Daily and seasonal variation in water relations of macchia shrubs and trees in France (Montpellier) and Turkey (Antalya)

Based upon two different research studies in the mediterranean regions of France and Turkey, drought resistance strategies were investigated in a broad group of species. The diurnal and seasonal patterns of the water relations of different lifeforms from the thermo-mediterranean to submediterranean lifezones were compared. Three sites near Montpellier, in Southern France, and five sites near Antalya, Turkey were used for this comparison. Xylem pressure potential and relative stomatal aperture were the key water relations parameters collected in France while these parameters as well as osmotic potential and leaf conductance were studied in Turkey.From the 26 different study species investigated in France, 7 distinct types of stomatal control were observed, with the deciduous lifeforms showing the least control, the sclerophyllous and coniferous evergreens the greatest control and the malacophyllous shrublets intermediate levels of control. Predawn water potential values provided a means of classifying species according to their temporal and spatial utilization of site water reserves. The comparison of turgor potentials (difference between water and osmotic potentials) gave an insight into leaf adaptations to site moisture. Species with high predawn water potentials generally maintain positive turgor even at midday during the summer, whereas species with low predawn values were frequently at zero turgor even at predawn. Phlomis grandiflora was the most extreme species with mid-summer predawns and midday water potentials of –6 MPa and osmotic potentials never more negative than –2.4 MPa.     

Stomatal responses to light in the facultative Crassulacean acid metabolism species, Portulacaria afra

Guard cell responses to light are mediated by guard cell chlorophyll and by a specific blue light photoreceptor. Gas exchange and epidermal peel techniques were employed to investigate these responses in the facultative Crassulacean acid metabolism (CAM) species, Portulacaria afra (L.) Jacq. In P. afra individuals performing C₃ metabolism, red light stimulated an increase in leaf conductance in intact leaves and stomatal opening in isolated epidermal peels, indicating the presence in guard cells of the chlorophyll-mediated response to light. Under a background of continuous red illumination, conductance exhibited transient increases following pulses of blue but not red light, indicating that the specific stomatal response to blue light was also operative. In contrast, in CAM individuals, conductance in gas exchange experiments and stomatal opening in epidermal peel experiments were not stimulated by red light. In CAM plants, conductance did not increase following blue light pulses administered over a range of temperatures, vapor pressure differences (VPD), ambient CO₂ concentrations and background red light intensities. These results indicate that P. afra does possess typical guard cell responses to light when performing C₃ metabolism. The metabolic pathways mediating these responses are either lost or inhibited when CAM is induced.     

Changes in water relations and in some physiological functions of bean under very light osmotic shock induced by polyethylene glycol

Bean plantlets ( Phaseolus vulgaris L. cv. Topcrop) were stressed at the age of 16–18 days by gradual (2–8%) or abrupt addition of 6% (w/v) polyethylene glycol Mw 6000 (PEG 6000) to Hoagland solution. Leaf conductance, photosynthesis, internal CO₂ partial pressure (C_i), relative water content (RWC), water content/dry weight (H₂O/DW), apoplastic PEG concentrations and weight of leaves, stems and roots were determined. Leaf conductance, photosynthesis and C_i were determined on non-detached primary leaves, and leaf potentials (water, osmotic and turgor potentials) were investigated in freshly detached (non-rehydrated) primary leaves, both in treated and control plants; RWC and osmotic potential were also assessed at the null turgor point. Low PEG 6000 concentrations induced early and evident decrease in leaf conductance and photosynthesis, whereas C_i decreased only moderately and tended to recover during advanced stress. There were moderate though significant decreases in RWC and H₂O/DW, no change or increases in water potential, no significant changes in osmotic potential and a moderate but significant increase in turgor potential. Even when referred to null turgor point, RWC significantly decreased and osmotic potential was unchanged. It was concluded that apoplastic PEG 6000 accumulation at evaporating sites would account for the early decrease in conductance which would also justify the unchanged or the prevalent increase in water potential and turgor potential. The subsequent PEG diffusion and concentration in the leaf apoplastic water would have induced the RWC and H₂O/DW decrease and the final turgor flexion documented.     

