattern of an increase in leaf density (1.2–2 times) and a reduction in leaf area (by 42–54%). This trend was accompanied by a reduction in the photosynthesis rate per 1 m2 (1.2–3.4 times) or per 1 mg of chlorophyll (by 29–63%). In two species, P. arundinacea and T. angustifolia, we detected a decrease in stomatal conductance by 2 and 4.8 times, respectively, in more arid conditions. Analysis of interspecific relations between plant productivity and leaf parameters has shown that plant biomass positively correlated with leaf area (r = 0.84, P < 0.01) and with the photosynthesis rate per leaf area unit (r = 0.97, P < 0.001) both in the regions of the Middle and South Urals. On the whole, structural changes in the leaves associated with adaptation of plant water relations to semiarid climate lead to a decrease in photosynthetic activity of leaves and, therefore, a decline of plant height and productivity of emergent plants growing in steppe wetland.

The Response of Plant Photosynthesis and Stomatal Conductance to Fine Particulate Matter (PM2.5) based on Leaf Factors Analyzing

The effects of particles on the photosynthesis of vegetation, which is a sink for fine particulate matter (PM2.5) deposition, are still not well understood. Here, we carried out indoor measurements to evaluate the variation dynamics of net photosynthetic rate and stomatal conductance of four plant species with different leaf characteristics under different PM2.5 levels. Then tree leaves were sampled and the groove proportion, leaf trichome density, stomatal density and stomatal size were quantitatively studied by scanning electron microscopy (SEM). The stomatal conductance of the 4 species had a close positive correlation with photosynthetic rate. Net photosynthetic rate and stomatal conductance declined over time at elevated PM2.5, and the rate of the decline became more rapid with higher concentration of PM2.5. The inhibiting effect might be caused by the closure of the stomata and the decrease of stomatal conductance, which was proved by the reduction of the stomatal size of under the condition of PM2.5 pollution. Leaf trichome and groove seemed to show a protective role for plants from PM2.5 exposure and be responsible for the difference of photosynthetic rate and stomatal conductance under the condition of PM2.5 pollution. The higher groove proportion and the presence of trichomes on the leaf surface in Neolitsea aurata and Lindera kwangtungensis absorbed some particulate matters and buffered the effect of PM2.5 pollution on stomata.

Interactive Regimes of Reduced Irrigation and Salt Stress Depressed Tomato Water Use Efficiency at Leaf and Plant Scales by Affecting Leaf Physiology and Stem Sap Flow.

Interactive effects of reduced irrigation and salt stress on leaf physiological parameters, biomass accumulation, and water use efficiency (WUE) of tomato plants at leaf and whole plant scales were investigated in a field experiment during 2016 and a greenhouse experiment during 2017. Experiment utilized two irrigation regimes (full, 2/3 of full irrigation) and four soil salt regimes (0, 0.3, 0.6, 0.9% in 2016 season; and 0, 0.2, 0.3, 0.4% in 2017 season). Three salts, sodium chloride, magnesium sulfate, and calcium sulfate (mass ratio of 2:2:1), were homogeneously mixed with soil prior to packing into containers (0.024 m3). Li-COR 6400 was used to measure tomato leaf physiological parameters. Instantaneous water use efficiency (WUEins, μmol mmol−1) and intrinsic water use efficiency (WUEint, μmol mol−1) were determined at leaf scale, yield water use efficiency (WUEY, g L−1), and dry biomass water use efficiency (WUEDM, g L−1) were determined at whole plant scale. Plants irrigated with 2/3 of full irrigation with zero soil-salt treatment had higher dry biomass and yield per plant, resulting in the highest WUEDM and WUEY at whole plant scale. Increasing soil salinity decreased dry biomass and yield, leading to greater decreases in whole plant WUEDM and WUEY under both irrigation treatments. At full irrigation, no decreases in stomatal conductance (gs, mol m−2 s−1) and slight increase in photosynthetic rate (Pn, μmol m−2 s−1) led to higher WUEint at leaf scale during both years. Under full and reduced irrigation, increasing soil salt content decreased Pn and transpiration rate (Tr, mmol m−2 s−1) and led to reductions in WUEins at the leaf scale. However, compared to full irrigation, reduced irrigation improved WUEins with a significant decline in Tr in no salt and 0.3% soil-salt treatments during both years. For soil salt content of 0.6%, stomatal limitation due to salt stress resulted in higher WUEint, but soil salt content of 0.9% decreased WUEint due to non-stomatal limitation. Soil salt content significantly decreased sap flow, with the maximum variation of daily sap flow per plant of 7.96–31.37 g/h in 2016 and 12.52–36.02 g h−1 in 2017. Sap flow rate was linearly related to air temperature (Ta, °C), solar radiation (Rs, W m−2), and vapor pressure deficit (VPD, kPa). These results advance knowledge on tomato response to abiotic stresses and could improve management of tomato production in water- and salt-stressed areas.

Seasonal patterns of gas exchange and water relations in juveniles of two Neotropical savanna tree species differing in leaf phenology

Ecophysiological studies on Neotropical savanna trees have shown that there is a resistance gradient of seasonal water stress in deciduous and evergreen species. Some species perform like drought avoiders, characterized by high transpiration rates during the wet season that can be sustained by evergreen deep rooted species during the dry season. Nevertheless, water deficit in superficial soil layers, fire damages and nutritional requirements could differentially affect juveniles of both phenological types determining their spatial patterns of distribution (evergreen species sparse in open grasslands and deciduous species gathered in groves). This study examined seasonal variations in water relations and photosynthesis in juveniles of two tree species differing in leaf phenology: Curatella americana (evergreen) and Cochlospermum vitifolium (deciduous). We wanted to test the hypothesis that the improved ability of evergreen deep rooted species like C. americana to colonize open savannas is related to drought tolerance and a conservative gas exchange pattern in juveniles that photosynthesize even during the dry season, while one might expect a water spender and avoider character in propagules of deciduous species like C. vitifolium that are active mostly during the rainy season. The studied species showed high transpiration rates during the rainy period, indicating little water availability limitations on surface soil layers. Even so, both species exhibited a distinctive decrease in stomatal conductance towards the end of the rainy season, an indication of water availability restrictions and/or a response to higher air evaporative demands in the shallow rooted juveniles. At the peak of the dry season, C. americana presented comparable transpiration rates with wet season measurements, indicating that the studied juveniles had reached more permanent (or stable) water sources at soil depths of 30 cm (approximate maximum root lengths measured). Yet, C. americana showed a significant decrease in water potentials from wet to dry seasons, and osmotic adjustment as a drought tolerance mechanism. Overall, the evergreen functional type (juvenile plants of C. americana) showed tolerance to water deficit, maintaining functional leaves in the dry season. On the other hand, in absence of a water deficit, juveniles of C. vitifolium showed higher photosynthetic rates that could be translated into high growth rates in wet habitats. However, the low water deficit tolerance of the juveniles of C. vitifolium seems to be the main handicap for its colonization potential of the open savanna environment. In conclusion, as predicted, the greater tolerance to water deficit of the evergreen studied species suggests that resistance to water stress determines the observed distribution pattern of tree species in the study area, where only the evergreen ones colonize the open savanna, acting in its adult stages as a focus for the recruitment of woody species.