24-Epibrassinolide alleviates drought effects in young Carapa guianensis plants,improving the hydraulic safety margin,gas exchange and antioxidant defence

• Climate change is increasing the frequency of extreme events such as droughts, limiting plant growth and productivity. Exogenous application of plant growth regulators, such as 24-epibrassinolide (EBR), might be a solution as this molecule is organic, eco-friendly, and biodegradable. This is the first research to examine possible roles of EBR on the hydraulic safety margin, physiological behaviour, and metabolism in Carapa guianensis Aubl. (Meliaceae) exposed to drought. C. guianensis is a widely distributed tree in tropical forests of the Amazon.
• The objective was to determine whether EBR can improve tolerance to water deficit in young C. guianensis by measuring hydraulic traits, nutritional, biochemical and physiological responses, and biomass. The experiment had four randomized treatments: two water conditions (control and water deficit) and two concentrations of EBR (0 and 100 nM EBR).
• EBR increased the water potential and hydraulic safety margin, increased CO₂ fixation, and improved stomatal performance. EBR also stimulated antioxidant defences (SOD, CAT, APX, and POX).
• Overall, tretreatment with EBR improved drought tolerance of young C. guianensis plants.

     

Lipid peroxidation, chloroplastic pigments and antioxidant strategies in Carapa guianensis (Aubl.) subjected to water-deficit and short-term rewetting

The effects of drought on membrane lipids and leaf pigments and the ability of andiroba (Carapa guianensis Aubl.) plants to attenuate oxidative damage through antioxidant enzymes or adjusting carotenoids and glycinebetaine (GB) were examined. Assessments were performed when pre-dawn leaf water potential (Ψ_pd) of water-stressed plants reached −1.35 and −3.21 MPa (15 and 27 days after withholding irrigation) and 12 h after resuming watering (short-term rewetting, day 28). Oxidative damages to lipids were evident on day 15, in which drought caused an increase of 47% in malondialdehyde (MDA) content. On day 27, MDA content did not differ between treatments. The activity of superoxide dismutase remained unchanged over experimental period, while significant increases in the ascorbate peroxidase (APX, 110%) and catalase (CAT, 50%) activities were observed only on day 27. GB content was 62% (day 15) and 112% (day 27) higher in water-stressed plants than in control. Regardless of Ψ_pd, both chlorophyll (Chl) a, Chl b and total carotenoids remained unchanged between well-watered and water-stressed plants, indicating that drought did not result in degradation of leaflet pigments. On day 28, Ψ_pd of water-stressed plants increased near to control plants and both activities of APX and CAT did not differ between treatments. Altogether, adjustments in APX and CAT activity and in the GB content were efficient strategies to prevent expressive oxidative damages in water-stressed andiroba plants.     

Variations in water status, gas exchange, and growth in Rosmarinus officinalis plants infected with Glomus deserticola under drought conditions

The influence of the arbuscular mycorrhizal fungus Glomus deserticola on the water relations, gas exchange parameters, and vegetative growth of Rosmarinus officinalis plants under water stress was studied. Plants were grown with and without the mycorrhizal fungus under glasshouse conditions and subjected to water stress by withholding irrigation water for 14 days. Along the experimental period, a significant effect of the fungus on the plant growth was observed, and under water stress, mycorrhizal plants showed an increase in aerial and root biomass compared to non-mycorrhizal plants. The decrease in the soil water potential generated a decrease in leaf water potential (psi(l)) and stem water potential (psi(x)) of mycorrhizal and non-mycorrhizal plants, with this decrease being lower in mycorrhizal water-stressed plants. Mycorrhization also had positive effects on the root hydraulic conductivity (Lp) of water stressed plants. Furthermore, mycorrhizal-stressed plants showed a more important decrease in osmotic potential at full turgor (psi(os)) than did non-mycorrhizal-stressed plants, indicating the capacity of osmotic adjustment. Mycorrhizal infection also improved photosynthetic activity (Pn) and stomatal conductance (g(s)) in plants under water stress compared to the non-mycorrhizal-stressed plants. A similar behaviour was observed in the photochemical efficiency of PSII (Fv/Fm) with this parameter being lower in non-mycorrhizal plants than in mycorrhizal plants under water stress conditions. In the same way, under water restriction, mycorrhizal plants showed higher values of chlorophyll content than did non-mycorrhizal plants. Thus, the results obtained indicated that the mycorrhizal symbiosis had a beneficial effect on the water status and growth of Rosmarinus officinalis plants under water-stress conditions.     