The evolutionary relationship between stomatal mechanism, crassulacean acid metabolism and C4 photosynthesis

Abstract. All of the features of crassulacean acid metabolism (CAM) and most characteristics of C₄ photosynthesis are exhibited by stomatal guard cells. It is proposed that CAM and possibly also C₄ photosynthesis result from the expression in photosynthetic cells of genetic information which is expressed only in guard cells of C₃ plants.     

Seasonal differences in photosynthesis between the C3 and C4 subspecies of Alloteropsis semialata are offset by frost and drought

The regional abundance of C₄ grasses is strongly controlled by temperature, however, the role of precipitation is less clear. Progress in elucidating the direct effects of photosynthetic pathway on these climate relationships is hindered by the significant genetic divergence between major C₃ and C₄ grass lineages. We addressed this problem by examining seasonal climate responses of photosynthesis in Alloteropsis semialata , a unique grass species with both C₃ and C₄ subspecies. Experimental manipulation of rainfall in a common garden in South Africa tested the hypotheses that: (1) photosynthesis is greater in the C₄ than C₃ subspecies under high summer temperatures, but this pattern is reversed at low winter temperatures; and (2) the photosynthetic advantage of C₄ plants is enhanced during drought events. Measurements of leaf gas exchange over 2 years showed a significant photosynthetic advantage for the C₄ subspecies under irrigated conditions from spring through autumn. However, the C₄ leaves were killed by winter frost, while photosynthesis continued in the C₃ plants. Unexpectedly, the C₄ subspecies also lost its photosynthetic advantage during natural drought events, despite greater water-use efficiency under irrigated conditions. This study highlights previously unrecognized roles for climatic extremes in determining the ecological success of C₃ and C₄ grasses.     

Diel leaf growth cycles in Clusia spp. are related to changes between C3 photosynthesis and crassulacean acid metabolism during development and during water stress

This study reports evidence that the timing of leaf growth responds to developmental and environmental constraints in Clusia spp. We monitored diel patterns of leaf growth in the facultative C₃-crassulacean acid metabolism (CAM) species Clusia minor and in the supposedly obligate CAM species Clusia alata using imaging methods and followed diel patterns of CO₂ exchange and acidification. Developing leaves of well-watered C. minor showed a C₃-like diel pattern of gas exchange and growth, with maximum relative growth rate (RGR) in the early night period. Growth slowed when water was withheld, accompanied by nocturnal CO₂ exchange and the diel acid change characteristic of CAM. Maximum leaf RGR shifted from early night to early in the day when water was withheld. In well-watered C. alata , similar changes in the diel pattern of leaf growth occurred with the development of CAM during leaf ontogeny. We hypothesize that the shift in leaf growth cycle that accompanies the switch from C₃ photosynthesis to CAM is mainly caused by the primary demand of CAM for substrates for nocturnal CO₂ fixation and acid synthesis, thus reducing the availability of carbohydrates for leaf growth at night. Although the shift to leaf growth early in the light is presumably associated with the availability of carbohydrates, source–sink relationships and sustained diurnal acid levels in young leaves of Clusia spp. need further evaluation in relation to growth processes.     

Rapid-cycling CAM; an hypothetical variant of photosynthetic metabolism

The currently recognized forms of CAM photosynthesis do not represent all possible variants on the theme. It is predicted here that there may exist an undiscovered variant in which the CO₂-acquiring and CO₂-reducing phases of CAM alternate over time periods shorter (possibly much shorter) than the diel cycle. The process would occur entirely in the photoperiod and may be likened to a form of C₄ photosynthesis in which the same photosynthetic cell alternates between performing the functions of a C₄ mesophyll and C₄ bundle sheath cell. Rapid-cycling CAM, which would be unlikely to be detected by methods commonly used to measure photosynthesis, could provide a CO₂-concentrating mechanism in both unicellular and multicellular plants.     

