Plasma membrane lipids in the resurrection plant Ramonda serbica following dehydration and rehydration

Plants of Ramonda serbica were dehydrated to 3.6% relative water content (RWC) by withholding water for 3 weeks, afterwards the plants were rehydrated for 1 week to 93.8% RWC. Plasma membranes were isolated from leaves using a two-phase aqueous polymer partition system. Compared with well-hydrated (control) leaves, dehydrated leaves suffered a reduction of about 75% in their plasma membrane lipid content, which returned to the control level following rewatering. Also the lipid to protein ratio decreased after dehydration, almost regaining the initial value after rehydration. Lipids extracted from the plasma membrane of fully-hydrated leaves were characterized by a high level of free sterols and a much lower level of phospholipids. Smaller amounts of cerebrosides, acylated steryl glycosides and steryl glycosides were also detected. The main phospholipids of control leaves were phosphatidylcholine and phosphatidylethanolamine, whereas sitosterol was the free sterol present in the highest amount. Following dehydration, leaf plasma membrane lipids showed a constant level of free sterols and a reduction in phospholipids compared with the well-hydrated leaves. Both phosphatidylcholine and phosphatidylethanolamine decreased following dehydration, their molar ratio remaining unchanged. Among free sterols, the remarkably high cholesterol level present in the control leaves (about 14 mol%) increased 2-fold as a result of dehydration. Dehydration caused a general decrease in the unsaturation level of individual phospholipids and total lipids as well. Upon rehydration the lipid composition of leaf plasma membranes restored very quickly approaching the levels of well-hydrated leaves.     

Chlorophyll fluorescence imaging of photosynthetic activity and pigment contents of the resurrection plants Ramonda serbica and Ramonda nathaliae during dehydration and rehydration

The desiccation-tolerant plants of the R. serbica and R. nathaliae are resurrection plants which are able to fully recover their physiological function after anabiosis. A comparison of chlorophyll fluorescence imaging and photosynthetic pigment contents responses of R. serbica and, for the first time, R. nathaliae to dehydration and rehydration were investigated. For this purpose, plants after collection from their natural habitats were kept fully watered for 14 days at natural condition. The experiment was conducted with mature leaves of both species. R. serbica and R. nathaliae plants were dehydrated to 5.88 % and 7.87 % relative water content (RWC) by withholding water for 15 days, afterwards the plants were rehydrated for 72 hours to 94.67 % and 97.02 % RWC. During desiccation, R. serbica plants preserved the chlorophyll content about 84 %, while R. nathaliae about 90 %. During dehydration when RWC were more than 40 %, photochemical efficiency of PSII for photochemistry, the Fv/Fm ratio, decreased about 40 % in R. nathaliae plants, but a strong reduction with 60 % was recorded for R. serbica. Following rehydration, the Fv/Fm ratio recovered more rapidly in R. nathaliae. The higher photosynthetic rates could also be detected via imaging the chlorophyll fluorescence decrease ratio Rfd, which possessed higher values after rehydration leaves of R. nathaliae as compared to R. serbica. The results showed that the photosynthetic activity and chlorophyll contents after rehydration are recovered more rapidly in R. nathaliae in comparison to R. serbica.     

Response of photosynthesis and respiration of resurrection plants to desiccation and rehydration

Using non-invasive techniques (CO₂ gas exchange, light scattering, light absorption, chlorophyll fluorescence, chlorophyll luminescence), we have analysed the response of respiration and photosynthesis to dehydration and rehydration of leaves of the resurrection plants Craterostigma plantagineum Hochst., Ramonda mykoni Reichb. and Ceterach officinarum Lam. et DC. and of the drought-sensitive mesophyte spinach (Spinacia oleracea L.). The following observations were made: (i) The rate of water loss during wilting of detached leaves of drought-tolerant resurrection plants was similar to that for leaves of the sensitive mesophyte, spinach. Leaves of Mediterranean xerophytes lost water much more slowly. (ii) Below a residual water content of about 20%, leaves of spinach did not recover turgor on rewatering, whereas leaves of the resurrection plants did. (iii) Respiration was less sensitive to the loss of water during wilting in the resurrection plants than in spinach. (iv) The sensitivity of photosynthesis to dehydration was similar in spinach and the resurrection plants. Up to a water loss of 50% from the leaves, photosynthesis was limited by stomatal closure, not by inhibition of reactions of the photosynthetic apparatus. Photosynthesis was inhibited and stomates reopened when loss of water became excessive. (v) After the leaves had lost 80% of their water or more, the light-dependent reactions of photosynthetic membranes were further inhibited by rewatering in spinach; they recovered in the resurrection plants. (vi) In desiccated leaves of the resurrection plants, slow rehydration reactivated mitochondrial gas exchange faster than photosynthetic membrane reactions. Photosynthetic carbon assimilation recovered only slowly.     

