Water use efficiency of two succulents with contrasting CO2 fixation pathways

Two succulents with similar growth forms but different types of photosynthesis, Cotyledon orbiculata (crassulacean acid metabolism, CAM) and Othonna opima (C₃ pathway), were investigated with respect to the modulation of water use efficiency (WUE) during the transition from the rainy season to subsequent drought. Environmental conditions were simulated in a controlled-environment experiment on the basis of data collected in the habitat of the two species in the southern Namib desert. Experiments included one or more periods of hot bergwind, which frequently occurs in this region. When water was readily available, daily net CO₂ fixation was similar in the two species. This result confirms that the daily CO₂ fixation of CAM plants is as high as that of morphologically similar C₃ plants adapted to the same habitat. As expected, both species reduced CO₂ fixation and water loss through transpiration during simulated hot bergwind periods and their WUE values increased. However, after the second hot bergwind period, nearly identical WUEs were recorded: 41.0 and 40.0 mmol mol^?1 for C. orbiculata and O. opima, respectively. Therefore the statement that a CAM plant is a better ‘water saver’ than a C₃ plant does not necessarily hold for CAM and C₃ plants with similar growth forms growing under the same environmental conditions.     

Temperature and water regulation of gas exchange of Opuntia polyacantha

Summary Opuntia polyacantha was collected from the shortgrass prairie in Colorado. Carbon dioxide and water vapor exchange was monitored in plants pretreated and analyzed under cool temperatures (20/15°C) and warm temperatures (35/15°C). Well watered plants under a 35/15 thermoperiod supported the fixation of atmospheric CO₂ during the night, early morning, and late afternoon. Plants under a 20/15 thermoperiod exhibited CO₂ uptake only during the afternoon. The fixation of CO₂ at night could be stimulated or induced by decreasing the night temperature. Plants from which water was withheld two or four weeks showed a decline in CO₂ fixation with the uptake at night exhibiting the greatest sensitivity. Under conditions of water stress the enhancement of CO₂ uptake at night by cool night temperatures was largely lost. Plants water stressed for 4 weeks recovered rapidly upon rewatering under warm or cool temperatures. Rates of CO₂ fixation were comparable to well watered plants within 24 h. The effects of temperature and water stress on gas exchange are compared to the in situ growth pattern of O. polyacantha and contrasted with the regulation of gas exchange observed in C₃ and C₄ grasses of the shortgrass prairie.This research was supported by funds from NSF Grants BMS 74-07894, GB-31862X, and GB-41233X     

Leaf photosynthesis of Haberlea rhodopensis before and during drought

Haberlea rhodopensis is a homoiochlorophyllous resurrection plant that shows a low rate of leaf net CO₂ uptake (4–6 μmol m^?2 s^?1) under saturating photosynthetic photon flux densities in air (21% O₂ and about 390 ppm CO₂). However, leaf net CO₂ uptake reaches values of 17–18 μmol m^?2 s^?1 under saturating CO₂ and light. H. rhodopensis leaves have a very low mesophyll CO₂ conductance that can partly explain the low rate of leaf net CO₂ uptake in normal air. Experimental evidences suggest that mesophyll conductance is not sensitive to temperature in the 20–35 °C range. In addition, it is shown that the (1) transpiration rate of H. rhodopensis is nearly linearly related to the vapour pressure difference between the leaf and the ambient air within the interval from 0.5 kPa to 2.5 kPa at a leaf temperature of 25 °C and (2) leaf net CO₂ uptake in normal air under saturating light does not change much with leaf temperature (between 20 °C and 30 °C). At a leaf relative water content of between 90% and 30%, the decrease of leaf net CO₂ assimilation during drought can be explained by a decrease of leaf CO₂ diffusional conductance. Accordingly the non-photochemical chlorophyll fluorescence quenching decreases only at relative water contents lower than 20%, indicating that photosynthetic activity maintains a trans-thylakoidal proton gradient over a wide range of leaf water contents. Moreover, PSII photochemistry (as estimated by the Fv/Fm ratio and the thermoluminescence B band intensity) is only affected at leaf relative water contents lower than about 20%, thus confirming that primary photosynthetic reactions are resistant to drought. Interestingly, the effect of leaf desiccation on photosynthetic capacity, measured at very high ambient CO₂ molar ratios under saturating PPFD, is identical to that observed for three non-resurrection C₃ mesophytes. This demonstrates that the photosynthetic apparatus of H. rhodopensis is not more resistant to desiccation when compared to other C₃ plants. Since the leaf area decreases by more than 50% when the leaf relative water content is reduced to about 40% during drought it is supposed, following Farrant et al. [Farrant, J.M., Vander, W.C., Lofell, D.A., Bartsch, S., Whittaker, A., 2003. An investigation into the role of light during desiccation of three angiosperms resurrection plants. Plant Cell Environ. 26, 1275–1286], that H. rhodopensis leaf cells avoid mechanical stress.     

