The impact of dehydration rate on the production and cellular location of reactive oxygen species in an aquatic moss

Background and Aims

The aquatic moss Fontinalis antipyretica requires a slow rate of dehydration to survive a desiccation event. The present work examined whether differences in the dehydration rate resulted in corresponding differences in the production of reactive oxygen species (ROS) and therefore in the amount of cell damage.

Methods

Intracellular ROS production by the aquatic moss was assessed with confocal laser microscopy and the ROS-specific chemical probe 2,7-dichlorodihydrofluorescein diacetate. The production of hydrogen peroxide was also quantified and its cellular location was assessed.

Key Results

The rehydration of slowly dried cells was associated with lower ROS production, thereby reducing the amount of cellular damage and increasing cell survival. A high oxygen consumption burst accompanied the initial stages of rehydration, perhaps due to the burst of ROS production.

Conclusions

A slow dehydration rate may induce cell protection mechanisms that serve to limit ROS production and reduce the oxidative burst, decreasing the number of damaged and dead cells due upon rehydration.     

快速脱水与复水过程中2种藓类植物的活性氧代谢与脂质过氧化特征

以来自不同水分生境的金发藓和湿地匐灯藓为材料,对二者在脱水与复水胁迫条件下的活性氧代谢、脂质过氧化损伤程度及其抗氧化系统应答的差异进行比较研究。结果显示:在脱水与复水过程中,(1)硅胶快速脱水更接近阳光直射条件下藓类植物的水分丧失。(2)随着含水量的变化,湿地匐灯藓虽然能够在复水后迅速修复细胞的完整性,但变化剧烈;金发藓则能够始终维持较低的膜透性。(3)2种藓类植物的丙二醛(MDA)含量变化均呈先升后降趋势,但金发藓的MDA含量明显低于湿地匐灯藓。(4)2种藓类植物的超氧阴离子自由基(O2.-)产生速率和过氧化氢含量(H2O2)的变化均与MDA含量变化相似,且金发藓活性氧水平明显高于湿地匐灯藓。(5)2种藓类植物的超氧化物歧化酶(SOD)、过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性受活性氧诱导亦呈先升后降的趋势,但金发藓抗氧化酶对活性氧迸发的应答更快,活性更强。(6)2种藓类植物的抗坏血酸(AsA)含量呈先降后升态势,但金发藓的含量低于湿地匐灯藓。研究表明,来自不同生境的2种藓类植物对脱水胁迫所致的氧化胁迫均具有很强的适应能力,尤其是复水过程中的修复能力,但不同藓类可能通过不同途径和机制来适应脱水所致的氧化胁迫;来自易发生水分亏缺生境的金发藓可能因具有更强抗氧化能力,从而获得比来自水分充沛生境的湿地匐灯藓更高的脱水耐性。     

Cellular localization of ROS and NO in olive reproductive tissues during flower development

Background  

Recent studies have shown that reactive oxygen species (ROS) and nitric oxide (NO) are involved in the signalling processes taking place during the interactions pollen-pistil in several plants. The olive tree (Olea europaea L.) is an important crop in Mediterranean countries. It is a dicotyledonous species, with a certain level of self-incompatibility, fertilisation preferentially allogamous, and with an incompatibility system of the gametophytic type not well determined yet. The purpose of the present study was to determine whether relevant ROS and NO are present in the stigmatic surface and other reproductive tissues in the olive over different key developmental stages of the reproductive process. This is a first approach to find out the putative function of these signalling molecules in the regulation of the interaction pollen-stigma.     