Chloroplastidic pigments,gas exchange,and carbohydrates changes during <Emphasis Type="Italic">Carapa guianensis</Emphasis> leaflet expansion

Changes in chloroplastidic pigments, gas exchange and carbohydrate concentrations were assessed during the rapid initial expansion of C. guianensis leaflet. Leaves at metaphyll stage were tagged and assessments were carried out 14, 17, 20, 23, 27, and 31 days later. Pigments synthesis, distribution and accumulation were uniform among leaflet sections (basal, median and apical). Chlorophyll (Chl) a, Chl b, Chl (a+b), and total carotenoids (Car) concentrations were significantly increased after 27 days from metaphyll, and the most expressive increases were parallel to lower specific leaflet area. Chl a/b was lower on day 14 and it was increased on subsequent days. Negative net photosynthesis rate (P _N), and the lowest stomatal conductance (g _s) and transpiration (E) were registered on day 14, following significant increases on subsequent days. The Chl (a+b) and Chl a effects on P _N were more expressive until day 20. Intercellular to ambient CO₂ concentration ratio (C _i/C _a) was higher on day 14 and lower on subsequent days, and no stomatal limitation to CO₂ influx inside leaflets was observed. Leaflet temperature was almost constant (ca. 35°C) during leaflet development. Sucrose and starch concentrations were increased in parallel to increases in P _N. Altogether, these results highlight the main physiological changes during C. guianensis leaflet expansion and they should be considered in future experiments focusing on factors affecting P _N in this species.     

Growth, sodium uptake and antioxidant responses of coastal plants differing in their ecological status under increasing salinity

In the present study, we compared the response to salinity of three plants from Brittany coast with contrasted ecological status: Limonium latifolium (salt marshes), Matricaria maritima (beach tops and sand dunes) and Crambe maritima (fixed dunes). Under controlled glasshouse conditions, the growth of the three plants decreased with increasing external salinity. L. latifolium and C. maritima exhibited the highest and lowest resistance to severe salt stress (400 mM), respectively. M. maritima could be considered as an intermediate species, since it tolerated salinity up to 200 mM. The same observation could be made with sodium absorption and acuumulation in plant tissues, the most tolerant species (L. latifolium being the least Na accumulator. Hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), commonly produced in conditions of stress, accumulated significantly in salt treated C. maritima and M. maritima while not in the tolerant L. latifolium. The latter used glutathione reductase to maintain constant H₂O₂ levels under salt stress while peroxidases were very low and ascorbate peroxidase did not respond to salinity stimulation. The medium tolerant halophyte M. maritima used peroxidases to protect from NaCl-induced H₂O₂, while the sensitive C. maritima failed to detoxify H₂O₂ despite a sharp increase in catalase activity. Results showed that the three coastal species differ in resistance to salinity. They also suggested that the level of plant resistance to salinity could be attributed to differing mechanisms to manage the accumulation of sodium and cope with the oxidative damages.     