Diurnal Pattern of Transpiration,Water Uptake and Water Budget of Succulents with Different CO2 Fixation Pathways

The water fluxes and the CO₂ exchange of three leaf succulents, Othonna opima, Cotyledon orbiculata and Senecio medley-woodii, with different leaf anatomy, growth form and CO₂ fixation pathways (C₃, CAM) were monitored with a gas exchange cuvette which was combined with a potometric system to quantify water uptake. Measurements, which are primarily valid for plants with a sufficient water supply, were made during 6 to 10 consecutive days under constant experimental conditions. Water uptake for 24 h exceeded water loss by transpiration only for a S, medley-woodii plant with 10 expanding but only 7 mature leaves. In this case the gained water evidently is put into leaf expansion. All other plants showed balanced transpiration and water uptake rates. O. opima and C. orbiculata have a similar life form, similar water storage volumes and the same natural habitat but their diurnal water uptake patterns differ significantly. In the C₃ plant O. opima water uptake increased when the transpiration increased or transpiration rates were higher than uptake rates and vice versa. On the contrary the CAM plant C. orbiculata transpired during the dark period at constant or decreasing rates but showed steadily increasing uptake rates. Senecio medley-woodii- and C. orbiculata are CAM plants with similar diurnal water uptake patterns with its maximum in uptake during or towards the end of the CO₂ dark fixation period. Water uptake of C. orbiculata was at its minimum at the end of the light period despite transpiration being maximal. The results were discussed considering the different CO₂ fixation pathways. In the investigated CAM succulents, C. orbiculata and S. medley-woodii, the CAM influenced water uptake throughout the whole day and not only during the CO₂ dark fixation period.     

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.     

Le vie fotosintetiche C3, C4 e CAM nel contesto ecologico

Abstract

Ecological aspects of C₃, C₄ and CAM photosynthetic pathways. - Three different photosynthetic CO₂ fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO₂ release. Both C₄ and CAM pathways behave as a CO₂ concentrating mechanism which prevent photorespiration. The CO₂-concentrating mechanism in C₄ plants is based on intracellular symplastic transport of C₄ dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO₂-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C₄ photosynthetic pathway as compared to the C₃ pathway permits higher rates of CO₂ fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO₂.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C₄ plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C₃; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.     

Changes of the chlorophyll fluorescence induction kinetics of C3 and CAM plants during day/night cycles

The induction kinetics of the 680 nm chlorophyll fluorescence were measured on attached leaves of Kalanchoë daigremontiana R. Hamet et Perr. (CAM plant), Sedum telephium L. and Sedum spectabile Bor. (C₃ plant in spring, CAM plant in summer) and Raphanus sativus L. (C₃ plant) at three different times during a 12/12 h day/night cycle. During the fluorescence transient the fluorescence intensity at the O, P and T-level (f_O, f_max, f_st,) was different for the plant species tested; this may be due to their different leaf structure, pigment composition and organization of their photosystems. The kinetics of the fluorescence induction depended on the time of preillumination or dark adaptation during the light/dark cycle but not on the type of primary CO₂ fixation mechanism (C₃ and CAM). For dark adapted leaves measured either at the end of the dark phase or after dark adaptation of plants taken from the light phase a higher P-level fluorescence, a higher variable fluorescence (P-O) and a larger complementary area were found than for leaves of plants taken directly from the light phase. This indicates the presence of largely oxidized photosystem 2 acceptor pools during darkness. During the light phase the fluorescence decline after the P-level was faster than during the dark phase; from this we conclude that the light adaptation of the photosynthetic apparatus (state 1state 2 transition, pH) during the induction period proceeded faster in plants taken from the light phase than in plants taken from the dark phase.Abbreviations C₃ plant plant with primary CO₂ fixation on ribulose-1,5-bis-phosphate (Calvin-Benson cycle) - CAM Crassulacean Acid Metabolism     