Dehydration rate and time of desiccation affect recovery of the lichenic algae <Emphasis Type="Italic">Trebouxia erici</Emphasis>: alternative and classical protective mechanisms

The mechanisms involved in desiccation tolerance of lichens and their photobionts are still poorly understood. To better understand these mechanisms we have studied dehydration rate and desiccation time in Trebouxia, the most abundant chlorophytic photobiont in lichen. Our findings indicate that the drying rate has a profound effect on the recovery of photosynthetic activity of algae after rehydration, greater than the effects of desiccation duration. The basal fluorescence (F′_o) values in desiccated algae were significantly higher after rapid dehydration, than after slow dehydration, suggesting higher levels of light energy dissipation in slow-dried algae. Higher values of PSII electron transport were recovered after rehydration of slow-dried Trebouxia erici compared to rapid-dried algae. The main component of non-photochemical quenching after slow dehydration was energy dependent (q _E), whereas after fast dehydration it was photoinhibition (q _I). Although q _E seems to play a role during desiccation recovery, no significant variations were detected in the xanthophyll cycle components. Desiccation did not affect PSI functionality. Classical antioxidant activities like superoxide dismutase or peroxidase decreased during desiccation and early recovery. Dehydrins were detected in the lichen-forming algae T. erici and were constitutively expressed. There is probably a minimal period required to develop strategies which will facilitate transition to the desiccated state in this algae. In this process, the xanthophyll cycle and classical antioxidant mechanisms play a very limited role, if any. However, our results indicate that there is an alternative mechanism of light energy dissipation during desiccation, where activation is dependent on a sufficiently slow dehydration rate.     

Hypothalamic S-Nitrosylation Contributes to the Counter-Regulatory Response Impairment following Recurrent Hypoglycemia

Aims

Hypoglycemia is a severe side effect of intensive insulin therapy. Recurrent hypoglycemia (RH) impairs the counter-regulatory response (CRR) which restores euglycemia. During hypoglycemia, ventromedial hypothalamus (VMH) production of nitric oxide (NO) and activation of its receptor soluble guanylyl cyclase (sGC) are critical for the CRR. Hypoglycemia also increases brain reactive oxygen species (ROS) production. NO production in the presence of ROS causes protein S-nitrosylation. S-nitrosylation of sGC impairs its function and induces desensitization to NO. We hypothesized that during hypoglycemia, the interaction between NO and ROS increases VMH sGC S-nitrosylation levels and impairs the CRR to subsequent episodes of hypoglycemia. VMH ROS production and S-nitrosylation were quantified following three consecutive daily episodes of insulin-hypoglycemia (RH model). The CRR was evaluated in rats in response to acute insulin-induced hypoglycemia or via hypoglycemic-hyperinsulinemic clamps. Pretreatment with the anti-oxidant N-acetyl-cysteine (NAC) was used to prevent increased VMH S-nitrosylation.

Results

Acute insulin-hypoglycemia increased VMH ROS levels by 49±6.3%. RH increased VMH sGC S-nitrosylation. Increasing VMH S-nitrosylation with intracerebroventricular injection of the nitrosylating agent S-nitroso-L-cysteine (CSNO) was associated with decreased glucagon secretion during hypoglycemic clamp. Finally, in RH rats pre-treated with NAC (0.5% in drinking water for 9 days) hypoglycemia-induced VMH ROS production was prevented and glucagon and epinephrine production was not blunted in response to subsequent insulin-hypoglycemia.

Conclusion

These data suggest that NAC may be clinically useful in preventing impaired CRR in patients undergoing intensive-insulin therapy.     

Protection of thylakoids against combined light and drought by a lumenal substance in the resurrection plant Haberlea rhodopensis

Background and Aims

Haberlea rhodopensis is a perennial, herbaceous, saxicolous, poikilohydric flowering plant that is able to survive desiccation to air-dried state under irradiance below 30 µmol m⁻² s⁻¹. However, desiccation at irradiance of 350 µmol m⁻² s⁻¹ induced irreversible changes in the photosynthetic apparatus, and mature leaves did not recover after rehydration. The aim here was to establish the causes and mechanisms of irreversible damage of the photosynthetic apparatus due to dehydration at high irradiance, and to elucidate the mechanisms determining recovery.

Methods

Changes in chloroplast structure, CO₂ assimilation, chlorophyll fluorescence parameters, fluorescence imaging and the polypeptide patterns during desiccation of Haberlea under medium (100 µmol m⁻² s⁻¹; ML) irradiance were compared with those under low (30 µmol m⁻² s⁻¹; LL) irradiance.

