On the Use of Leaf Spectral Indices to Assess Water Status and Photosynthetic Limitations in Olea europaea L. during Water-Stress and Recovery

Diffusional limitations to photosynthesis, relative water content (RWC), pigment concentrations and their association with reflectance indices were studied in olive (Olea europaea) saplings subjected to water-stress and re-watering. RWC decreased sharply as drought progressed. Following rewatering, RWC gradually increased to pre-stress values. Photosynthesis (A), stomatal conductance (g_s), mesophyll conductance (g_m), total conductance (g_t), photochemical reflectance index (PRI), water index (WI) and relative depth index (RDI) closely followed RWC. In contrast, carotenoid concentration, the carotenoid to chlorophyll ratio, water content reflectance index (WCRI) and structural independent pigment index (SIPI) showed an opposite trend to that of RWC. Photosynthesis scaled linearly with leaf conductance to CO₂; however, A measured under non-photorespiratory conditions (A_1%O2) was approximately two times greater than A measured at 21% [O₂], indicating that photorespiration likely increased in response to drought. A_1%O2 also significantly correlated with leaf conductance parameters. These relationships were apparent in saturation type curves, indicating that under non-photorespiratory conditions, CO₂ conductance was not the major limitations to A. PRI was significant correlated with RWC. PRI was also very sensitive to pigment concentrations and photosynthesis, and significantly tracked all CO₂ conductance parameters. WI, RDI and WCRI were all significantly correlated with RWC, and most notably to leaf transpiration. Overall, PRI correlated more closely with carotenoid concentration than SIPI; whereas WI tracked leaf transpiration more effectively than RDI and WCRI. This study clearly demonstrates that PRI and WI can be used for the fast detection of physiological traits of olive trees subjected to water-stress.     

Variability in water use efficiency at the leaf level among Mediterranean plants with different growth forms

Assessing natural variability of leaf water use efficiency in plants adapted to extreme conditions of the Mediterranean climate represents an important step in the evaluation of the usefulness of some plant ecophysiological traits under water stress. Eleven Mediterranean species naturally inhabiting the Balearic Islands and corresponding to different growth forms (herbs, semi-deciduous shrubs, woody evergreen shrubs and woody evergreen semi-shrubs) were subject to progressive soil water depletion. Leaf intrinsic water use efficiency was measured by gas exchange at four different degrees of water stress. Under well watered conditions, differences in leaf intrinsic water use efficiency (A _N/g _s) among growth forms were limited to woody evergreen semi-shrubs, which presented the highest values. Under water stress conditions, differences became more evident, with a trend for an increase in A _N/g _s from woody evergreen shrubs, through semi-deciduous shrubs and herbaceous to woody evergreen semi-shrubs. The observed variation in A _N/g _s correlated with several physiological (leaf water potential, soil to leaf hydraulic conductance and stomatal conductance) and morphological (stomatal density) parameters, displaying a general relationship for all growth forms. This suggests that the capacity for withstanding water limitation is adaptive for all Mediterranean species. However, when A _N/g _s was related to leaf mass area, this relationship was not generally applicable, and depended on growth forms, suggesting that different growth forms display specific morphological adjustments in response to water shortage.     

Chemical and hydraulic signals regulate stomatal behavior and photosynthetic activity in maize during progressive drought

The objectives of this study were to investigate stomatal regulation in maize seedlings during progressive soil drying and to determine the impact of stomatal movement on photosynthetic activity. In well-watered and drought-stressed plants, leaf water potential (Ψ _leaf), relative water content (RWC), stomatal conductance (g _s), photosynthesis, chlorophyll fluorescence, leaf instantaneous water use efficiency (iWUE_leaf), and abscisic acid (ABA) and zeatin-riboside (ZR) accumulation were measured. Results showed that g _s decreased significantly with progressive drought and stomatal limitations were responsible for inhibiting photosynthesis in the initial stages of short-term drought. However, after 5 days of withholding water, non-stomatal limitations, such as damage to the PSII reaction center, became the main limiting factor. Stomatal behavior was correlated with changes in both hydraulic and chemical signals; however, changes in ABA and ZR occurred prior to any change in leaf water status. ABA in leaf and root tissue increased progressively during soil drying, and further analysis found that leaf ABA was negatively correlated with g _s (R ² = 0.907, p < 0.05). In contrast, leaf and root ZR decreased gradually. ZR in leaf tissue was positively correlated with g _s (R ² = 0.859, p < 0.05). These results indicate that ABA could induce stomatal closure, and ZR works antagonistically against ABA in stomatal behavior. In addition, the ABA/ZR ratio also had a strong correlation with g _s, suggesting that the combined chemical signal (the interaction between ABA and cytokinin) plays a role in coordinating stomatal behavior. In addition, Ψ _leaf and RWC decreased significantly after only 3 days of drought stress, also affecting stomatal behavior.     