Sugar ratios, glutathione redox status and phenols in the resurrection species Haberlea rhodopensis and the closely related non-resurrection species Chirita eberhardtii

Because of their unique tolerance to desiccation, the so‐called resurrection plants can be considered as excellent models for extensive research on plant reactions to environmental stresses. The vegetative tissues of these species are able to withstand long dry periods and to recover very rapidly upon re‐watering. This study follows the dynamics of key components involved in leaf tissue antioxidant systems under desiccation in the resurrection plant Haberlea rhodopensis and the related non‐resurrection species Chirita eberhardtii. In H. rhodopensis these parameters were also followed during recovery after full drying. A well‐defined test system was developed to characterise the different responses of the two species under drought stress. Results show that levels of H₂O₂ decreased significantly both in H. rhodopensis and C. eberhardtii, but that accumulation of malondialdehyde was much more pronounced in the desiccation‐tolerant H. rhodopensis than in the non‐resurrection C. eberhardtii. A putative protective role could be attributed to accumulation of total phenols in H. rhodopensis during the late stages of drying. The total glutathione concentration and GSSG/GSH ratio increased upon complete dehydration of H. rhodopensis. Our data on soluble sugars suggest that sugar ratios might be important for plant desiccation tolerance. An array of different adaptations could thus be responsible for the resurrection phenotype of H. rhodopensis.     

Prompt response of superoxide dismutase and peroxidase to dehydration and rehydration of the resurrection plant <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis>

The relic endemic nature of Haberlea rhodopensis, which grows in Balkan Peninsula, in combination with its high vegetative desiccation-tolerance, makes this species a good model to study mechanisms behind plant adaptation to severe drought stress. The aim of this study was to evaluate the antioxidant protection provided by Superoxide dismutase (SOD) and Peroxidase (PO) in H. rhodopensis after exposure to and recovery from dehydration at different developmental stages. During dehydration the electrolyte leakage from leaf tissue increased more significantly in post-flowering plants than in flowering plants, while upon subsequent rehydration this parameter showed a very fast decrease to the basic value of fresh leaves and did not depend on developmental stage. Like other higher plant species, SOD and PO demonstrated in H. rhodopensis an ability to adjust their activity very promptly to changing water supply. In addition, the leaves of this resurrection species retained significant activities of SOD and PO even in air-dried state, considered as the most severe form of water stress. The enhanced activity of antioxidant enzymes may either enable the scavenging of the active oxygen species produced at very severe water deficit, and/or carry a potential for resurrection on subsequent rehydration. Upon stress treatment total activities of both enzymes were higher in flowering than post-flowering plants which reveals that developmental stage might be a factor affecting plant stress tolerance. This work identified for the first time SOD isoforms of H. rhodopensis. Native PAGE showed at least six multiple isoforms in the protein extract from leaf tissue of flowering plants, and the differential visualization revealed that four of them were Cu, Zn-SOD isoforms, one was Mn-SOD and one Fe-SOD. These findings provide a good starting point for future study of the SOD gene family of this rare resurrection plant at the molecular level.     

Induction of crassulacean acid metabolism in Mesembryanthemum crystallinum increases reproductive success under conditions of drought and salinity stress

Summary Mesembryanthemum crystallinum L., an inducible crassulacean acid metabolism (CAM) plant, was grown for approximately 5 weeks following germination in well-watered, non-saline soil in a controlled-environment chamber. During this time, plants were characterized by C₃ photosynthetic carbon metabolism. After the initial 5 weeks, CAM was induced by a combination of high soil salinity and reduced soil water content. One group of plants was allowed to engage in CAM by being continuously exposed to normal CO₂-containing air (about 350–400 ppm). A second group of plants was deprived of ambient CO₂ each night (12 h dark period) until completion of their life cycle, thereby minimizing potential carbon gain via dark CO₂ fixation. The capacity to express CAM under conditions of drought and salinity stress markedly improved reproductive success: plants kept in normal CO₂-containing air produced about 10 times more seeds than plants kept in CO₂-free air during dark periods. Seeds from plants deprived of ambient CO₂ overnight had more negative ¹³C values than seeds from plants kept in normal air.     