Key Results

Well-watered plants (control) at 100 µmol m⁻² s⁻¹ were not damaged. Plants desiccated at LL or ML had similar rates of water loss. Dehydration at ML decreased the quantum efficiency of photosystem II photochemistry, and particularly the CO₂ assimilation rate, more rapidly than at LL. Dehydration induced accumulation of stress proteins in leaves under both LL and ML. Photosynthetic activity and polypeptide composition were completely restored in LL plants after 1 week of rehydration, but changes persisted under ML conditions. Electron microscopy of structural changes in the chloroplast showed that the thylakoid lumen is filled with an electron-dense substance (dense luminal substance, DLS), while the thylakoid membranes are lightly stained. Upon dehydration and rehydration the DLS thinned and disappeared, the time course largely depending on the illumination: whereas DLS persisted during desiccation and started to disappear during late recovery under LL, it disappeared from the onset of dehydration and later was completely lost under ML.

Conclusions

Accumulation of DLS (possibly phenolics) in the thylakoid lumen is demonstrated and is proposed as a mechanism protecting the thylakoid membranes of H. rhodopensis during desiccation and recovery under LL. Disappearance of DLS during desiccation in ML could leave the thylakoid membranes without protection, allowing oxidative damage during dehydration and the initial rehydration, thus preventing recovery of photosynthesis.Key words: Haberlea rhodopensis, resurrection plant, electron microscopy, blue–green fluorescence, chlorophyll fluorescence     

Rehydration of the Lichen Ramalina lacera Results in Production of Reactive Oxygen Species and Nitric Oxide and a Decrease in Antioxidants

Lichens are slow-growing associations of fungi and unicellular green algae or cyanobacteria. They are poikilohydric organisms whose lifestyle in many cases consists of alternating periods of desiccation, with low metabolic activity, and hydration, which induces increase in their metabolism. Lichens have apparently adapted to such extreme transitions between desiccation and rehydration, but the mechanisms that govern these adaptations are still poorly understood. In this study, the effect of rehydration on the production of reactive oxygen species and nitric oxide as well as low-molecular-weight antioxidants was investigated with the lichen Ramalina lacera. Rehydration of R. lacera resulted in the initiation of and a rapid increase in photosynthetic activity. Recovery of photosynthesis was accompanied by bursts of intracellular production of reactive oxygen species and nitric oxide. Laser-scanning confocal microscopy using dichlorofluorescein fluorescence revealed that formation of reactive oxygen species following rehydration was associated with both symbiotic partners of the lichen. The rate and extent of reactive oxygen species production were similar in the light and in the dark, suggesting a minor contribution of photosynthesis. Diaminofluorescein fluorescence, indicating nitric oxide formation, was detected only in fungal hyphae. Activities associated with rehydration did not have a deleterious effect on membrane integrity as assessed by measurement of electrolyte leakage, but water-soluble low-molecular-weight antioxidants decreased significantly.     

Thermoluminescence studies on the function of Photosystem II in the desiccation tolerant lichen Cladonia convoluta

The effect of desiccation and rehydration on the function of Photosystem II has been studied in the desiccation tolerant lichen Cladonia convoluta by thermoluminescence. We have shown that in functional fully hydrated thalli thermoluminescence signals can be observed from the recombination of the S₂₍₃₎Q_B ^– (B band), S₂Q_A ^– (Q band), Tyr-D⁺Q_A ^– (C band) and Tyr-Z⁺(His⁺)Q_A ^– (A band) charge stabilization states. These thermoluminescence signals are completely absent in desiccated thalli, but rapidly reappear on rehydration. Flash-induced oscillation in the amplitude of the thermoluminescence band from the S₂₍₃₎Q_B ^– recombination shows the usual pattern with maxima after 2 and 6 flashes when rehydration takes place in light. However, after rehydration in complete darkness, there is no thermoluminescence emission after the 1 st flash, and the maxima of the subsequent oscillation are shifted to the 3rd and 7th flashes. It is concluded that desiccation of Cladonia convoluta converts PS II into a nonfunctional state. This state is characterized by the lack of stable charge separation and recombination, as well as by a one-electron reduction of the water-oxidizing complex. Restoration of PS II function during rehydration can proceed both in the light and in darkness. After rehydration in the dark, the first charge separation act is utilized in restoring the usual oxidation state of the water-oxidizing comples.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DT desiccation tolerant - PS II Photosystem II - TL thermoluminescence - P₆₈₀ reaction center Chl of PS II - Q_A and Q_B puinone electron acceptors of PS II - S₀,...,S₄ the redox states of the water-oxidizing complex - Tyr-Z and Tyr-D redox-active tyrosine electron donors of PS II     