Role of phenolics in the antioxidative status of the resurrection plant Ramonda serbica during dehydration and rehydration

Ramonda serbica plants dehydrated for 14 days reached a relative water content of 4.2% and entered into anabiosis prior to being rehydrated for 48 h. Total ascorbate (AsA + DHA) and glutathione (GSH + GSSG) contents increased during dehydration and approached control values by the end of rehydration. Reduced ascorbate (AsA) and glutathione (GSH) were consumed during the first 13 days of dehydration when guaiacol-, syringaldazine- and phenolic peroxidases (EC 1.11.1.7) increased. At the end of dehydration AsA and GSH accumulated whereas peroxidases decreased to half the value of controls. In this period, plants of R. serbica face a phase of reduced metabolism and, thus, of reduced consumption of antioxidants. During rehydration, both AsA and GSH were utilized reaching, after 48 h, about 20 and 40% of their total pools, respectively; moreover peroxidases increased showing the recovery of metabolic activities. In the dehydration process total phenolic acids decreased, but accumulated after 5 h of rehydration and returned to control values at the end of rehydration. In R. serbica leaves, the most representative phenolic acids were protocatechuic, p -hydroxybenzoic and chlorogenic acids. Most concentrated phenolic acids, such as protocatechuic and chlorogenic acids, accumulated during the first period of rehydration when AsA decreased. These results suggest a role of ascorbate in inhibiting oxidation when phenolic peroxidases remain at low levels. As a consequence of this inhibition, ascorbate was oxidized and when most of it was consumed, oxidation of phenols resumed.     

Unusual composition of thylakoid membranes of the resurrection plant Boea hygroscopica: Changes in lipids upon dehydration and rehydration

Boea hygroscopica is a resurrection plant that is able to pass from biosis to anabiosis and vice versa following slow dehydration, but loses this ability following a rapid water loss. Fresh leaves were detached from plants grown in well-watered conditions and subjected to either rapid or slow dehydration and rehydration. Upon rehydration only slowly dried leaves revived. Analysis of thylakoid membranes revealed a rather small amount of total lipids (1,4–2 μmol g^?1 dry weight) in comparison with other flowering plants. The main glycolipid was digalactosyldiacylglycerol (DGDG) rather than monogalactosyldiacylglycerol (MGDG) as is common in higher plants. Linoleic acid was the main fatty acid (30–40 mol% of total fatty acids), while linolenic acid was present from 14 to 26 mol%. In both the fresh and rehydrated leaves nearly all lipid components were present in similar amounts. Following dehydration the DGDG/MGDG molar ratio, which was 1.1 in control and rehydrated leaves, doubled by the end of the rapid drying period and was three times as high in slowly dried leaves. The total polar lipid/free sterol molar ratio as well as the free fatty acid level assumed the highest values in the rapidly dehydrated leaves. A shift towards the more unsaturated fatty acids was observed in all lipid classes upon dehydration irrespective of whether it was slow or rapid. Our data show only small differences between rapidly and slowly dehydrated leaves which can be correlated to the capacity of slowly dehydrated leaves to revive.     

The effect of nitrogen and sulphur on changes in nitrogen fractions of barley plants at various early stages of growth and on yield and amino acid composition of grain

Summary In a pot experiment the effects were estimated of varying nitrogen and sulphur applications on the total and nitrate nitrogen, free amino acid and amide and total amino acid concentrations of barley plants at five early stages of growth.Variations with time in contents of total and nitrate nitrogen differed at low and high nitrogen levels. Concentrations of free amino acids and amides were influenced by light intensity preceding sampling, age of plant and by levels of nitrogen and sulphur. At the fifth harvest (55 days old plants), high nitrogen addition without sulphur caused a fivefold increase in asparagine content and lesser increases in glycine and serine. Alterations in total amino acid contents with time were largely dependent on variations in total nitrogen. Sulphur deficiency, as judged by contents of S-amino acids and amides, became more pronounced with time. A positive interaction was found between effects of nitrogen and sulphur on the yield and cystine and methionine content of grain. Absence of sulphur reduced the cystine content of grain much more than that of methionine, particularly at the high nitrogen level.     

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.     

Desiccation and rehydration dynamics in the epiphytic resurrection fern Pleopeltis polypodioides

The epiphytic resurrection—or desiccation-tolerant (DT)—fern Pleopeltis polypodioides can survive extreme desiccation and recover physiological activity within hours of rehydration. Yet, how epiphytic DT ferns coordinate between deterioration and recovery of their hydraulic and photosynthetic systems remains poorly understood. We examined the functional status of the leaf vascular system, chlorophyll fluorescence, and photosynthetic rate during desiccation and rehydration of P. polypodioides. Xylem tracheids in the stipe embolized within 3–4 h during dehydration. When the leaf and rhizome received water, tracheids refilled after ∼24 h, which occurred along with dramatic structural changes in the stele. Photosynthetic rate and chlorophyll fluorescence recovered to predesiccation values within 12 h of rehydration, regardless of whether fronds were connected to their rhizome. Our data show that the epiphytic DT fern P. polypodioides can utilize foliar water uptake to rehydrate the leaf mesophyll and recover photosynthesis despite a broken hydraulic connection to the rhizome.     

