Leaf water potential, stomatal resistance, and photosynthetic response to water stress in peach seedlings

Individual groups of peach (Prunus persica [L.] Batsch) seedlings stressed to −17, −26 and −36 bars recovered to control levels within 1, 3, and 4 days, respectively. Stomatal resistance was significantly correlated with both leaf water potential and net photosynthesis. In seedlings stressed to −52 bars, leaf water potential and stomatal resistance recovered sooner than net photosynthesis, despite recovery of 0₂ evolution at a rate similar to leaf water potential. Therefore, some nonstomatal factor other than reduction in photochemical activity must be responsible for the lag in recovery of CO₂ assimilation following irrigation.     

Genotypic variation in the photosynthetic competence of Sorghum bicolor seedlings subjected to polyethylene glycol-mediated drought stress

Eleven varieties of Sorghum bicolor, subjected to PEG-mediated drought stress were compared for their photosynthetic performance. The varieties differed in their relative water content over a range of PEG concentrations (0-25%). CO2 assimilation, stomatal conductance and the quantum yield of PSII electron transport decreased with increasing PEG concentrations in all varieties. However the intercellular CO2 concentration showed a nonlinear PEG concentration-dependent change. At lower PEG concentrations there was a decrease in the levels of intercellular CO2 concentration in all varieties that could be attributed to stomatal closure. At higher PEG concentrations, some varieties showed an increase in the intercellular CO2 concentration, indicating an inhibition of photosynthetic activity due to non-stomatal effects, while others did not. It was seen that the varieties differed in the stress thresholds at which stomatal and metabolic limitations to photosynthesis occur. These differences in the photosynthetic adaptation of Sorghum varieties could be useful in identifying genotypes showing large differences in photosynthetic adaptation, which could be useful in mapping photosynthetic traits for drought stress tolerance.     

Patterns of dynamic irradiance affect the photosynthetic capacity and growth of dipterocarp tree seedlings

In the deeply shaded understorey of S.E. Asian rain forests the growth and survival of dipterocarp seedlings is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain in understorey plants. To test the sensitivity of photosynthesis and growth to variation in the pattern of dynamic irradiance, dipterocarp tree seedlings (Shorea leprosula and Hopea nervosa) were grown for 370 days under shaded forest light treatments of equal total daily photosynthetic photon flux density (approximately 3.3 mol m(-2) day(-1)), but characterised by either long flecks (LF) or short flecks (SF). Seedling growth was more than 4-fold greater under LF, compared with SF, in both species. Variation in the relative growth rates (RGR) and light saturated rates of photosynthesis (A(max)) were strongly positively correlated with the mean duration of sunflecks. Variation in RGR was strongly correlated with greater unit leaf rate growth, indicating that photosynthetic carbon gain per unit leaf area was greater under LF. The accumulation of starch in leaves over the diurnal period was 117% greater in both species under LF, compared with SF. Greater carbon gain in seedlings under LF is likely to have resulted from the combination of (1) greater A(max) (S. leprosula 35%, H. nervosa 40%), (2) more efficient dynamic photosynthesis, and (3) greater incident photosynthetic quantum yield, compared with seedlings receiving the SF irradiance treatment. The pattern of dynamic irradiance received by seedlings may significantly impact their growth and survival to a previously unrecognised extent, with important consequences for regeneration processes and hence forest structure and composition.     

Cotton genotypic variation in the photosynthetic response to irradiance

The photosynthetic response of 8 cotton (Gossypium hirsutum L.) genotypes to changing irradiance was investigated under field conditions during the 1998 through 2000 growing seasons. Equations developed to describe the response of net photosynthetic rate (P_N) to photosynthetic photon flux density (PPFD) demonstrated that, across all irradiances, the two okra leaf-type genotypes photosynthesized at a greater rate per unit leaf area than all of the six normal leaf-type genotypes. This superior photosynthetic performance of the okra leaf-type genotypes can be partially explained by their 13 % greater leaf chlorophyll content relative to that of the normal leaf-type genotypes. The 37 % reduction in leaf size brought upon by the okra leaf trait may have concentrated the amount of photosynthetic machinery per unit leaf area. Nevertheless, the lack of sufficient canopy leaf surface area suppressed the potential yield development that could accompany the higher P_N per unit leaf area.     

