Protective mechanism of desiccation tolerance in Reaumuria soongorica: Leaf abscission and sucrose accumulation in the stem

Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub, is widely found in semi-arid areas in northwestern China and can survive severe desiccation of its vegetative organs. In order to study the protective mechanism of desiccation tolerance in R. soongorica, diurnal patterns of net photosynthetic rate (Pn), water use efficiency (WUE) and chlorophyll fluorescence parameters of Photosystem II (PSII), and sugar content in the source leaf and stem were investigated in 6-year-old plants during progressive soil drought imposed by the cessation of watering. The results showed that R. soongorica was characterized by very low leaf water potential, high WUE, photosynthesis and high accumulation of sucrose in the stem and leaf abscission under desiccation. The maximum Pn increased at first and then declined during drought, but intrinsic WUE increased remarkably in the morning with increasing drought stress. The maximal photochemical efficiency of PSII (Fv/Fm) and the quantum efficiency of noncyclic electric transport of PSII(Φ_PSII) decreased significantly under water stress and exhibited an obvious phenomenon of photoinhibition at noon. Drought stressed plants maintained a higher capacity of dissipation of the excitation energy (measured as NPQ) with the increasing intensity of stress. Conditions of progressive drought promoted sucrose and starch accumulation in the stems but not in the leaves. However, when leaf water potential was less than −21.3 MPa, the plant leaves died and then abscised. But the stem photosynthesis remained and, afterward the plants entered the dormant state. Upon rewatering, the shoots reactivated and the plants developed new leaves. Therefore, R. soongorica has the ability to reduce water loss through leaf abscission and maintain the vigor of the stem cells to survive desiccation. Supported by the Program of the Research of Vegetation Restoration in Arid Areas of Lanzhou (Grant No. 03-2-27) and the National Natural Science Foundation of China (Grant No. 30270243)     

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (P _N), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.     

Ecophysiological responses of Caragana korshinskii Kom. under extreme drought stress: Leaf abscission and stem survives

Caragana korshinskii Kom. is a perennial xerophytic shrub, well known for its ability to resist drought. In order to study ecophysiological responses of C. korshinskii under extreme drought stress and subsequent rehydration, diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem II as well as Chl content were analyzed. Plant responses to extreme drought included (1) leaf abscission and using stem for photosynthesis, (2) improved instantaneous water-use efficiency, (3) decreased photosynthetic rate and partly closed stomata owing to leaf abscission and low water status, (4) decreased maximum photochemical efficiency of photosystem II (PSII) (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of noncyclic electron transport of PSII, and Chl a and Chl b. Four days after rehydration, new leaves budded from stems. In the rewatered plants, the chloroplast function was restored, the gas exchange and Chl fluorescence returned to a similar level as control plant. The above result indicated that maintaining an active stem system after leaf abscission during extreme drought stress may be the foundation which engenders these mechanisms rapid regrowth for C. korshinskii in arid environment.     

Antitranspirant functions of stem periderms and their influence on corticular photosynthesis under drought stress

Transpiration and photosynthesis of current-year stems and adult leaves of different deciduous tree species were investigated to estimate their probable influence on carbon balance. Peridermal transpiration of young stems was found to be rather small as compared to the transpiration of leaves (stem/leaf like 1/5–1/20). A characteristic that was mainly attributable to the lower peridermal conductance to water and CO₂, which made up only 8–28% of stomatal conductance. Water vapour conductance was significantly lower in stems, but also non-responsive to PAR, which led to a comparatively higher water use efficiency (WUE, ratio assimilation/transpiration). Thus, although corticular photosynthesis reached only 11–37% of leaf photosynthesis, it may be a means of improving the carbon balance of stems under limited water availability. The influence of drought stress on primary photosynthetic reactions was also studied. Under simulated drought conditions the drying time needed to provoke a 50% reduction (t ₅₀) in dark- and light-adapted PSII efficiency (Fv/Fm, ΔF/Fm′) was up to ten times higher in stems than in leaves. Nevertheless, up to a relative water deficit (RWD) of around 40–50% dark-adapted PSII efficiency of leaves and stems was rather insensitive to dehydration, showing that the efficiency of open PS II reaction centres is not impaired. Thus, it may be concluded that in stems as well as in leaves the primary site of drought damage is at the level of dark enzyme reactions and not within PSII. However, enduring severe drought caused photoinhibitory damage to the photosynthetic apparatus of leaves and stems; thereby RWD₅₀ values (= RWD needed to provoke a 50% reduction in Fv/Fm ad ΔF/Fm′) were comparably lower in stems as compared to leaves, indicating a possibly higher drought sensitivity of the cortex chlorenchyma.     