Chilling-induced changes in the antioxidant status of basil plants

Six cultivars of basil, ‘Genovese’, ‘Purpurascens’, ‘Cinnamon’, ‘Crispum’, ‘Citriodora’, and ‘Siam Queen’, at the age of 8 weeks, were subjected to low temperature (6 °C for 8 days) or 18 °C (control). Content of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), total phenolics, and l-ascorbic acid were assessed in basil leaves after low temperature exposure. Activity of peroxidase (POD), and catalase (CAT) enzymes and 2,2-diphenyl-1-picrylhydrazyl (DPPH^·) radical scavenging activity were also determined. The greatest increase in H₂O₂ was observed for lettuce leaf basil, by 104 % in comparison to the control, while most noticeable increase in the content of MDA was noted for lemon basil (by 77 %). Chilling treatment resulted in higher POD activity in two cultivars: Thai and green basil, changes in CAT activity was negligible for almost all tested genotypes, with an exception of Thai basil, for which activity of this enzyme dropped. Chilling induced the increase of l-ascorbic acid in most tested basil cultivars, but total phenolic content increased significantly only in lettuce leaf basil. Higher ability in scavenging free radicals was shown in basil treated with 6 °C, especially the red basil cultivar. For this genotype, DPPH^· radical scavenging activity was the highest among tested cultivars and was parallel to the highest content of phenolics. The results indicated overproduction of H₂O₂, deterioration of membrane integrity, and activation of enzymatic and/or non-enzymatic defence mechanisms in basil with an evidence of genotypic variation as the response to low temperature.     

Brassinosteroids improve photosystem II efficiency,gas exchange,antioxidant enzymes and growth of cowpea plants exposed to water deficit

Water deficit is considered the main abiotic stress that limits agricultural production worldwide. Brassinosteroids (BRs) are natural substances that play roles in plant tolerance against abiotic stresses, including water deficit. This research aims to determine whether BRs can mitigate the negative effects caused by water deficiency, revealing how BRs act and their possible contribution to increased tolerance of cowpea plants to water deficit. The experiment was a factorial design with the factors completely randomised, with two water conditions (control and water deficit) and three levels of brassinosteroids (0, 50 and 100 nM 24-epibrassinolide; EBR is an active BRs). Plants sprayed with 100 nM EBR under the water deficit presented significant increases in Φ_PSII, q_P and ETR compared with plants subjected to the water deficit without EBR. With respect to gas exchange, P _N, E and g _s exhibited significant reductions after water deficit, but application of 100 nM EBR caused increases in these variables of 96, 24 and 33%, respectively, compared to the water deficit + 0 nM EBR treatment. To antioxidant enzymes, EBR resulted in increases in SOD, CAT, APX and POX, indicating that EBR acts on the antioxidant system, reducing cell damage. The water deficit caused significant reductions in Chl a, Chl b and total Chl, while plants sprayed with 100 nM EBR showed significant increases of 26, 58 and 33% in Chl a, Chl b and total Chl, respectively. This study revealed that EBR improves photosystem II efficiency, inducing increases in Φ_PSII, q_P and ETR. This substance also mitigated the negative effects on gas exchange and growth induced by the water deficit. Increases in SOD, CAT, APX and POX of plants treated with EBR indicate that this steroid clearly increased the tolerance to the water deficit, reducing reactive oxygen species, cell damage, and maintaining the photosynthetic pigments. Additionally, 100 nM EBR resulted in a better dose–response of cowpea plants exposed to the water deficit.     

Ethylene reduces gas exchange and growth of lettuce plants under hypobaric and normal atmospheric conditions

Elevated levels of ethylene occur in controlled environment agriculture and in spaceflight environments, leading to adverse plant growth and sterility. The objectives of this research were to characterize the influence of ethylene on carbon dioxide (CO₂) assimilation (C_A), dark period respiration (DPR) and growth of lettuce ( Lactuca sativa L. cv. Buttercrunch) under ambient and low total pressure conditions. Lettuce plants were grown under variable total gas pressures of 25 kPa (hypobaric) and 101 kPa (ambient) pressure. Endogenously produced ethylene accumulated and reduced C_A, DPR and plant growth of ambient and hypobaric plants. There was a negative linear correlation between increasing ethylene concentrations [from 0 to around 1000 nmol mol⁻¹ (ppb)] on C_A, DPR and growth of ambient and hypobaric plants. Declines in C_A and DPR occurred with both exogenous and endogenous ethylene treatments. C_A was more sensitive to increasing ethylene concentration than DPR. There was a direct, negative effect of increasing ethylene concentration reducing gas exchange as well as an indirect ethylene effect on leaf epinasty, which reduced light capture and C_A. While the C_A was comparable, there was a lower DPR in hypobaric than ambient pressure plants – independent of ethylene and under non-limiting CO₂ levels (100 Pa pCO₂, nearly three-fold that in normal air). This research shows that lettuce can be grown under hypobaria (≅25% of normal earth ambient total pressure); however, hypobaria caused no significant reduction of endogenous ethylene production.     