The Effect of Sodium Chloride on the Balance between the C3- and C4-Carbon Fixation Pathways

Young leaves of salt-depleted Aeluropus litoralis Parl. plants show CO₂ fixation by the C₃-carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO₂ fixation by the C₄-pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose-1,5-diphosphate carboxylase.     

Photosynthetic response of Tempranillo grapevine to climate change scenarios

Photosynthesis in C₃ plants is CO₂ limited and therefore any increase in Rubisco carboxylation substrate may increase net CO₂ fixation, unless plants experience acclimation or other limitations. These aspects are largely unexplored in grapevine. Photosynthesis analysis was used to assess the stomatal, mesophyll, photochemical and biochemical contributions to the decreasing photosynthesis observed in Tempranillo grapevines (Vitis vinifera) from veraison to ripeness, modulated by CO₂, temperature and water availability. Photosynthesis and photosystem II photochemistry decreased from veraison to ripeness. The elevated CO₂ and temperature increased photosynthesis, but transiently, in both well irrigated (WI) and water‐stressed plants. Photosynthetic rates were maxima 1 week after the start of elevated CO₂ and temperature treatments, but differences with treatments of ambient conditions disappeared with time. There were not marked changes in leaf water status, leaf chlorophyll or leaf protein that could limit photosynthesis at ripeness. Leaf total soluble sugars remained at ripeness as high as 2 weeks after the start of treatments. On the other hand, and as expected, CO₂ diffusional limitations impaired photosynthesis in grapevine plants grown under water scarcity, stomatal and mesophyll conductances to CO₂ decreased and in turn low chloroplastic CO₂ concentrations limited photosynthetic CO₂ fixation. In summary, photochemistry and photosynthesis from veraison to ripeness in Tempranillo grapevine were dominated by a developmental‐related decreasing trend that was only transiently influenced by elevated CO₂ concentrations.     

δ13C of CO2 respired in the dark in relation to δ13C of leaf metabolites: comparison between Nicotiana sylvestris and Helianthus annuus under drought

The variations of δ¹³C in leaf metabolites (lipids, organic acids, starch and soluble sugars), leaf organic matter and CO₂ respired in the dark from leaves of Nicotiana sylvestris and Helianthus annuus were investigated during a progressive drought. Under well‐watered conditions, CO₂ respired in the dark was ¹³C‐enriched compared to sucrose by about 4‰ in N. sylvestris and by about 3‰ and 6‰ in two different sets of experiments in H. annuus plants. In a previous work on cotyledonary leaves of Phaseolus vulgaris, we observed a constant ¹³C‐enrichment by about 6‰ in respired CO₂ compared to sucrose, suggesting a constant fractionation during dark respiration, whatever the leaf age and relative water content. In contrast, the ¹³C‐enrichment in respired CO₂ increased in dehydrated N. sylvestris and decreased in dehydrated H. annuus in comparison with control plants. We conclude that (i) carbon isotope fractionation during dark respiration is a widespread phenomenon occurring in C₃ plants, but that (ii) this fractionation is not constant and varies among species and (iii) it also varies with environmental conditions (water deficit in the present work) but differently among species. We also conclude that (iv) a discrimination during dark respiration processes occurred, releasing CO₂ enriched in ¹³C compared to several major leaf reserves (carbohydrates, lipids and organic acids) and whole leaf organic matter.     

Physiological and biochemical aspects and molecular mechanisms of plant adaptation to the elevated concentration of atmospheric CO<Subscript>2</Subscript>