干旱胁迫下红砂幼苗非结构性碳水化合物动态变化特征

该试验以荒漠区主要建群种红砂幼苗为研究对象,设置适宜水分(CK)、轻度干旱(MD)、中度干旱(SD)和重度干旱(VSD)4个胁迫处理(即田间持水量的80%、60%、40%和20%),采用盆栽控水试验,分别测定干旱胁迫15、30、45和60 d时红砂幼苗的叶、茎、粗根和细根中非结构碳水化合物(NSC)及其组分的含量,分析不同胁迫强度下不同干旱持续时间红砂幼苗NSC的动态变化及各组分差异,以揭示红砂NSC对干旱胁迫的响应机制。结果表明:(1)干旱胁迫强度和胁迫持续时间对红砂幼苗不同器官NSC及其组分均有显著影响,其中胁迫持续时间对NSC动态变化的影响尤为显著。(2)干旱胁迫初期,红砂叶中的NSC含量呈下降趋势,而茎中的NSC含量呈上升趋势,粗根和细根中NSC含量在各胁迫处理下基本保持稳定。(3)干旱胁迫后期,红砂叶和茎中的可溶性糖、淀粉和NSC含量逐渐增加,而粗根和细根中的淀粉和NSC含量呈下降趋势(中度干旱除外),且这一时期重度干旱处理下各器官可溶性糖和NSC的含量明显高于CK。研究发现,重度干旱胁迫能显著诱导提高红砂幼苗不同器官中的NSC含量,并通过分解根中淀粉和增加叶片中可溶性糖含量的方式来调节细胞渗透势平衡,以维持细胞活力,进而保持红砂在干旱胁迫后期的存活。     

土壤水分对湿地松幼苗光合特征的影响

通过设置常规供水(CK)、轻度干旱(T1)、水饱和(T2)、水淹(T3)4个处理组,研究湿地松当年实生幼苗在不同土壤水分条件下的光合生理响应及叶绿素荧光特性。结果表明:不同水分处理对湿地松幼苗的叶片气体交换参数、水分利用效率(WUE)、光合色素、叶绿素荧光参数等指标有不同的影响;其中,T3的光合色素含量最低;T2、T3组的湿地松幼苗表现出较低的净光合速率(Pn)、电子传递速率(ETR)和PSⅡ光化学的量子效应(ΦPSⅡ),但与其他耐水淹植物相比,T3条件下的湿地松幼苗仍具有较高的Pn,说明湿地松幼苗具有较强的耐水淹能力;在T1条件下,湿地松幼苗具有较高的WUE和较低的蒸腾速率(Tr)、气孔导度(Gs)以对抗干旱的逆境,其Pn、PSⅡ最大光化学效率(Fv/Fm)、ETR和ΦPSII均有所下降,但仍能维持在相对正常的水平。研究证明,湿地松幼苗具有一定的耐淹耐旱特性,可运用于三峡库区消落带的植被重建。     

Shade mitigates photoinhibition and enhances water use efficiency in kiwifruit under drought