Protection of the photosynthetic apparatus against dehydration stress in the resurrection plant Craterostigma pumilum

The group of homoiochlorophyllous resurrection plants evolved the unique capability to survive severe drought stress without dismantling the photosynthetic machinery. This implies that they developed efficient strategies to protect the leaves from reactive oxygen species (ROS) generated by photosynthetic side reactions. These strategies, however, are poorly understood. Here, we performed a detailed study of the photosynthetic machinery in the homoiochlorophyllous resurrection plant Craterostigma pumilum during dehydration and upon recovery from desiccation. During dehydration and rehydration, C. pumilum deactivates and activates partial components of the photosynthetic machinery in a specific order, allowing for coordinated shutdown and subsequent reinstatement of photosynthesis. Early responses to dehydration are the closure of stomata and activation of electron transfer to oxygen accompanied by inactivation of the cytochrome b₆f complex leading to attenuation of the photosynthetic linear electron flux (LEF). The decline in LEF is paralleled by a gradual increase in cyclic electron transport to maintain ATP production. At low water contents, inactivation and supramolecular reorganization of photosystem II becomes apparent, accompanied by functional detachment of light‐harvesting complexes and interrupted access to plastoquinone. This well‐ordered sequence of alterations in the photosynthetic thylakoid membranes helps prepare the plant for the desiccated state and minimize ROS production.     

ABA Increases the Desiccation Tolerance of Photosynthesis in the Afromontane Understorey Moss Atrichum androgynum

The effect of pretreatment with abscisic acid (ABA) on the physiologyof the moss Atrichum androgynum during a desiccation–rehydrationcycle was examined. During rehydration following desiccationfor 16 h, net CO₂fixation recovered much more slowly than photosystemII (PSII) activity, conditions conducive to the formation ofreactive oxygen species (ROS) in the photosynthetic apparatus.Pretreatment with ABA increased the rate of recovery of photosynthesisand PSII activity, and also doubled non-photochemical quenching(NPQ). Increased NPQ activity will reduce ROS formation, andmay explain in part how ABA hardens the moss to desiccation.In ABA-pretreated, but not untreated mosses, desiccation significantlyincreased the concentration of soluble sugars. Sugar accumulationmay promote vitrification of the cytoplasm and protect membranesduring desiccation. Starch concentrations in freshly collectedA. androgynum were only approx. 40 mg g^-1dry mass; they roseslightly during desiccation but were only slightly affectedby ABA pretreatment. ABA did not reduce chlorophyll breakdownduring desiccation. Copyright 2001 Annals of Botany Company Moss, desiccation, abscisic acid, photosynthesis, chlorophyll fluorescence     

Increased activities of superoxide dismutase and catalase are not the mechanism of desiccation tolerance induced by hardening in the moss Atrichum androgynum

Abstract

The effects of treatments that increase desiccation tolerance were tested on the activity of the enzymes superoxide dismutase (SOD) and catalase (CAT) in the moss Atrichum androgynum subjected to a drying/wetting cycle. Hardening by both abscisic acid (ABA) pretreatment and partial dehydration significantly increased the rate of recovery of photosynthesis during rehydration following desiccation. Hardening treatments had little effect on SOD activity. In non-hardened plants, SOD activity increased three-fold during desiccation for 32 h at 52% rh, but hardened material tended to display smaller increases in activity. During rehydration, SOD activities rapidly declined to their initial values in all treatments. Hardening by partial dehydration, but not ABA, reduced CAT activity. After desiccation for 32 h, material from all treatments displayed about half the initial CAT activity, and activity did not change during subsequent rehydration. Results show that, while the induction of SOD appears to play a role in desiccation tolerance, a similar induction occurred in both hardened and non-hardened mosses. Induction of greater activities of enzymes that scavenge reactive oxygen species is not responsible for the added tolerance induced by hardening treatments.     