Cotton genotypic variation in the photosynthetic response to irradiance

The photosynthetic response of 8 cotton (Gossypium hirsutum L.) genotypes to changing irradiance was investigated under field conditions during the 1998 through 2000 growing seasons. Equations developed to describe the response of net photosynthetic rate (P_N) to photosynthetic photon flux density (PPFD) demonstrated that, across all irradiances, the two okra leaf-type genotypes photosynthesized at a greater rate per unit leaf area than all of the six normal leaf-type genotypes. This superior photosynthetic performance of the okra leaf-type genotypes can be partially explained by their 13 % greater leaf chlorophyll content relative to that of the normal leaf-type genotypes. The 37 % reduction in leaf size brought upon by the okra leaf trait may have concentrated the amount of photosynthetic machinery per unit leaf area. Nevertheless, the lack of sufficient canopy leaf surface area suppressed the potential yield development that could accompany the higher P_N per unit leaf area.This revised version was published online in March 2005 with corrections to the page numbers.     

Arbuscular mycorrhizal influence on growth,photosynthetic pigments,osmotic adjustment and oxidative stress in tomato plants subjected to low temperature stress

The influence of the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, on characteristics of growth, photosynthetic pigments, osmotic adjustment, membrane lipid peroxidation and activity of antioxidant enzymes in leaves of tomato (Lycopersicon esculentum cv Zhongzha105) plants was studied in pot culture under low temperature stress. The tomato plants were placed in a sand and soil mixture at 25°C for 6 weeks, and then subjected to 8°C for 1 week. AM symbiosis decreased malondialdehyde (MDA) content in leaves. The contents of photosynthetic pigments, sugars and soluble protein in leaves were higher, but leaf proline content was lower in mycorrhizal than non-mycorrhizal plants. AM colonization increased the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) in leaves. The results indicate that the AM fungus is capable of alleviating the damage caused by low temperature stress on tomato plants by reducing membrane lipid peroxidation and increasing the photosynthetic pigments, accumulation of osmotic adjustment compounds, and antioxidant enzyme activity. Consequently, arbuscular mycorrhiza formation highly enhanced the cold tolerance of tomato plant, which increased host biomass and promoted plant growth.     

Hormonal changes in papaya seedlings subjected to progressive water stress and re-watering

Changes on abscisic acid (ABA), jasmonic acid (JA) and indole-3-acetic acid (IAA) levels were investigated in papaya seedlings (Carica papaya L.) cv. “Baixinho de Santa Amalia” under progressive water stress and subsequent rehydration. Also, the behaviour of leaf gas exchange and leaf growth was determined under stress condition. The results indicated that ABA and JA differ in their pattern of change under water stress. ABA continuously increased in leaves and roots during the whole period of stress whereas JA showed a sharp increase and a later decrease in both organs. Re-watering reduced rapidly (24 h) leaf and root ABA to control levels whereas the influence on JA levels could not be assessed. Drought and recovery did not alter IAA levels in leaf and root tissues of papaya seedlings. In addition, water stress reduced stomatal conductance, photosynthetic rate, transpiration rate, the percentage of attached leaves and leaf growth. Rehydration reverted in few days the effects of stress on leaf growth and gas exchange parameters. Overall, the data suggest that ABA could be involved in the induction of several progressive responses such as the induction of stomatal closure and leaf abscission to reduce papaya water loss. In addition, the pattern of accumulation of JA is compatible with a triggering signal upstream ABA. The unaltered levels of IAA could suggest a certain adaptive ability of papaya to maintain active physiological processes under progressive drought stress.     