We tested the hypotheses that a reduction of incident light of 50 % over sun-acclimated leaves of water stressed kiwifruit (Actinidia deliciosa var. deliciosa) would (1) reduce stomatal limitations to carbon supply and (2) mitigate the inactivation of the primary photochemistry associated with photosystem (PS) II, thereby this increases carbon gain and water-use efficiency (WUE). Groups of field-grown vines were either shaded or left naturally exposed and subjected to progressive water stress in order to study moderately and severely droughted vines, while other groups were well irrigated. Daily variations in leaf gas exchange and midday chlorophyll (Chl) a fluorescence were determined once plants had −0.6 MPa (moderate stress) and −1.0 MPa pre-down leaf water potential (severe stress). Variations in Chl pigment content and specific leaf area (SLA) are also discussed. Results reveal that 50 % shade application maintained efficiency of PSII close to 0.8 even under severe drought so that to prevent its large decline (0.65) recorded in sunlit leaves. Under moderate stress level stomata behaviour dominated upon metabolic impairments of PSII. Reduction of irradiance increased WUE (15–20 %) in droughted vines, representing a valuable intervention to save photosynthetic apparatus and improve WUE in vines experiencing typical Mediterranean summer stresses.     

Photosynthetic limitations and volatile and non‐volatile isoprenoids in the poikilochlorophyllous resurrection plant Xerophyta humilis during dehydration and rehydration

We investigated the photosynthetic limitations occurring during dehydration and rehydration of Xerophyta humilis, a poikilochlorophyllous resurrection plant, and whether volatile and non‐volatile isoprenoids might be involved in desiccation tolerance. Photosynthesis declined rapidly after dehydration below 85% relative water content (RWC). Raising intercellular CO₂ concentrations during desiccation suggest that the main photosynthetic limitation was photochemical, affecting energy‐dependent RuBP regeneration. Imaging fluorescence confirmed that both the number of photosystem II (PSII) functional reaction centres and their efficiency were impaired under progressive dehydration, and revealed the occurrence of heterogeneous photosynthesis during desiccation, being the basal leaf area more resistant to the stress. Full recovery in photosynthetic parameters occurred on rehydration, confirming that photosynthetic limitations were fully reversible and that no permanent damage occurred. During desiccation, zeaxanthin and lutein increased only when photosynthesis had ceased, implying that these isoprenoids do not directly scavenge reactive oxygen species, but rather protect photosynthetic membranes from damage and consequent denaturation. X. humilis was found to emit isoprene, a volatile isoprenoid that acts as a membrane strengthener in plants. Isoprene emission was stimulated by drought and peaked at 80% RWC. We surmise that isoprene and non‐volatile isoprenoids cooperate in reducing membrane damage in X. humilis, isoprene being effective when desiccation is moderate while non‐volatile isoprenoids operate when water deficit is more extreme.     

铝胁迫对蓼科植物生长和光合、蒸腾特性的影响

采用水培试验,设置5种铝处理浓度,研究了铝对3种蓼科植物酸模叶蓼、杠板归和辣蓼叶片光合、蒸腾和叶绿素荧光参数的影响。结果表明,高铝处理(400μmol.L-1)显著抑制3种蓼科植物地上部和根系生长,并且导致3种蓼科植物叶片叶绿素含量、Chla/Chlb、净光合速率(Pn)、水分利用效率(WUE)、PSII光合电子传递量子效率(φPSII)和光化学猝灭系数(qP)显著下降。中低铝处理(25~100μmol.L-1)时,与对照相比,酸模叶蓼生物量显著增加,杠板归显著减少,辣蓼先增加后减少。其中,酸模叶蓼和辣蓼叶绿素含量、Chla/Chlb、Pn、蒸腾速率(Tr)、胞间CO2浓度(Ci)、PSII最大光化学效率(Fv/Fm)、qP均未发生显著变化,但辣蓼WUE、φPSII和非光化学猝灭系数(NPQ)显著下降,酸模叶蓼无显著变化;而杠板归除Ci、Fv/Fm外,其余叶片光合、蒸腾及叶绿素荧光参数均出现显著下降。上述结果表明,酸模叶蓼在中低铝处理条件下可通过保持较高的叶绿素含量、Chla/Chlb、WUE、Pn、PSII反应中心光化学反应效率以及提高非辐射能量耗散来增强其对铝的耐性。     

Water use efficiency as a function of leaf age and position within the cotton canopy