Reconstitution of chlorophylls and photosynthetic CO2 assimilation upon rehydration of the desiccated poikilochlorophyllous plant Xerophyta scabrida (Pax) Th. Dur. et Schinz

Resynthesis of the photosynthetic apparatus and resumption of CO₂ assimilation upon rehydration is reported for the monocotyledonous and poikilochlorophyllous desiccation-tolerant (PDT) plant Xerophyta scabrida (Pax) Th. Dur. et Schinz (Velloziaceae). During desiccation there was a complete breakdown of chlorophylls whereas the total carotenoid content of air-dried leaves was reduced to about 22% of that of functional leaves. The prerequisites for the resynthesis of photosynthetic pigments and functional thylakoids were the reappearance of turgor and maximum leaf water content at 2 and 10 h after rehydration, respectively. The period of increased initial respiration after rewetting leaves (rehydration respiration) lasted up to 30 h and was thus 6 to 10 times longer than in homoiochlorophyllous desiccation-tolerant plants (HDTs) in which chlorophylls are retained during desiccation. Accumulation of chlorophylls a + b and total carotenoids (xanthophylls and carotene) started 10 h after rehydration. Normal levels of chlorophyll and carotenoids were obtained 72 h after rehydration. Values for the variable-fluorescence decrease ratio (Rfd690 values), an indicator of photochemical activity, showed that photochemical function started 10 h after rehydration, but normal values of 2.7 were reached only 72 h after rehydration. Net CO₂ assimilation started 24 h after rewetting and normal rates were reached after 72 h, at the same time as normal values of stomatal conductance were obtained. The increasing rates of net CO₂ assimilation were paralleled by decreasing values of the intercellular CO₂ concentration. All photosynthetic parameters investigated showed values normal for functional chloroplasts by 72 h after the onset of rehydration. Fully regreened leaves of the presumed C₃ plant X. scabrida exhibited a net CO₂ assimilation rate which was in the same range as that of other C₃ plants and higher than that of recovered HDT plants. The fundamental difference between air-dried PDT plants, such as X. scabrida, which have to resynthesize the photosynthetic pigment apparatus, and air-dried HDT plants, which only undergo a functional recovery, is discussed.Abbreviations c -carotene - c_i intercellular CO₂ concentration - Car x + c total carotenoid content x + c - Chl a + b total chlorophyll a + b content - g_s stomatal conductance - HDT homoiochlorophyllous desiccation tolerant - LWC leaf-water content - P_N net photosynthesis rate - PDT poikilochloro phyllous desiccation tolerant - R_d dark respiration - Rfd variable fluorescence decrease ratio (Rfd = fd/fs) - x xanthophylls The senior author thanks the Deutschem Akademischem Auslandsdienst (Bonn, Germany), Soros Foundation (Budapest, Hungary) and European Community (Brussels, Belgium) for providing fellowships for research periods at Karlsruhe. The research was also supported by the Hungarian Scientific Research Foundation (OTKA I/848, OTKA I/3.1545 and OTKA I/4.F.5359). We wish to thank Professor T. Pocs (Eger, Hungary — Morogoro, Tanzania) for collecting the plant material and to the linguist Mr. A. Jackson for correcting the English.     