Exogenous glycine betaine modulates ascorbate peroxidase and catalase activities and prevent lipid peroxidation in mild water-stressed Carapa guianensis plants

The hypothesis that application of exogenous glycine betaine (GB_EX) may attenuate the effects of mild water deficit in leaf gas exchange and lipid peroxidation in Carapa guianensis was examined. For this reason, 110-d old plants were sprayed with 0, 25, and 50 mM GB_EX and then subjected to two watering regimes. In the first, irrigation was continuously performed to maintain the soil near to field capacity (watered plants). In the second, irrigation was withheld and water deficit resulted from progressive evapotranspiration (water-stressed plants). Treatment comparisons were assessed when predawn leaflet water potential (Ψ_pd) of stressed plants reached ?1.28 ± 0.34 MPa. Regardless of the watering regime, significant (P<0.05) increases in foliar glycine betaine (GB_Leaf) concentration were observed in response to increasing GB_EX; however, such increases were more expressive in stressed plants. The net photosynthetic rate, stomatal conductance to water vapor, and intercellular to ambient CO₂ concentration ratio were significantly lower in water-stressed plants independently of GB_EX concentration sprayed on leaves. The application of 25 and 50 mM GB_EX caused significant (P<0.05) increases in ascorbate peroxidase (APX) activity in stressed plants, while significant (P<0.05) increases in catalase activity was observed just in the stressed plants treated with 50 mM GB_EX. Malondialdehyde concentrations did not differ between watered and stressed plants regardless of GBEX concentration. In conclusion, C. guianensis was able to incorporate GBEX through their leaves and the resulting increases in GB_Leaf attenuated lipid peroxidation in stressed plants through positive modulation of APX and CAT activities.     

Changes in leaf gas exchange, water relations, biomass production and solute accumulation in Phragmites australis under hypoxic conditions

The physiological responses to hypoxic stress were studied in the common reed, Phragmites australis (Cav.) Trin. ex Steudel. Growth, leaf gas exchange, water (and ion) relations and osmotic adjustment were determined in hydroponically grown plants exposed to 10, 20 and 30 days of oxygen deficiency. The highest growth of reed seedlings was found in normoxic (aerobic) conditions. Treatment effects on biomass production were relatively consistent within each harvest. Leaf water potential and osmotic potential declined significantly as hypoxia periods increased. However, leaf turgor pressure showed a consistent pattern of increase, suggesting that reed plants adjusted their water status by osmotic adjustment in response to root hypoxia. After 20 and 30 days in the low oxygen treatment, net CO₂ assimilation and stomatal conductance were positively associated and the former variable also had a strong positive relationship with transpiration. Short-term hypoxic stress had a slight effect on the ionic status (K⁺, Ca²⁺ and Mg²⁺) of reed plants. In contrast, soluble sugar concentrations increased more under hypoxic conditions as compared to normoxia. These findings indicate that hypoxia slightly affected the physiological behavior of reed plants.     

Diurnal changes in water deficit and recovery patterns of intact rice leaves under natural and controlled environment conditions

In the natural monsoon environment, the leaves of Bala and Jaya cultivars experienced more than 10 per cent LWD for about 8 and 6 1/2 h in the diurnal cycle, respectively. The maximum LWD of around 14 per cent occurred between 14.30 to 15.30 at the time of minimum relative humidity (EH) and maximum air temperature (Ta). On the other hand, under controlled environment conditions, LWD values of both the cultivars remained below 10 per cent in the entire diurnal cycle. There was a close relationship between LWD and Ta (r = 0.90 for Bala and 0.91 for Jaya) as well as RH (r = -0.95 for Bala and -0.85 for Jaya). The results suggested that RWC values obtained on an interval of 30 min were not enough to observe more than the general trend as a complete recovery of stress achieved within 25 min of rewatering the stressed leaves. Calculations indicated that the determination of LWD with the techniques using excised leaf are likely to incur error as rice leaf may loose water at a rate of 0.3 MPa min⁻¹.     