The gas exchange properties of whole plant canopies are an integral part of crop productivity and have attracted much attention in recent years. However, insufficient information exists on the coordination of transpiration and CO₂ uptake for individual leaves during the growing season. Single-leaf determinations of net photosynthesis (Pn), transpiration (E) and water use efficiency (WUE) for field-grown cotton (Gossypium hirsutum L.) leaves were recorded during a 2-year field study. Measurements were made at 3 to 4 day intervals on the main-stem and first three sympodial leaves at main-stem node 10 from their unfolding through senescence. Results indicated that all gas exchange parameters changed with individual main-stem and sympodial leaf age. Values of Pn, E and WUE followed a rise and fall pattern with maximum rates achieved at a leaf age of 18 to 20 days. While no significant position effects were observed for Pn, main-stem and sympodial leaves did differ in E and WUE particularly as leaves aged beyond 40 days. For a given leaf age, the main-stem leaf had a significantly lower WUE than the three sympodial leaves. WUE's for the main-stem and three sympodial leaves between the ages of 41 to 50 days were 0.85, 1.30, 1.36 and 1.95 μmol CO₂ mmol⁻¹ H₂O, respectively. The mechanisms which mediated leaf positional differences for WUE were not strictly related to changes in stomatal conductance (g_s·H₂O) since decreases in g_s·H₂O with leaf age were similar for the four leaves. However, significantly different radiant environments with distance along the fruiting branch did indicate the possible involvement of mutual leaf shading in determining WUE. The significance of these findings are presented in relation to light competition within the plant canopy during development.     

五种高速公路边坡绿化植物的生理特性及抗旱性综合评价

采用盆栽控制土壤水分,测定不同程度水分胁迫梯度下5种高速公路边坡绿化植物的叶水势、光合作用参数及叶绿素荧光参数,并以水分胁迫下各生理指标的平均变化速率为原始数据进行抗旱性综合评价。结果表明:(1)随着水分胁迫的加强,5种植物的ΨL、Pn、Tr、GS、Fv、Fm、Fv/Fm、Fv/Fo均逐渐下降,Fo逐渐升高,WUE先升后降,Ci先降后升,不同生理指标的变化幅度及拐点有一定差异;平均变化速率相对较小的是八宝景天和马蔺,较大的是肥皂草和太行菊;(2)根据水分胁迫下Gs和Ci变化方向的差异判断八宝景天、马蔺以W2作为Pn降低由气孔限制转为非气孔因素的分界线,五叶地锦、肥皂草和太行菊以W1为分界线,八宝景天、马蔺在W2时WUE最高,五叶地锦、肥皂草和太行菊在W1时最高;(3)5种植物抗旱性大小排序为:八宝景天>马蔺>五叶地锦>太行菊>肥皂草,聚类分析结果为:八宝景天、马蔺为强抗旱植物,五叶地锦为中抗旱植物,太行菊、肥皂草为弱抗旱植物。     

Interactive effects of photon fluence rates and drought on CAM‐cycling in Delosperma tradescantioides (Mesembryanthemaceae)

The ability of photosynthesis and CAM to acclimate to low (220 µmol m^?2 s^?1; LL) and relatively high (550 µmol m^?2 s^?1; HL) photosynthetic photon flux densities (PPFD) was investigated in the CAM-cycling species Delosperma tradescantioides by means of CO₂ gas exchange and chlorophyll fluorescence analysis. Furthermore, the influence of short-term drought on malic acid accumulation and the activity of photosystem II (PSII) was studied to assess the possible interactions between drought and the prevailing PPFD in this species. HL plants showed features of sun versus shade acclimation relative to LL plants. Nocturnal malic acid accumulation (Δ-malate) and leaf water content also tended to be higher in HL plants. Irrespective of the PPFD during growth, the weak Δ-malate doubled within 3 days of drought. Despite largely restricted CO₂ uptake, photosynthetic activity as estimated from fluorescence analysis declined only ca 5%. After 7 days of drought, when plants showed CAM-idling and Δ-malate had decreased again, potential carbon assimilation was still ca 84% of that in well-watered plants and remained relatively constant throughout the day. Decarboxylation of malic acid accounted for ca 23% of potential assimilation assuming total oxidation of a maximum portion of this organic acid. Drought did not affect predawn maximum photochemical efficiency (F_v/F_m). Nonphotochemical quenching (qN) increased (24%) in response to desiccation and resulted in a more or less constant reduction state of PSII. This increase in qN resulted mainly from the change in its fast-relaxing component (qNF), while the slow component (qNS) was significant only at or above saturating PPFD in both HL and LL plants. The photon response characteristics of PSII, which differed between LL and HL plants, were unaffected by short-term drought. Photon harvesting and photon use were always adjusted to guarantee a low reduction state of PSII. Results suggest that in both LL and HL plants CAM-cycling may help to stabilize photosynthesis but to a large extent by other means than simply providing internally derived CO₂.     