Sap salinity effects on xylem conductivity in two mangrove species

Xylem sap salinity and conductivity were examined in two mangrove ecosystem tree species . For Avicennia germinans , extracted xylem sap osmotic potentials ranged from −0.24 to −1.36 MPa versus −0.14 to −0.56 MPa for Conocarpus erectus. Xylem sap of Conocarpus did not vary in osmotic potential between sites nor between predawn and midday. In Avicennia , values were more negative at midday than predawn, and also more negative at hypersaline than hyposaline sites. After removing embolisms, specific conductivity ( K _s) was measured as a function of salinity of the artificial xylem sap perfusion. For both species the lowest K _s values, about 70% of the maximum K _s, were obtained when stems were perfused with deionized water (0 m m ; 0.0 MPa) or with a 557-m m saline solution (−2.4 MPa). Higher K _s values were obtained in the range from −0.3 to −1.2 MPa, with a peak at −0.82 ± 0.08 MPa for Avicennia and −0.75 ± 0.08 MPa for Conocarpus . The variations in K _s values with minima both at very low and very high salt concentrations were consistent with published results for swelling and shrinking of synthetic hydrogels, suggesting native hydrogels in pit membranes of vessels could help regulate conductivity.     

Comparative ecophysiology of C3 and C4 plants

Abstract. In this review we relate the physiological significance of C₄ photosynthesis to plant performance in nature. We begin with an examination of the physiological consequences of the C₄ pathway on photosynthesis, then discuss the ecophysiological performance of C₄ plants in contrasting environments. We then compare the performance of C₃ and C₄ plants when they occur together in similar habitats, and finally discuss the distribution of C₄ photosynthesis with respect to the physical environment, phylogeny, and life form.     

Effects of chloride and sodium on gas exchange parameters and water relations of Citrus plants

Gas exchange parameters, water relations and Na⁺/Cl^- content were measured on leaves of one-year-old sweet orange ( Citrus sinensis [L.] Osbeck cv. Hamlin) seedlings grown at increasing levels of salinity. Different salts (NaCl, KCl and NaNO₃) were used to separate the effects of Cl⁻ and Na⁺ on the investigated parameters. The chloride salts reduced plant dry weight and increased defoliation. Accumulation of Cl⁻ in the leaf tissue caused a sharp reduction in photosynthesis and stomatal conductance. By contrast, these parameters were not affected by leaf Na⁺ concentrations of up to 478 m M in the tissue water. Leaf water potentials reached values near −1.8 MPa at high NaCl and KCl supplies. This reduction was offset by a decrease in the osmotic potential so that turgor was maintained at or above control values. The changes in osmotic potential were closely correlated with changes in leaf proline concentrations. Addition of Ca²⁺ (as calcium acetate) increased growth and halved defoliation of salt stressed plants. Furthermore, calcium acetate decreased the concentration of Cl⁻ and Na⁺ in the leaves, and increased photosynthesis and stomatal conductance. Calcium acetate also counteracted the reductions in leaf water and osmotic potentials induced by salinity. In addition, calcium acetate inhibited the accumulation of proline in the leaves which affected the reduction in osmotic potential. These results indicate that adverse effects of salinity in Citrus leaves are caused by accumulation of chloride.     

Expression of Modes of Photosynthesis (C3, CAM) in Clusia criuva Camb. in a CerradolGallery Forest Transect

Abstract: The large majority of the Ca . 150 species of the neotropical shrub and tree genus Clusia have the potential to perform Crassulacean acid metabolism (CAM). They are either obligate CAM plants or C₃/CAM intermediate plants. Only a very small percentage of the plants studied so far are apparently obligate C₃ species. Among these was C. criuva , until recent laboratory studies showed that it may also have a certain CAM capacity under artificial stress conditions. Measurements of stomatal conductance (porometry) and chlorophyll fluorescence variables of C. criuva occurring along a transect from deep shade inside a gallery forest across the semi-shaded ecotone towards a cerrado and into the exposed cerrado itself in central Brazil now show that it can adapt its photosynthetic apparatus to effective performance of C₃ photosynthesis under highly different photosynthetic photon flux densities. In addition, however, it does have a certain potential for CAM and cannot be considered as a strictly obligate C₃ plant. Should a basic capacity for performing CAM be a general property of the genus, the quest for CAM traits in other remaining putatively obligate C₃ species of the genus ought to be pursued.     