Diurnal changes in needle gas exchange in alpine Pinus pumila during snow-melting and summer seasons

Pinus pumila (Pallas) Regel. is a dominant dwarf tree in alpine regions of Japan. The possible factors limiting the net photosynthetic rate (P_n) of the needles of P. pumila were examined in the snow-melting (May) and the summer (August) seasons. In August, in situ maximum P_n was 20 mol kg^–1 needle s^–1 in the current-year needles and 25 mol kg^–1 needle s^–1 in the 1-year-old needles. Diurnal trends of P_n in August were positively related to fluctuations in photosynthetic photon flux density (PPFD) and no midday depression of P_n was found, indicating that a decrease in PPFD rather than an increase in needle-to-air vapor pressure deficit (W) might cause the reduction of P_n. Both stomatal conductance (g_s) and P_n were lower in May than in August. In May, P_n and g_s were almost zero in the morning, but gradually increased with decreasing W, reaching maximum P_n values (4 mol kg^–1 needle s^–1) and g_s (60 mmol kg^–1 needle s^–1) at 16.00 hours. The daytime P_n in May was positively related to g_s. Relative water content in the exposed needles above the snow in May was 83%, which was far above the lethal level. This indicates that the water flow from stems or soils to needles was enough to compensate for a small amount of water loss due to the low g_s in May, although the water supplied to needles would be impeded by the low temperatures. Thus, the reduced g_s in May would be important for avoiding needle desiccation, and would result in a reduced P_n.     

Genetic diversity and mating system in a tropical tree,Carapa guianensis (Meliaceae)

Carapa guianensis Aublet. (Meliaceae) is an abundant, canopy tree species often exported as fine wood products. The populations in the Atlantic coastal plain of Costa Rica have undergone a wide variety of human-induced changes in distribution and abundance, including isolation due to deforestation. Using protein electrophoretic analysis of seeds, we investigated the genetic variability and population differentiation of nine populations and the outcrossing rate in two populations that differed in density by a factor of 50%. Average polymorphism (35%) and heterozygosity (0.12) across populations were relatively low in comparison with other tropical tree species. Only 4.6% of the total genetic variability could be attributed to population differentiation even though populations were separated by as much as 70 km. However, genetic differentiation did increase with distance between pairs of compared populations. There were no apparent differences in genetic variation between logged and unlogged forests. The outcrossing rate for each population was not significantly different from 1.0, but there was evidence of nonrandom mating in the lower density population. High levels of gene flow are probably maintained by seed dispersal as seeds travel well throughout flooded forests and rivers. High population density and synchronous flowering also contribute to high outcrossing rates. These genetic characteristics suggest that Carapa guianensis is a good candidate for natural forest management where it occurs at high density. However, current management practices need to account for the rapid rate of deforestation occurring around managed natural forest sites that will result in the elimination of intervening populations.     

云锦杜鹃夏季气体交换和水分利用效率日变化

云锦杜鹃净光合速率和气孔导度日变化曲线为"双峰"型,光合效率午间明显降低,主要由非气孔限制引起。表观量子效率和实际光化学效率的降低是非气孔限制形成和发展的深层原因。蒸腾速率的日变化为"单峰型",午间最高。水分利用效率早晚较高、午间较低。净光合速率、蒸腾速率、气孔导度和水分利用效率分别与一些环境因子的相关性达到0.01或0.05显著水平。利用多元逐步回归方法分别得到了净光合速率、胞间CO2浓度、蒸腾速率、气孔导度和水分利用效率与环境因子的最优方程。     

Changes in leaf gas exchange, water relations, biomass production and solute accumulation in Phragmites australis under hypoxic conditions