Antioxidant responses and water status in Brassica seedlings subjected to boron stress

The effect of boron (B) on growth, water status and oxidative damage was investigated in the leaves and roots of 7-day-old seedlings of Brassica juncea var. Varuna. For this seedlings of Brassica were grown in solution culture with variable boron supply (0.033, 0.33, 3.3 and 33 mg B L^?1) under controlled conditions in green house. Photosynthetic pigments were found to be decreased more under excess (3.3 and 33 mg B L^–1) than deficient boron supply (0.033 mg B L^–1) when compared to control (0.33 mg B L^–1). Accumulation of hydrogen peroxide and thiobarbituric acid reactive substances content in both leaves and roots under deficient and excess boron supply suggested oxidative damage due to excessive production of reactive oxygen species. Increased activity of antioxidative enzymes: superoxide dismutase, catalase and peroxidase along with polyphenol oxidase was observed in leaves and roots under boron deficiency and excess than in control. Increased proline concentration, decreased total water content and water saturation deficit also indicated the water deficit condition in leaves and roots of boron-stressed Brassica seedlings.     

Mycorrhizal promotion of host stomatal conductance in relation to irradiance and temperature

Colonization of roots and soil by arbuscular mycorrhizal (AM) fungi sometimes promotes stomatal conductance (g _s) of the host plant, but scientists have had difficulty predicting or manipulating the response. Our objective was to test whether the magnitude of AM influence on g _s is related to environmental conditions: irradiance, air temperature or leaf temperature. Stomatal conductances of two groups of uncolonized sorghum plants were compared to g _s of plants colonized by Glomus intraradices (Gi) or Gigaspora margarita (Gm) in 31 morning and afternoon periods under naturally varying greenhouse conditions. Stomatal conductance of Gi and Gm plants was often markedly higher than g _s of similarly sized nonAM plants. AM promotion of g _s was minimal at the lowest irradiances and lowest air and leaf temperatures, but was substantial at intermediate irradiance and temperatures. AM promotion was again low or absent at the highest irradiances and temperatures. Magnitude of AM promotion of g _s was not a function of absolute g _s. Promotion of g _s by Gi and Gm was remarkably similar. Differing phosphorus fertilization did not affect g _s.     

The influence of stomatal morphology and distribution on photosynthetic gas exchange

The intricate and interconnecting reactions of C₃ photosynthesis are often limited by one of two fundamental processes: the conversion of solar energy into chemical energy, or the diffusion of CO₂ from the atmosphere through the stomata, and ultimately into the chloroplast. In this review, we explore how the contributions of stomatal morphology and distribution can affect photosynthesis, through changes in gaseous exchange. The factors driving this relationship are considered, and recent results from studies investigating the effects of stomatal shape, size, density and patterning on photosynthesis are discussed. We suggest that the interplay between stomatal gaseous exchange and photosynthesis is complex, and that a disconnect often exists between the rates of CO₂ diffusion and photosynthetic carbon fixation. The mechanisms that allow for substantial reductions in maximum stomatal conductance without affecting photosynthesis are highly dependent on environmental factors, such as light intensity, and could be exploited to improve crop performance.     