Physiological and anatomical changes induced by drought in two olive cultivars (cv Zalmati and Chemlali)

Photosynthetic gas exchange, vegetative growth, water relations and fluorescence parameters as well as leaf anatomical characteristics were investigated on young plants of two Olea europaea L. cultivars (Chemlali and Zalmati), submitted to contrasting water availability regimes. Two-year-old olive trees, grown in pots in greenhouse, were not watered for 2 months. Relative growth rate (RGR), leaf water potential (Ψ_LW) and the leaf relative water content (LWC) of the two cultivars decreased with increasing water stress. Zalmati showed higher values of RGR and LWC and lower decreased values of Ψ_LW than Chemlali, in response to water deficit, particularly during severe drought stress. Water stress also caused a marked decline on photosynthetic capacity and chlorophyll fluorescence. The net photosynthetic rate, stomatal conductance, transpiration rate, the maximal photochemical efficiency of PSII (F _v/F _m) and the intrinsic efficiency of open PSII reaction centres (F′ _v/F′ _m) decreased as drought stress developed. In addition, drought conditions, reduced leaf chlorophyll and carotenoids contents especially at severe water stress. However, Zalmati plants were the less affected when compared with Chemlali. In both cultivars, stomatal control was the major factor affecting photosynthesis under moderate drought stress. At severe drought-stress levels, the non-stomatal component of photosynthesis is inhibited and inactivation of the photosystem II occurs. Leaf anatomical parameters show that drought stress resulted in an increase of the upper epidermis and palisade mesophyll thickness as well as an increase of the stomata and trichomes density. These changes were more characteristic in cv. ‘Zalmati’. Zalmati leaves also revealed lower specific leaf area and had higher density of foliar tissue. From the behaviour of Zalmati plants, with a smaller reduction in relative growth rate, net assimilation rate and chlorophyll fluorescence parameters, and with a thicker palisade parenchyma, and a higher stomatal and trichome density, we consider this cultivar more drought-tolerant than cv. Chemlali and therefore, very promising for cultivation in arid areas.     

砂藓RcPLD基因的抗旱功能分析

砂藓(Racomitrium canescens)是一种具有极强耐脱水性的苔藓植物,编码磷脂酶D的基因RcPLD能够在砂藓的脱水和复水过程中产生显著的表达响应,它可能参与了砂藓的强耐脱水性功能。该研究使用已克隆的RcPLD编码序列构建拟南芥(Arabidopsis thaliana)过量表达转基因株系rcpld-oe,初步考察过表达株系的干旱胁迫耐受能力及其相关的生理生化指标,分析RcPLD增强拟南芥抗旱性的机制。结果表明:(1)利用已克隆的RcPLD编码序列构建了植物中的过表达载体,成功构建了RcPLD的过表达转基因拟南芥株系rcpld-oe,并获得了多个T_3代rcpld-oe纯合体株系。(2)在正常生长条件下,rcpld-oe株系T_3代纯合体植株比野生型拟南芥植株体积小,但营养生长期较长,抽薹较晚,莲座叶衰老速率较慢;在干旱处理条件下,rcpld-oe株系表现出比野生型拟南芥更强的干旱耐受能力。(3)在干旱胁迫处理过程中,rcpld-oe株系莲座叶的水分散失速率降低,可能在一定程度上降低了干旱对膜完整性的损伤和光合作用的抑制,但其渗透调节物质含量的变化相对较小。研究发现,在干旱胁迫条件下,rcpld-oe植株莲座叶的水分散失速率和光合作用抑制程度显著降低,从而表现出明显强于野生型的干旱耐受能力,这为后续RcPLD功能的深入研究和更多砂藓抗旱功能基因的挖掘奠定了基础。     