The physiological responses to hypoxic stress were studied in the common reed, Phragmites australis (Cav.) Trin. ex Steudel. Growth, leaf gas exchange, water (and ion) relations and osmotic adjustment were determined in hydroponically grown plants exposed to 10, 20 and 30 days of oxygen deficiency. The highest growth of reed seedlings was found in normoxic (aerobic) conditions. Treatment effects on biomass production were relatively consistent within each harvest. Leaf water potential and osmotic potential declined significantly as hypoxia periods increased. However, leaf turgor pressure showed a consistent pattern of increase, suggesting that reed plants adjusted their water status by osmotic adjustment in response to root hypoxia. After 20 and 30 days in the low oxygen treatment, net CO₂ assimilation and stomatal conductance were positively associated and the former variable also had a strong positive relationship with transpiration. Short-term hypoxic stress had a slight effect on the ionic status (K⁺, Ca²⁺ and Mg²⁺) of reed plants. In contrast, soluble sugar concentrations increased more under hypoxic conditions as compared to normoxia. These findings indicate that hypoxia slightly affected the physiological behavior of reed plants.     

The effect of strobilurins on leaf gas exchange, water use efficiency and ABA content in grapevine under field conditions

Strobilurins are one of the most important classes of agricultural fungicides. In addition to their anti-fungal effect, strobilurins have been reported to produce simultaneous effects in plant physiology. This study investigated whether the use of strobilurin fungicide improved water use efficiency in leaves of grapevines grown under field conditions in a Mediterranean climate in southern Spain. Fungicide was applied three times in the vineyard and measurements of leaf gas exchange, plant water status, abscisic acid concentration in sap ([ABA]), and carbon isotope composition in leaves were performed before and after applications. No clear effect on stomatal conductance, leaf water potential and intrinsic water use efficiency was found after three fungicide applications. ABA concentration was observed to increase after fungicide application on the first day, vanishing three days later. Despite this transient effect, evolution of [ABA] matched well with the evolution of leaf carbon isotope ratio, which can be used as a surrogate for plant water use efficiency. Morning stomatal conductance was negatively correlated to [ABA]. Yield was enhanced in strobilurin treated plants, whereas fruit quality remained unaltered.     

Ultraviolet-B and water stress effects on growth,gas exchange and oxidative stress in sunflower plants

The effects and interaction of drought and UV-B radiation were studied in sunflower plants (Helianthus annuus L. var. Catissol-01), growing in a greenhouse under natural photoperiod conditions. The plants received approximately 1.7 W m(-2) (controls) or 8.6 W m(-2) (+UV-B) of UV-B radiation for 7 h per day. The UV-B and water stress treatments started 18 days after sowing. After a period of 12 days of stress, half of the water-stressed plants (including both UV-B irradiated or non-irradiated) were rehydrated. Both drought and UV-B radiation treatments resulted in lower shoot dry matter per plant, but there was no significant interaction between the two treatments. Water stress and UV-B radiation reduced photosynthesis, stomatal conductance and transpiration. However, the amplitude of the effects of both stressors was dependent on the interactions. This resulted in alleviation of the negative effect of drought on photosynthesis and transpiration by UV-B radiation as the water stress intensified. Intercelluar CO(2) concentration was initially reduced in all treatments compared to control plants but it increased with time. Photosynthetic pigments were not affected by UV-B radiation. Water stress reduced photosynthetic pigments only under high UV-B radiation. The decrease was more accentuated for chlorophyll a than for chlorophyll b. As a measure for the maximum efficiency of photosystem II in darkness F (v)/F (m) was used, which was not affected by drought stress but initially reduced by UV-B radiation. Independent of water supply, UV-B radiation increased the activity of pirogalol peroxidase and did not increase the level of malondialdehyde. On the other hand, water stress did not alter the activity of pirogalol peroxidase and caused membrane damage as assessed by lipid peroxidation. The application of UV-B radiation together with drought seemed to have a protective effect by lowering the intensity of lipid peroxidation caused by water stress. The content of proline was not affected by UV-B radiation but was increased by water stress under both low and high UV-B radiation. After 24 h of rehydration, most of the parameters analyzed recovered to the same level as the unstressed plants.