Physiological and growth responses of two dogwoods to short-term drought stress and re-watering

Cornus florida and its cultivars have attracted many attentions by its colorful ornamental features. Suitable moisture condition is a major factor in the success of introduction. However, little is known about dogwoods drought adaptation to seasonal water deficit, and recovery potential from the following rainfall. In this paper, treatment of continuous drought lasted 19 days, followed by re-watering for 7 days was performed on 10-month-old seedlings of C. florida, comparing with native C. kousa. Meantime, well-watered seedlings of two species were regarded as controls. Soil relative water content (SRWC) in stressed seedlings of both dogwoods decreased significantly with drought stress prolonged, and recovered to the normal level after re-watering. As the response to drought stress, significant decline in internal carbon dioxide concentration (C_i), remarkable increment in intrinsic water use efficiency (WUE_i) in C. florida, significant increment in chlorophyll content in C. kousa, and notable decline in leaf relative water content (LRWC), maximum quantum efficiency of PSII photochemistry (F_v/F_m), photochemical quenching (qP), as well as significant increment in malondialdehyde (MDA) content, superoxide dismutase (SOD) activity, soluble sugar content (SSC) in both dogwoods were observed. However, most of physiological variables recovered to the level of control after re-watering. Furthermore, drought stress promoted root volume, root area, root biomass, whereas inhibited seedling height, basal diameter, aboveground biomass, resulting in increase of root/shoot ratio. Our findings indicate that, although C. florida has a weaker performance than C. kousa under drought stress, it can recover to the normal level after re-watering. These results suggest that C. florida and its cultivars possess drought adaptive potential for introducing to southern China.     

Global response patterns of plant photosynthesis to nitrogen addition: A meta‐analysis

A mechanistic understanding of plant photosynthetic response is needed to reliably predict changes in terrestrial carbon (C) gain under conditions of chronically elevated atmospheric nitrogen (N) deposition. Here, using 2,683 observations from 240 journal articles, we conducted a global meta‐analysis to reveal effects of N addition on 14 photosynthesis‐related traits and affecting moderators. We found that across 320 terrestrial plant species, leaf N was enhanced comparably on mass basis (N_mass, +18.4%) and area basis (N_area, +14.3%), with no changes in specific leaf area or leaf mass per area. Total leaf area (TLA) was increased significantly, as indicated by the increases in total leaf biomass (+46.5%), leaf area per plant (+29.7%), and leaf area index (LAI, +24.4%). To a lesser extent than for TLA, N addition significantly enhanced leaf photosynthetic rate per area (A_area, +12.6%), stomatal conductance (g_s, +7.5%), and transpiration rate (E, +10.5%). The responses of A_area were positively related with that of g_s, with no changes in instantaneous water‐use efficiency and only slight increases in long‐term water‐use efficiency (+2.5%) inferred from ¹³C composition. The responses of traits depended on biological, experimental, and environmental moderators. As experimental duration and N load increased, the responses of LAI and A_area diminished while that of E increased significantly. The observed patterns of increases in both TLA and E indicate that N deposition will increase the amount of water used by plants. Taken together, N deposition will enhance gross photosynthetic C gain of the terrestrial plants while increasing their water loss to the atmosphere, but the effects on C gain might diminish over time and that on plant water use would be amplified if N deposition persists.     

Influence of leaf water status on stomatal response to humidity,hydraulic conductance,and soil drought in Betula occidentalis

Whole-canopy measurements of water flux were used to calculate stomatal conductance (g _s ) and transpiration (E) for seedlings of western water birch (Betula occidentalis Hook.) under various soil-plant hydraulic conductances (k), evaporative driving forces (ΔN; difference in leaf-to-air molar fraction of water vapor), and soil water potentials (Ψ_s). As expected, g _s dropped in response to decreased k or Ψ_S, or increased ΔN(> 0.025). Field data showed a decrease in mid-day g _s with decreasing k from soil-to-petiole, with sapling and adult plants having lower values of both parameters than juveniles. Stomatal closure prevented E and Ψ from inducing xylem cavitation except during extreme soil drought when cavitation occurred in the main stem and probably roots as well. Although all decreases in g _s were associated with approximately constant bulk leaf water potential (ψ_l), this does not logically exclude a feedback response between Ψ_L and g _s . To test the influence of leaf versus root water status on g _s , we manipulated water status of the leaf independently of the root by using a pressure chamber enclosing the seedling root system; pressurizing the chamber alters cell turgor and volume only in the shoot cells outside the chamber. Stomatal closure in response to increased ΔN, decreased k, and decreased Ψ_S was fully or partially reversed within 5 min of pressurizing the soil. Bulk Ψ_L remained constant before and after soil pressurizing because of the increase in E associated with stomatal opening. When ΔN was low (i.e., < 0.025), pressurizing the soil either had no effect on g _s , or caused it to decline; and bulk Ψ_L increased. Increased Ψ_l may have caused stomatal closure via increased backpressure on the stomatal apparatus from elevated epidermal turgor. The stomatal response to soil pressurizing indicated a central role of leaf cells in sensing water stress caused by high ΔN, low k, and low Ψ_S. Invoking a prominent role for feedforward signalling in short-term stomatal control may be premature.     