Physiological markers of stress susceptibility in maize and triticale under different soil compactions and/or soil water contents

Differences between two maize and two triticale genotypes grown in low soil compaction (LSC), moderate soil compaction (MSC) and severe soil compaction (SSC) and with a limited (D) or excess (W) soil water content were observed as a decrease in shoot (S) and root (R) biomass, leaf greening (SPAD) and increase in membrane injury (LI), root and leaf water potential (ψ), photosynthesis (Pn), transpiration (E) and stomata conductance (g_S). Close correlations between ψ_L and ψ_R, and between differences ψ_L and ψ_R (Δψ) were found. Drought or waterlogging with LSC conditions in both maize genotypes resulted in higher WUE than in control plants (LSC C), but under the SSC WUE declined. However, for triticale differences in WUE, between treatments were small and insignificant. In general, changes in markers were greater for genotypes sensitive to the soil compaction (Ankora, CHD-12) than in resistant ones (Tina, CHD-247) and were higher in seedlings grown under SSC conditions.

Abbreviations: ψ_R, ψ_L: root and leaf water potential; C: control; D: drought; E: transpiration rate; FWC: field water capacity; g_S: stomatal conductance; LSC, MSC, SSC: low, moderate and severe soil compaction; P_n: photosynthesis rate; W: waterlogging     

Photosynthetic activity of homoiochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> during dehydration and rehydration

The functional state of the photosynthetic apparatus of flowering homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and subsequent rehydration was investigated in order to characterize some of the mechanisms by which resurrection plants survive drought stress. The changes in the CO₂ assimilation rate, chlorophyll fluorescence parameters, thermoluminescence, fluorescence imaging and electrophoretic characteristics of the chloroplast proteins were measured in control, moderately dehydrated (50% water content), desiccated (5% water content) and rehydrated plants. During the first phase of desiccation the net CO₂ assimilation decline was influenced by stomatal closure. Further lowering of net CO₂ assimilation was caused by both the decrease in stomatal conductance and in the photochemical activity of photosystem II. Severe dehydration caused inhibition of quantum yield of PSII electron transport, disappearance of thermoluminescence B band and mainly charge recombination related to S₂Q_A⁻ takes place. The blue and green fluorescence emission in desiccated leaves strongly increased. It could be suggested that unchanged chlorophyll content and amounts of chlorophyll–proteins, reversible modifications in PSII electron transport and enhanced probability for non-radiative energy dissipation as well as increased polyphenolic synthesis during desiccation of Haberlea contribute to drought resistance and fast recovery after rehydration.     

Desiccation of the resurrection plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> at high temperature

Haberlea rhodopensis plants, growing under low irradiance in their natural habitat, were desiccated to air-dry state at a similar light intensity (about 30 μmol m⁻² s⁻¹) under optimal (23/20°C, day/night) or high (38/30°C) temperature. Dehydration of plants at high temperature increased the rate of water loss threefold and had a more detrimental effect than either drought or high temperature alone. Water deficit decreased the photochemical activity of PSII and PSI and the rate of photosynthetic oxygen evolution, and these effects were stronger when desiccation was carried out at 38°C. Some reduction in the amount of the main PSI and PSII proteins was observed especially in severely desiccated Haberlea leaves. The results clearly showed that desiccation of the homoiochlorophyllous poikilohydric plant Haberlea rhodopensis at high temperature had more damaging effects than desiccation at optimal temperature and in addition recovery was slower. Increased thermal energy dissipation together with higher proline and carotenoid content in the course of desiccation at 38°C compared to desiccation at 23°C probably helped in overcoming the stress.     