Effects of dehydration and rehydration on the intravascular space in horses

• 1.1. The resistance of sub-tropical horses, and desert-dwelling horses to 72 hr dehydration/24 hr rehydration was investigated via changes in red cell parameters and plasma protein concentration.
• 2.2. Red cell count, haemoglobin and haematocrit increased up to 48 hr dehydration. Between 48 and 72 hr dehydration these parameters decreased, implying a fluid shift onto the intravascular space from the interstitium/hindgut. Most parameters had regained baseline values by 24 hr rehydration.
• 3.3. Mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration and total plasma protein were not significantly different between breeds at, or between most stages of hydration.
• 4.4. Protection of plasma volume during dehydration/rehydration was aided by maintaining intravascular protein (especially albumin) levels. Red cells were transiently dehydrated and overhydrated but resisted osmolysis.

     

Proteome analysis of leaves from the resurrection plant <Emphasis Type="Italic">Boea hygrometrica</Emphasis> in response to dehydration and rehydration

Resurrection plants differ from other species in their unique ability to survive desiccation. In order to understand the mechanisms of desiccation tolerance, proteome studies were carried out using leaves of the resurrection plant Boea hygrometrica to reveal proteins that were differentially expressed in response to changes in relative water content. This opportunity was afforded by the rare ability of excised B. hygrometrica leaves to survive and resume metabolism following desiccation in a manner similar to intact plants. From a total of 223 proteins that were reproducibly detected and analyzed, 35% showed increased abundance in dehydrated leaves, 5% were induced in rehydrated leaves and 60% showed decreased or unchanged abundance in dehydrated and rehydrated leaves. Since the induction kinetics fall into clearly defined patterns, we suggest that programmed regulation of protein expression triggered by changes of water status. Fourteen dehydration responsive proteins were analyzed by mass spectrometry. Eight proteins were classified as playing a role in reactive oxygen species scavenging, photosynthesis and energy metabolism. In agreement with these findings, glutathione content and polyphenol oxidase activity were found to increase upon dehydration and rapid recovery of photosynthesis was observed.     

The effect of nitrogen nutrition on growth and biomass partitioning of annual plants originating from habitats of different nitrogen availability

Summary The hypothesis was tested that faster growth of nitrophilic plants at high nitrogen (N) nutrition is counterbalanced by faster growth of non-nitrophilic plants at low N-nutrition. Ten annual plant species were used which originated from habitats of different N-availability. The species' preference for N was quantified by the N-number of Ellenberg (1979), a relative measure of nitrophily. The plants were cultivated in a growth cabinet at five levels of ammonium-nitrate supply. At low N-supply, the relative growth rate (RGR) was independent of nitrophily. At high N-supply, RGR tended to be higher in nitrophilic than in non-nitrophilic species. However, the response of RGR to N-supply was strongly and positively correlated with the nitrophily of species. Increasing N-supply enhanced partitioning to leaf weight per total biomass (LWR) and increased plant leaf area per total biomass (LAR). Specific leaf weight (SLW) and LWR were both higher in non-nitrophilic than in nitrophilic species at all levels of N-nutrition. NAR (growth per leaf area or net assimilation rate) increased with nitrophily only under conditions of high N-supply. RGR correlated positively with LAR, irrespective of N-nutrition. Under conditions of high N-supply RGR correlated with SLW negatively and with NAR positively.     

大花旋蒴苣苔对脱水与复水的生理响应

本试验以陕西、四川(下称西部)、安徽和浙江(下称东部)4省区大花旋蒴苣苔(Boea clarkeana Hemsl.)的离体叶片为材料,通过对不同脱水时间(0、4、8、12、24和48 h)和脱水48 h后不同复水时间(4、8、12、24和48 h)条件下大花旋蒴苣苔生理指标变化的研究,比较了中国东部和西部地区大花旋蒴苣苔的耐脱水特性。结果表明:4种群大花旋蒴苣苔的相对含水量在脱水过程中呈显著下降趋势(P0.05);相对含水量下降的总体趋势表现为浙江种群安徽种群四川种群陕西种群;在复水过程中,4种群相对含水量基本恢复到实验前水平;超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CAT)3种抗氧化酶活性以及MDA、脯氨酸、可溶性糖和可溶性蛋白的含量在脱水过程中均表呈显著上升趋势(P0.05),在复水过程中均呈显著下降趋势(P0.05)。通过对4种群生理指标的比较研究发现,4种群的耐脱水性整体表现为陕西种群四川种群安徽种群浙江种群,即西部种群的耐旱性强于东部地区。     

Ultrastructural responses of the desiccation tolerant plants Xerophyta viscosa and X. retinervis to dehydration and rehydration

This paper compares the changes in water content, chlorophyll a fluorescence and leaf ultrastructure during dehydration and rehydration in two desiccation tolerant plants Xerophyta viscosa and X. retinervis. Both species showed decreasing quantum efficiency of photosystem 2 (F_v/F_m) with decreasing water content. Extreme water loss observed after 25 d of dehydration resulted in considerable damage of leaf tissue ultrastructure. After rehydration, both species need several days to reconstitute their photosynthetic machinery.