Changes at panicle emergence in the water relations of a wetland and a dryland Japonica rice cultivar under wetland conditions

The diurnal and seasonal changes in plant water relations of two Japonica rice ( Oryza sativa L.) cultivars, Nipponbare and Tachiminori, were studied under flooded conditions at Kyoto University. The dryland cv. Tachiminori maintained higher predawn and midday leaf osmotic potentials relative to the wetland cv. Nipponbare during the vegetative stage, but the ranking was reversed after flowering. The relationship between leaf water potential and leaf osmotic potential showed that prior to panicle emergence Nipponbare was able to adjust osmotically to maintain turgor, whereas after heading there was little turgor maintenance. Tachiminori showed little difference in osmotic adjustment before and after panicle emergence. Fertilizer treatment during panicle development also helped to maintain the degree of osmotic adjustment in both cultivars.     

A mechanistic model for photosynthesis based on the multisubstrate ordered reaction of ribulose 1,5 bisphosphate carboxylase

Abstract. A mechanistic model of photosynthesis is developed based on the characteristics of ribulose 1,5-bisphosphate (RuBP) carboxylase and the assimilation of CO₂ as an ordered reaction with RuBP binding before CO₂. An equation is derived which considers the effects of light (for regeneration of RuBP) and CO₂. Taking values for the maximum turnover of RuBP carboxylase at substrate saturation, the maximum carboxylation efficiency (maximum increase in rate per increase in CO₂ concentration) and the minimum quantum requirement for the C₃ pathway, photosynthesis in the absence of photorespiration is simulated. In the model, at varying concentrations of CO₂, the efficiency of light utilization approaches a maximum value as photon flux density decreases. Similarly, with a given maximum carboxyation capacity, at varying photon flux densities the carboxylation efficiency approaches a constant maximum value (equal to V _max/ K _{m CO2}) as CO₂ is decreased. However, a decrease in the state of activation of RuBP carboxylase under low light results in a lower carboxylation efficiency. Limits on the rate of photosynthesis, as the maximum capacity for regeneration of RuBP is reduced relative to carboxylation potential, or as the maximum capacity of the carboxylase varies, are considered.     

Pressure-volume relationships in non-rehydrated tissue at various water deficits

Abstract. The purpose of this paper is to examine a technique for estimating the weight at full saturation (W_s) from pressure-volume (P-V) analysis of non-rehydrated plant tissue at various water deficits. Tissue samples are typically rehydrated prior to P-V analysis to determine W_s, necessary to calculate many tissue water parameters. However, several studies have indicated that artificial rehydration may significantly alter P-V relationships, such as the plateau effect, resulting in erroneous measurements of tissue elasticity and osmotic potentials. The results of this study suggest that linear regression of P-V data at and above the turgor loss point may be used to extrapolate W_s from non-rehydrated samples at various moisture deficits, thus eliminating the plateau effect and other potential rehydration problems. Determination coefficients and standard errors of the Y-intercept indicated a strong linear relationship between tissue fresh weight and water potential (Ψ), and a high degree of predictability of W_s in all but one of the species-treatment combinations evaluated in this study, despite predawn Ψ as low as - 1.0 MPa.     