Inflorescences vs leaves: a distinct modulation of carbon metabolism process during Botrytis infection

Plant growth and survival depends critically on photo assimilates. Pathogen infection leads to changes in carbohydrate metabolism of plants. In this study, we monitored changes in the carbohydrate metabolism in the grapevine inflorescence and leaves using Botrytis cinerea and Botrytis pseudo cinerea. Fluctuations in gas exchange were correlated with variations in chlorophyll a fluorescence. During infection, the inflorescences showed an increase in net photosynthesis (Pn) with a stomatal limitation. In leaves, photosynthesis decreased, with a non‐stomatal limitation. A decrease in the effective photosystem II (PSII) quantum yield (ΦPSII) was accompanied by an increase in photochemical quenching (qP) and non‐photochemical quenching (qN). The enhancement of qP and ΦPSII could explain the observed increase in Pn. In leaves, the significant decline in ΦPSII and qP, and increase in qN suggest that energy was mostly oriented toward heat dissipation instead of CO₂ fixation. The accumulation of glucose and sucrose in inflorescences and glucose and fructose in the leaves during infection indicate that the plant's carbon metabolism is differently regulated in these two organs. While a strong accumulation of starch was observed at 24 and 48 hours post‐inoculation (hpi) with both species of Botrytis in the inflorescences, a significant decrease with B. cinerea at 24 hpi and a significant increase with B. pseudo cinerea at 48 hpi were observed in the leaves. On the basis of these results, it can be said that during pathogen attack, the metabolism of grapevine inflorescence and leaf is modified suggesting distinct mechanisms modifying gas exchange, PSII activity and sugar contents in these two organs.     

Endogenous Control of Photosynthetic Activity during Progressive Drought: Influence of Final Products of Photosynthesis

Photosynthetic activities and the redox states of photosystem I (PSI) and photosystem II (PSII) in intact leaves of cucumber plants (Cucumis sativus L.), as well as the sucrose and starch contents were examined under conditions of ongoing soil water deficit imposed by the cessation of watering. As the soil drought progressed, the maximum rate of photosynthetic CO₂ fixation was shown to decrease. These changes in the maximum photosynthetic rate occurred synchronously with changes in the maximum quantum yield of photosynthesis. Under soil water deficit, the reduced form of PSII primary acceptor Q _A was accumulated only at photon flux densities of about 100 mol/(m² s). At such photon flux densities, the changes in nonphotochemical quenching (qN) induced by soil water deficit were opposite to changes in photochemical quenching parameter (1 – qP). Irrespective of the duration of soil drought, the relationship between steady-state concentrations of photochemically inactive reaction centers of PSI and PSII (the fractions of P700 and Q _A in the oxidized and reduced state, respectively) was almost linear, which provides evidence for the concerted operation of both photosystems. The conditions of soil water deficit promoted sucrose accumulation in the source leaf, which was paralleled by a substantial decrease in the amount of starch in the same leaf. The highest content of sucrose in the leaf after a 7-day drought was correlated with the largest decrease in photosynthetic activity. It is concluded that the progressive drought triggers an endogenous mechanism that regulates photosynthesis through feedback relations, namely, the inhibition of photosynthesis by its end products.     

干旱胁迫对夏蜡梅光合特性的影响

以2年生夏蜡梅(Sinocalycanthus chinensis)苗木为材料,通过盆栽试验,研究土壤干旱胁迫对夏蜡梅叶片光合特性的影响,结果显示:随着干旱胁迫程度的加重,夏蜡梅叶片净光合速率、蒸腾速率、气孔导度显著降低;胞间CO2浓度在轻度和中度胁迫下显著低于对照,但在重度胁迫下显著高于对照;水分利用效率在胁迫下提高,且以中度胁迫为最大.温度升高使叶片净光合速率和水分利用效率降低、蒸腾速率升高,加重了干旱对光合作用的不利影响.对照及轻度和中度干旱胁迫处理的净光合速率、气孔导度和水分利用效率日变化曲线均为双峰型,但重度胁迫下净光合速率和气孔导度日变化转变为峰值很小的单峰型;各处理的蒸腾速率日变化曲线为单峰型并以午间最高.表明夏蜡梅光合作用对干旱胁迫有一定的适应能力,但重度胁迫对其造成严重影响.