光照对鄂东南2种落叶阔叶树种幼苗生长、光合特性和生物量分配的影响

通过设置4个光照梯度(25%、12%、6%和3%自然光)模拟鄂东南低山丘陵地区落叶阔叶林林下的光环境,研究了2种耐荫性不同的树种幼苗--麻栎(Quercus acutissima)和化香(Platycarya strobilacea)不同光强下的存活率、光合特性、生长和生物量分配,探讨了低光环境中耐荫性不同的树种幼苗维持自身碳平衡的机制和权衡"存活-生长"选择的生活史策略。结果表明:(1)低光下的2个树种幼苗的生长、光合特性和生物量分配具有显著性差异。(2)各个光照梯度下麻栎幼苗都生长良好,存活率保持在35%以上,而化香幼苗遭遇高的死亡率,80d后3%和6%自然光下的幼苗全部死亡;低光环境中麻栎幼苗比化香幼苗具有更大的表观光量子(AQY)和最大净光合效率(Pmax),更低的光补偿点(LCP)和暗呼吸效率(Rd),即耐荫性较强的麻栎幼苗比耐荫性较弱的化香幼苗具有更高的低光碳同化率和碳捕获能力。(3)2个树种幼苗的成活率与RGR呈负相关关系,各个光照梯度下耐荫性较弱的化香幼苗的相对生长率(RGR)显著高于耐荫性较强的麻栎幼苗,而两个树种幼苗的净同化率(NAR)无明显差异。相对于麻栎幼苗较高的根生物量比(RMR),化香幼苗将更多的生物量分配给叶部,因而具有较高的叶生物量比(LMR)、叶面积比(LAR)和比叶面积(SLA)。不同耐荫性的幼苗生长及生物量分配方式的差异是植物"存活-生长"权衡后的结果,耐荫性弱的化香幼苗具有较高的生长潜力和较弱的自我保护能力,而耐荫性强的麻栎幼苗具有更高的低光碳储量,能够维持更好的低光碳平衡,具有竞争优势。     

The protective role of selenium in rape seedlings subjected to cadmium stress

The effect of selenium (Se) on rape (Brassica napus) seedlings subjected to cadmium (Cd) stress was studied in vitro by investigating plant growth and changes in fatty acid composition, activity of antioxidative enzymes and DNA methylation pattern. Physiological experiments were carried out on seedlings cultured for 2 weeks on Murashige-Scoog (MS) media with Cd concentrations of 0, 400 and 600 μM, and on corresponding media supplied with Se (2 μM). Exposure to increasing Cd concentrations reduced the fresh weight of the upper part (hypocotyls+cotyledons) of the seedlings more strongly than that of the root system, which was accompanied by higher Cd accumulation in these tissues. In the upper part, Cd exposure led to significant changes in the biochemical parameters: fatty acid unsaturation of plasmalemma decreased, the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPOX) diminished and that of ascorbate peroxidase (APX) increased. In contrast, the roots showed an increase in fatty acid unsaturation and in the activity of antioxidative enzymes. In both parts of rape seedlings H₂O₂ level and lipid peroxidation increased. Se addition to medium considerably reversed the Cd-induced decrease in fresh mass as well as the changes in lipid unsaturation and peroxidation. Se applied separately or in combination with Cd did not significantly affect the activity of antioxidative enzymes in the roots, but diminished it in the upper part. Moreover, the presence of Se in medium prevented changes in the DNA methylation pattern triggered in rape seedlings by high Cd concentrations. Two possible mechanisms for the action of Se were considered: (1) removal of Cd from metabolically active cellular sites, and (2) reduction of oxygen radicals.     

Moderate water stress causes different stomatal and non‐stomatal changes in the photosynthetic functioning of Phaseolus vulgaris L. genotypes

The impact of moderate water deficit on the photosynthetic apparatus of three Phaseolus vulgaris L. cultivars, Plovdiv 10 (P10), Dobrudjanski Ran (DR) and Prelom (Prel), was investigated. Water shortage had less impact on leaf hydration, RWC (predawn and midday) and predawn water potential in Prel. RWC and Ψ_p were more reduced in P10, while there was no osmotic adjustment in any cultivar. Although drought drastically reduced stomatal opening in P10 and DR, reduced A_max indicated non‐stomatal limitations that contributed to the negligible P_n. These limitations were on potential thylakoid electron transport rates of PSI and II, pointing to photosystem functioning as a major limiting step in photosynthesis. This agrees with decreases in actual photochemical efficiency of PSII (F_v′/F_m′), quantum yield of photosynthetic non‐cyclic electron transport (?_e) and energy‐driven photochemical events (q_P), although the impact on these parameters would also include down‐regulation processes. When compared to DR, Prel retained a higher functional state of the photosynthetic machinery, justifying reduced need for photoprotective mechanisms (non‐photochemical quenching, zeaxanthin, lutein, β‐carotene) and maintenance of the balance between energy capture and dissipative pigments. The highest increases in fructose, glucose, arabinose and sorbitol in Prel might be related to tolerance to a lower oxidative state. All cultivars had reduced A_max due to daytime stomatal closure in well‐watered conditions. Under moderate drought, Prel had highest tolerance, higher leaf hydration and maintenance of important photochemical use of energy. However, water shortage caused appreciable non‐stomatal limitations to photosynthesis linked to regulation/imbalance at the metabolic level (and growth) in all cultivars. This included damage, as reflected in decreased potential photosystem functioning, pointing to higher sensitivity of photosynthesis to drought than is commonly assumed.     