Ultrastructural changes in the pyrenoid of the lichenParmelia sulcata stored under controlled conditions

Summary Observations have been made on ultrastructural changes in pyrenoids of the algal cells in the lichenParmelia sulcata subjected to a range of controlled desiccation and rehydration regimes. Weaker staining of parts of the proteinaceous pyrenoid matrix, interpreted as damage, occurred within 48 hours of transfering moist thalli to 53% r.h. in a 12 hours light 12 hours dark (20°C 14°C) regime. No specific damage resulted from direct dehydration at 0% r.h. or on transfer after 48 hours from 53% r.h. to 0% r.h. Material stored dry under different conditions,e.g., 0%, 53% or 53% to 0% r.h., for 72 hours showed recovery of the pyrenoid matrix when rehydrated in water (30 minutes) or by storage at 100% r.h. for 24 hours. After storage at 53% r.h. or 0% r.h. for 3 months, damage was more extreme, being greater in material originally dried at 53% r.h. Recovery in water, to the original appearance, only occurred after rehydration for 24 hours at 100% r.h. After 3 months desiccation, damage, due to the initial 48 hours at 53% r.h. was still apparent. Severe damage involved expansion of the pyrenoid. As rehydration restored pyrenoids to their original dimensions, pyrenoid proteins probably became dispersed rather than degraded during desiccation.     

Tolerance to oxidative stress induced by desiccation in Porphyra columbina (Bangiales, Rhodophyta)

Unravelling the mechanisms underlying desiccation tolerance is crucial in order to understand the position of algal species in the intertidal zone. The alga Porphyra columbina lives in the uppermost part of the rocky intertidal zones around the world and was selected as a model for this study. Naturally desiccated plants were collected during low tide and studied for morphological changes, oxidative burst induction, biomolecule oxidation, antioxidant responses, and photosynthetic status. Naturally hydrated plants collected during high tides were used for comparative purposes. In addition, changes induced by desiccation were assessed in vitro and the capacity to recover from desiccation was determined by rehydrating the fronds in seawater. The global results show that desiccation induces morphological and cellular alterations accompanied by a loss of ～96% of the water content. Overproduction of reactive oxygen species (ROS) was induced by desiccation and two peaks of H(2)O(2) were detected at 1 and 3 h of desiccation. However, during in vitro rehydration post-desiccation, the ROS quickly returned to the basal levels. At the biomolecular level, only a low production of oxidized proteins was recorded during desiccation, whereas the activity of diverse antioxidant enzymes increased. However, this activity diminished to near basal levels during rehydration. The photosynthetic efficiency (F(v)/F(m)) during desiccation declined by 94-96% of the values recorded in hydrated plants. This reduction was generated by the low levels of trapped energy flux per cross-section (TRo/CS), electron transport flux per CS (ETo/CS), and density of reaction centres (RC/SCo) as well as the chlorophyll content. The inverse pattern was observed for the levels of phycocyanin and phycoerythrin content. F(v)/F(m) and the photosynthetic indicators were restored to normal levels after only 5 min of rehydration. The results indicate that desiccation in P. columbina causes overproduction of ROS that is efficiently attenuated. The morphological and photosynthetic changes could be operating as tolerance mechanisms due to the fact that these responses principally prevent biomolecular alteration and cellular collapse. Thus, the activation of different physiological mechanisms helps to explain the high tolerance to desiccation of P. columbina and, at least in part, the position of this species at the highest level in the intertidal zone.     

Hardening by partial dehydration and ABA increase desiccation tolerance in the cyanobacterial lichen Peltigera polydactylon

* BACKGROUND AND AIMS: The ability of partial dehydration and abscisic acid pretreatments to increase desiccation tolerance in the cyanobacterial lichen Peltigera polydactylon was tested. * METHODS: Net photosynthesis and respiration were measured using infrared gas analysis during a drying and rehydration cycle. At the same time, the efficiency of photosystem two was measured using chlorophyll fluorescence, and the concentrations of chlorophyll a were spectrophotometrically assayed. Heat production was also measured during a shorter drying and rehydration cycle using differential dark microcalorimetry. * KEY RESULTS: Pretreating lichens by dehydrating them to a relative water content of approx. 0.65 for 3 d, followed by storing thalli hydrated for 1 d in the light, significantly improved their ability to recover net photosynthesis during rehydration after desiccation for 15 but not 30 d. Abscisic acid pretreatment could substitute for partial dehydration. The improved rates of photosynthesis during the rehydration of pretreated material were not accompanied by preservation of photosystem two activity or chlorophyll a concentrations compared with untreated lichens. Partial dehydration and ABA pretreatments appeared to have little direct effect on the desiccation tolerance of the mycobiont, because the bursts of respiration and heat production that occurred during rehydration were similar in control and pretreated lichens. * CONCLUSIONS: Results indicate that the photobiont of P. polydactylon possesses inducible tolerance mechanisms that reduce desiccation-induced damage to carbon fixation, and will therefore improve the supply of carbohydrates to the whole thallus following stress. In this lichen, ABA is involved in signal transduction pathways that increase tolerance of the photobiont.     