A survey of photosynthetic carbon metabolism in 4 ecotypes of Phragmites australis in northwest China: Leaf anatomy, ultrastructure, and activities of ribulose 1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase and glycollate oxidase

Four ecotypes of Phragmites australis from different habitats in northwest China were examined to compare their photosynthetic characteristics. In a swamp ecotype, the Δ ¹³C value of leaf materials was −34.0‰, and bundle sheath cells contained a small amount of organelles and round-shaped chloroplasts, as being similar to typical C₃ plants. In a dune ecotype, the Δ ¹³C value was −20.9‰ and bundle sheath cells contained oval-shaped chloroplasts with poorly-developed grana. In light and heavy salt meadow ecotypes, Δ ¹³C values were −30.6‰ and −35.6‰, respectively. The shape of bundle sheath chloroplasts in the light salt meadow ecotype was intermediate between those of the swamp and dune ecotypes. Abundance of bundle sheath organelles in the heavy salt meadow ecotype was intermediate. The swamp ecotype had photosynthetic enzyme activities typical of C₃ type plants, whereas the dune ecotype had an increased activity of phosphoenolpyruvate carboxylase (PEPC), a key C₄ enzyme, and a decreased ribulose 1,5-bisphosphate carboxylase (Rubisco) activity. The light salt meadow and heavy salt meadow ecotypes had substantial activities of PEPC, which indicates potential for C₄ photosynthesis. These data suggest that this species evolved the C₃-like ecotype in swamp environments and the C₄-like C₃-C₄ intermediate in dune desert environments, and C₃-like C₃-C₄ intermediates in salt environments.     

Elevated pyruvate,orthophosphate dikinase (PPDK) activity alters carbon metabolism in C3 transgenic potatoes with a C4 maize PPDK gene

Changes in carbon metabolism and δ¹³C value of transgenic potato plants with a maize pyruvate,orthophosphate dikinase (PPDK; EC 2.7.9.1) gene are reported. PPDK catalyzes the formation of phospho enol pyruvate (PEP), the initial acceptor of CO₂ in the C₄ photosynthetic pathway. PPDK activities in the leases of transgenic potatoes were up to 5.4‐fold higher than those of control potato plants (wild‐type and treated control plants). In the transgenic potato plants, PPDK activity in leaves was negatively correlated with pyruvate content (r²= 0.81), and was positively correlated with malate content (r²= 0.88). A significant increase in the δ¹³C value was observed in the transgenic potato plants, suggesting a certain contribution of PEP carboxylase as the initial acceptor of atmospheric CO₂. These data suggest that elevated PPDK activity may alter carbon metabolism and lead to a partial operation of C₄‐type carbon metabolism. However, since parameters associated with CO₂ gas exchange were not affected, the altered carbon metabolism had only a small effect on the total photosynthetic characteristics of the transgenic plants.     

Photosynthetic carbon assimilation in the blue-green alga Coccochloris peniocystis

Abstract. Cells of the blue-green alga Coccochloris peniocystis , grown at air levels of CO₂, were exposed to [^l4C]bicarbonate in the light for periods of 0.5 to 2.0 s followed by exposure to unlabelled bicarbonate for longer periods of time in the light. The kinetics of tracer movement during these pulse-chase experiments demonstrate that the principal mechanism of CO₂ fixation in this alga is the C₃-pathway although an appreciable amount of the C₄ acid aspartate is found as one of the initial products of photosynthesis. Degradation of the labelled aspartate revealed that after 20 s of illumination, over 95% of the radioactivity was located in the β-carboxyl of this C₄ acid. This alga possesses little, if any, capacity for either the enzymatic decarboxylation of C₄ acids or the regeneration of phosphoenolpyruvate (PEP) from pyruvate mediated by the enzyme pyruvate, P_i dikinase. These data further demonstrate the lack of a functional C₄-pathway in this alga.     