Effect of low irradiance on the photosynthetic performance and spiking of Phalaenopsis

Lowering irradiance can delay the flower stalk, i.e., spike development, in order to schedule flowering time of Phalaenopsis; however, the effect on photosynthetic performance and spiking inhibition remains poorly understood. We compared light and shade treatments of Phalaenopsis aphrodite subsp. formosana in order to determine how limiting light affects day-night changes in the photosynthetic capacity of leaves and the carbon pool of leaves and stems resulting in delayed spiking. The low irradiance treatment [20 μmol(photon) m^?2 s^?1] for six weeks did not affect potential functions of photosynthetic apparatus estimated by chlorophyll a fluorescence analysis, but it significantly reduced the net CO₂ uptake and O₂ evolution rates, carbohydrate and organic acid concentrations, and amplitudes of CAM activity in new and fully expanded leaves of Phalaenopsis and delayed the spiking compared with the control kept at 150 μmol(photon) m^?2 s^?1. The shortened stem contained a remarkably high sucrose concentration, accounting for more than 80% of total soluble sugars for both treatments throughout the day. Moreover, the sucrose concentration was unaffected by the lowering of irradiance. The relationship between the sucrose content and spiking seemed to be loose; the major factor(s) for spiking in Phalaenopsis remained to be ascertained as the flower stalk bud is attached to the shortened stem.     

The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance

Dipterocarps dominate the canopy of lowland tropical rain forest in South‐east Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth depends largely upon the increased irradiance that can occur with the creation of canopy gaps. However, the response of dipterocarp seedlings to increased irradiance and their subsequent establishment in the canopy may be influenced by the availability of other resources, such as nutrient availability. We investigated the influence of nitrogen supply on aspects of the photosynthetic physiology and growth of seedlings of four dipterocarp species (Shorea leprosula, Shorea johorensis, Shorea oleosa and Dryobalanops lanceolata) growing under low irradiance, during transfer from low to high irradiance, and during subsequent growth at high irradiance. All four species increased growth and photosynthetic capacity in response to N‐supply at high irradiances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N‐supply were exposed to a large increase in irradiance, all species showed some degree of initial photodamage (measured through chlorophyll fluorescence), the extent of which was similar between species but differed markedly depending on the pre‐exposure growth irradiance and N‐supply. Greater photodamage occurred in seedlings grown at lower compared with higher N‐supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to recover from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N‐supply. Under high N‐supply all species apart from S. oleosa increased photosynthetic capacity per unit chlorophyll following exposure to high irradiance by increasing photosynthetic capacity per unit leaf area while, under low N‐supply, an increase in photosynthetic capacity per unit leaf only occurred in D. lanceolata. Our results suggest that variations in N‐availability may have a much greater impact on the relative competitiveness of dipterocarp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N‐availability in the regeneration dynamics and distribution of canopy‐dominating dipterocarp species.     