The review of publications concerning the impact of increasing CO₂ concentration in the Earths atmosphere (C_a) on higher terrestrial plants. The physiological changes in plants induced by increasing C_a, including growth and biochemical composition, the characteristics of photosynthesis and respiration, as well as the molecular mechanisms of the regulation of the activity of most important biosynthetic enzymes at early and late stages of the exposure to elevated C_a are under consideration. Various concepts of metabolic regulation during acclimation to increasing CO₂ concentration are critically reviewed. The pathways of possible involvement of carbonic anhydrase-mediated systems of CO₂ transport and concentration during C₃ photosynthesis of higher plants, the metabolic and signal mechanisms of photosynthesis inhibition by carbohydrates and the role of ethylene at elevated C_a are presented. The effect of elevated C_a on plant development and source-sink relations, as well as its interaction with other environmental factors, such as mineral, primarily nitrogen nutrition, light, temperature, and water regime, are discussed in with the context of potential forecasting of the consequences of increase in C_a and temperature for the activities of various higher plant forms in the rapidly changing climate.Translated from Fiziologiya Rastenii, Vol. 52, No. 1, 2005, pp. 129–145.Original Russian Text Copyright © 2005 by Romanova.     

Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Summary Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C₃ photosynthesis. ¹⁴CO₂ uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C₃ during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO₂ uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO₂ uptake was absent. The shift from CAM to CAM-idling or C₃ in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO₂ uptake. Whereas during the summer, rewatering quickly resulted in early morning CO₂ uptake, but nocturnal CO₂ uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C₃ pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.     

Light and dark CO2 fixation in Clusia uvitana and the effects of plant water status and CO2 availability

Summary In well-watered plants of Clusia uvitana, a species capable of carbon fixation by crassulacean acid metabolism (CAM), recently expanded leaves gained 5 to 13-fold more carbon during 12 h light than during 12 h dark periods. When water was withheld from the plants, daytime net CO₂ uptake strongly decreased over a period of several days, whereas there was a marked increase in nocturnal carbon gain. Photosynthetic rates in the chloroplasts were hardly affected by the water stress treatment, as demonstrated by measurements of chlorophyll a fluorescence of intact leaves, indicating efficient decarboxylation of organic acids and refixation of carbon in the light. Within a few days after rewatering, plants reverted to the original gas exchange pattern with net CO₂ uptake predominantly occurring during daytime. The reversible increase in dark CO₂ fixation was paralleled by a reversible increase in the content of phosphoenolpyruvate (PEP) carboxylase protein. In wellwatered plants, short-term changes in the degree of dark CO₂ fixation were induced by alterations in CO₂ partial pressure during light periods: a decrease from 350 to 170 bar CO₂ caused nocturnal carbon gain, measured in normal air (350 bar), to increase, whereas an increase to 700 bar CO₂, during the day, caused net dark CO₂ fixation to cease. The increased CAM activity in response to water shortage may, at least to some extent, be directly related to the reduced carbon gain during daytime.     

Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.

Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO₂. Three chambers were maintained at current CO₂ concentration (ambient treatment), three at twice ambient CO₂, or approximately 720 μmol mol^?1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C₃ and C₄ grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO₂, with no differences in the relative responses of C₃/C₄ grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO₂ on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO₂‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO₂, but only Stipa comata (C₃) plants exhibited significant reductions in N under elevated compared to ambient CO₂ chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO₂ enriched environments.     

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.     

Evaluation of the potential to measure photosynthetic rates in C3 plants (Flaveria pringlei and Oryza sativa) by combining chlorophyll fluorescence analysis and a stomatal conductance model

The response curves of leaf photosynthesis to varying light, temperature and leaf-to-air vapour pressure deficit were measured in the C₃ plants Flaveria pringlei and Oryza sativa in normal air with a computerized open infrared gas analysis (IRGA) system, and the photochemical efficiency of photosystem II, described as (1–F,/F′_m) after Genty. Briantais & Baker (1989, Biochimica et Biophysica Acta 990, 87–92), was simultaneously measured with a modulated fluorometer. A model was written for rates of CO₂ fixation as a function of the true rate of O₂ evolution measured by fluorescene analysis (Jo₂), mesophyll conductance and intercellular CO₂ partial pressure. A second model was developed for rates of CO₂ fixation as a function of Jo₂, mesophyll conductance and stomatal conductance. In the latter case, leaf stomatal conductance was simulated using the stomatal model proposed by Leuning (1995, Plant, Cell and Environment 18 , 339–355). The rates of CO₂ fixation predicted from the models were similar to rates measured by IRGA. The results indicate that there is potential to measure CO₂ fixation in C₃ plants by combining the non-invasive measurement of Jo₂ by chlorophyll fluorescence analysis with the stomatal conductance model.