Seed desiccation limits removal by ants

Ants collect and disperse seeds that bear an attractive nutritive body called the elaiosome. In mesic habitats, many myrmecochorous plant species have elaiosomes that are usually soft and desiccation-sensitive. The aim of this study was to link the desiccation rate of two species of seeds (Chelidonium majus and Viola odorata) to the seed-removing behaviour of the ant Myrmica rubra. In laboratory experiments seeds of both species lost one-third of their weight in 24 h. Concurrently, seed removal rates decreased sharply (92%) for Viola odorata over 24 h and slowly for Chelidonium majus, which retained one-third of its attractiveness after one month of desiccation. Seeds recovered their initial weight almost entirely after being soaked in water for 18 h. This rehydration partially (Viola odorata) or totally (Chelidonium majus) restored the attractiveness of the seeds. In this paper, we show that the window of seed attractiveness during which ant–plant interactions occur is desiccation-dependent and plant-specific.     

The use of aeroponics to investigate antioxidant activity in the roots of <Emphasis Type="Italic">Xerophyta viscosa</Emphasis>

In order to ultimately understand the whole plant mechanism of attaining desiccation tolerance, we undertook to investigate the root tissues of the resurrection plant Xerophyta viscosa, as previous work has only been conducted on the leaf tissues of resurrection plants. An aeroponic plant growth system was designed and optimised to observe the root’s response to desiccation without the restrictions of a soil medium, allowing easy access to roots. Successful culture of both X.viscosa and the control, Zea mays, was achieved and dehydration stress was implemented through reduction of nutrient solution spraying of the roots. After drying to the air dry state (achieved after 7 days for roots and 10 days for shoots), rehydration was achieved by resumption of root spraying. X.viscosa plants survived desiccation and recovered but Z. mays did not. The activity of the antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase and quantities of ascorbate and glutathione were determined during root desiccation. There was an initial decline in activity in all enzymes upon drying to 80% RWC, but activity thereafter remained constant, at rates indicative of potential metabolic activity, to the air-dry state. This data suggests that these enzymes are not denatured by desiccation of the root tissue. Ascorbate and glutathione content remained constant at concentrations of 70 and 100 μM, respectively during drying. Thus root tissues appear to retain antioxidant potential during drying, for use in recovery upon rehydration, as has been reported for leaf tissues of this and other resurrection plants.     

Differences in physiological adaptation of <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> Friv. leaves and roots during dehydration–rehydration cycle

The ecophysiological responses of the homoiochlorophyllous desiccation-tolerant (HDT) plant Haberlea rhodopensis showed that this plant could tolerate water deficit and both leaves and roots had high ability to survive severe desiccation. The changes and correlation between CO₂ assimilation, stomatal conductance, contents of photosynthetic pigments, root respiration and specific leaf area during dehydration–rehydration cycle were investigated. The physiological activity of leaves and roots were examined in fully hydrated (control) plants and during 72 h of dehydration, as well as following 96 h of rehydration every 6 and 24 h. After 6 h of dehydration, the stomatal conductance declined and the intercellular CO₂ concentration increased. The reduction in CO₂ assimilation rate was observed after 54 h of dehydration. There was a good correlation between the root respiration and water content. Our results showed that the plasticity of adaptation in leaves and roots were different during extreme water conditions. Roots were more sensitive and reacted faster to water stress than leaves, but their activity rapidly recovered due to immediate and efficient utilization of periodic water supply.