低温胁迫对六种苗木生理特性的影响

研究了低温胁迫对大花五桠果(Dillenia turbinata)、双翼豆(Peltophorum tonkinense)、海南山竹子(Garcinia oblingifolia)、盆架子(Alstonia scholaris)、非洲桃花心木(Khaya senegalensis)和海南红豆(Ormosia pinnata)幼苗的影响.结果表明:低温胁迫下6种苗木叶片的相对电导率比对照有所增加;除了0℃处理的双翼豆幼苗外,低温胁迫的幼苗叶片内的脯氨酸含量大于对照;不同低温胁迫的幼苗叶片的蛋白质和叶绿素含量变幅较小;大花五桠果、海南山竹子、非洲桃花心木叶片的丙二醛(MDA)含量随着温度的降低而上升,而其他树种幼苗的丙二醛含量变化幅度不大;各低温胁迫的大花五桠果叶片的超氧物歧化酶(SOD)活性显著高于对照,双翼豆叶片的SOD活性低于对照,而其他幼苗叶片的SOD活性变化较小.采用主成分分析法进行抗寒综合评价表明,6种苗木的抗寒能力排序为海南红豆＞盆架子＞非洲桃花心木＞海南山竹子＞双翼豆＞大花五桠果.     

Diurnal variation in leaf water potential,stomatal conductance,and irradiance of winter crop under different moisture levels

Mustard (Brassica juncea) Coss., chickpea (Cicer arietinum L. and barley (Hordeum vulgare) L. were grown under different moisture levels. Diurnal changes in leaf water potential showed lower values and higher fluctuation in mustard, chickpea and barley grown with no irrigation as compared to one supplemental irrigation. Diurnal maximum of adaxial stomatal conductance in mustard and barley was higher under one irrigation treatment. In mustard stomatal conductance of abaxial surface of leaf remained higher than adaxial surface of leaf throughout the day, whereas the reverse was true in barley. Also the leaf and soil temperature and reflectance were slightly higher in all the three crops under no irrigation.     

Growth characteristics and antioxidant metabolism of moongbean genotypes differing in photosynthetic capacity subjected to water deficit stress

Abstract

Two genotypes (Pusa 9531 and PS 16) of moongbean [Vigna radiata (L) Wilczek], differing in photosynthetic capacity were grown for 30 days in earthen pots at three field capacities (100, 75 and 50%), and the possible role of biochemical alterations and antioxidant metabolism in conferring photosynthetic capacity was determined by measuring Rubisco activity, photosynthetic traits, lipid peroxidation and assaying activities of the central components of antioxidant defence system. Growth, Rubisco activity, photosynthetic traits and soluble protein content decreased significantly with decreasing field capacity (FC) from 100 to 50%. Levels of TBARS, H₂O₂, electrolyte leakage and proline contents increased with decreasing FC. Activities of SOD and GR increased in both genotypes with decreasing FC; the CAT and APX activities over-expressed only at mild (75%) FC but not at severe (50%) FC. There were found genotype-dependent alterations in growth, photosynthetic traits, Rubisco activity and antioxidant metabolism when exposed to water deficit. Decline in efficiency of the H₂O₂-decomposing system at severe drought was responsible for oxidative damage occurring in both the genotypes. The differential responses of antioxidative enzymes in the two genotypes were the result of their ability to protect photosynthetic apparatus and alleviate water deficit stress.     

Effects of plant growth regulators on the antioxidant system in seedlings of two maize cultivars subjected to water stress

The effects of brassinolide, uniconazole and methyl jasmonate on the antioxidant system were studied in seedlings of drought-resistant (PAN 6043) and drought-sensitive (SC 701) cultivars of Zea mays L. When seedlings treated with the three regulators were subjected to water stress (–1.0 MPa PEG 6000 solution), the activities of superoxide dismutase, catalase and ascorbate peroxidase, as well as the ascorbic acid and total carotenoid contents, increased in the resistant cultivar, whereas the levels remained unaltered in the sensitive cultivar. The increased tolerance to drought stress induced by the growth regulators in the resistant cultivar seems to be due to the maintenance of increased antioxidant enzyme activity and antioxidant substance levels.