The Genus Clusia L.: Molecular Evidence for Independent Evolution of Photosynthetic Flexibility

Abstract: Members of the Clusiaceae genus Clusia (tropical trees and shrubs) belong to the small group of dicotyledonous trees which are able to perform crassulacean acid metabolism (CAM). Most of the species are able to switch between C₃ and CAM modes of photosynthesis and only a few are restricted to either C₃ or CAM. In order to discover possible phylogenetic relationships with regard to the mode of photosynthesis, we investigated 17 species of the genus Clusia, and one species each of the Clusiaceae genera Oedematopus and Hypericum on the basis of internal transcribed spacer (ITS) sequences between the 18S and 26S coding regions of nuclear ribosomal DNA. Little length variation was detected in the ITS region of Clusia species. ITS1 sequences ranged from 255 to 260 bp and ITS2 sequences from 208 to 210 bp. Neighbour-joining and parsimony analyses of these sequences resulted in considerable differences in cluster formation when compared to a classification based on morphological characteristics. The molecular data also give no indication of a group-specific evolution of modes of photosynthesis, i.e., C₃ and CAM. We thus conclude that CAM has evolved independently several times within the genus Clusia.     

C4 photosynthesis at low temperatures

Abstract. C₄ plants grown in optimum conditions are, by comparison to C₃, capable of higher maximum dry-matter yields and greater efficiencies of water and nitrogen use, yet they are rare outside the subtropics. Both latitudinal and altitudinal limits of C₄ distributions correlate most closely with a mean minimum temperature of 8-10°C during the period of active growth. The possibility that the C₄ process is inherently incapable of functioning at low temperatures is examined. The reversible effects of chilling on the quantum efficiency of C₄ photosynthesis and the functioning of the individual steps in the C₄ cycle are examined. Chilling also produces an irreversible loss of capacity to assimilate CO₂ which is directly proportional to the light received during chilling. It is suggested that the reversible reduction in capacity to assimilate CO₂ and the lack of an alternative pathway for the utilization of lightgenerated reducing power may make C₄ species more prone to chilling-dependent photoinhibition. Laboratory studies and limited field observations suggest that this damage would be most likely to occur during photosynthetic induction at the temperatures and light levels encountered on clear, cool mornings during the spring and early summer in cool climates. Even those C₄ species occurring naturally in cool climates do not appear fully capable of tolerating these conditions; indeed their growth patterns suggest that they may be adapted by avoiding 'rather than enduring' such conditions.     

The appearance of a new molecular species of phosphoenolpyruvate carboxylase (PEPC) and the rapid induction of CAM in Sedum telephium L.

Abstract. When detached leaves of Sedum telephium are incubated in the absence of water, a rapid switch from C₃ photosynthesis to CAM (as indicated by the onset of day-to-night fluctuations in titratable acidity. ΔH⁺) occurs within the first dark period. The C₃-CAM switch in intact plants occurs within 3 5d. Extractable activity of phospho enol pyruvate carboxylase (PEPC) increases five-fold in intact plants during CAM induction; however, during rapid CAM induction in detached leaves, there is only a very small increase in PEPC activity. Fractionation by anion exchange chromatography of crude extracts from leaves of intact plants subjected to water deficit shows that CAM induction is associated with the appearance of a molecular species of PEPC termed PEPC I. PEPC I is barely detectable in well-watered plants which are not performing CAM. The major form in these plants is termed PEPC II. In leaves from intact plants, there is a significant positive correlation between PEPC I activity and ΔH⁺ during a period of increasing water deficit. PEPC I exhibits day to night fluctuations in malate sensitivity, being less sensitive during the dark period. In contrast, PEPC II is more sensitive to inhibition by malate and has no day to night fluctuation in sensitivity. In detached leaves deprived of water, a small increase in PEPC I capacity is detected at the end of the first dark period (20 h after the start of treatment). The results suggest that PEPC I is required for attainment of maximum nocturnal malic acid synthesis. There is a significant correlation between leaf water status (relative water content), ΔH⁺, total PEPC and PEPC I activity suggesting that the internal water status of the plant may be a trigger for CAM induction. Abscisic acid applied to detached leaves does not cause nocturnal acidification.