Drought stress response in Jatropha curcas: Growth and physiology

Tolerance to drought remains poorly described for Jatropha curcas accessions from different geographical and climatic origins. To address this issue we studied the response of two J. curcas accessions, one from Indonesia (wet tropical climate) and the other from Cape Verde islands (semi-arid climate). Potted seedlings (with 71 days) of both accessions were subjected to continuous well watered conditions (control) or to a drought stress period followed by re-watering. To mimic natural conditions in which drought stress develops gradually, stress was imposed progressively by reducing irrigation (10% reduction every 2 days, on a weight base), for a period of 28 days, until a field capacity of 15% (maximum stress) was achieved, followed by one week under well-watered conditions. We measured soil and plant water status, growth and biomass partitioning, leaf morphology, leaf gas exchange and chlorophyll a fluorescence. Both accessions maintained high leaf relative water content (70–80%) even at maximum stress. Net photosynthesis (A_n) was not affected by mild to moderate stress but it abruptly dropped at severe stress. This was due to reduced stomatal conductance, which showed earlier decline than A_n. Plant growth (stem elongation, leaf emergence and total leaf area) was reduced, minimizing water loss, but no significant differences were found between accessions. Drought stress did not reduce chlorophyll contents but led to reduced chlorophyll a/b. Both accessions showed fast recovery of both stomatal and photochemical parameters suggesting a good tolerance to water stress. Both J. curcas accessions showed a-dehydration-avoidant behaviour, presenting a typical water saving strategy due to strict stomatal regulation, regardless of their provenance.     

Stomatal and photosynthetic responses of olive (Olea europaea L.) leaves to water deficits

The leaf gas exchange of mature olive trees (Olea europaea L.) was characterized over a wide range of water deficits in the field during 1998, in Cordoba, Spain. Leaf photosynthesis (A) and stomatal conductance (g_l) responded diurnally and seasonally to variations in tree water status and evaporative demand. In the absence of water stress, A and g_l were generally high during autumn and low in days of high vapour pressure deficits (VPD). Leaf A varied between 0 and 2 µmol m^?2 s^?1 under severe water deficits that lowered the stem water potential (Ψ_x) to ?8·0 MPa, but recovered rapidly following rehydration. Transpiration efficiency (TE) was curvilinearly related to VPD and not influenced by water deficits except in cases of severe water stress, where low TE values were observed at Ψ_x below ?4 MPa. Three models of leaf conductance were calibrated and validated with the experimental data; two were based on the model proposed by Leuning (L) and the other was derived from the widely used Jarvis (J) model. The L models performed better than the J model in two validation tests. The scatter of the predictions and the limited accuracy of all three models suggest that, in addition to the physiological and environmental variables considered, there are additional endogenous factors influencing the g_l of olive leaves.     

Linking the patterns in soil moisture to leaf water potential, stomatal conductance, growth, and mortality of dominant shrubs in the Florida scrub ecosystem

Patterns in soil moisture availability affect plant survival, growth and fecundity. Here we link patterns in soil moisture to physiological and demographic consequences in Florida scrub plants. We use data on different temporal scales to (1) determine critical soil moisture content that leads to loss of turgor in leaves during predawn measurements of leaf water status (Ψ _crit), (2) describe the temporal patterns in the distribution of Ψ _crit, (3) analyze the strength of relationship between rainfall and soil moisture content based on 8 years of data, (4) predict soil moisture content for 75 years of rainfall data, and (5) evaluate morphological, physiological and demographic consequences of spring 2006 drought on dominant shrubs in Florida scrub ecosystem in the light of water-uptake depth as determined by stable isotope analysis (δ¹⁸O). Based on 1998–2006 data, the soil moisture content at 50 cm depth explained significant variation in predawn leaf water potential of two dominant shrubs, Quercus chapmanii and Ceratiola ericoides (r ²?=?0.69). During 8 years of data collection, leaves attained Ψ _crit only during the peak drought of 2000 when the soil moisture fell below 1% by volume at 50 and 90 cm depth. Precipitation explained a significant variation in soil moisture content (r ²?=?0.62). The patterns in predicted soil moisture for 75 year period, suggested that the frequency of drought occurrence has not increased in time. In spring 2006, the soil reached critical soil moisture levels, with consequences for plant growth and physiological responses. Overall, 24% of plants showed no drought-induced damage, 51% showed damage up to 50%, 21% had intense leaf shedding and 2% of all plants died. Over the drought and recovery period (May–October 2006), relative height growth was significantly lower in plants with greater die-back. All species showed a significant depression in stomatal conductance, while all but deep-rooted palms Sabal etonia and Serenoa repens showed significantly lower predawn (Ψ _pd) and mid-day (Ψ _md) leaf water potential in dry compared to wet season. Plants experiencing less severe die-back exhibited greater stomatal conductance, suggesting a strong relationship between physiology and morphology. Based on results we suggest that the restoration efforts in Florida scrub should consider the soil moisture requirements of key species.     

Effects of NaCl salinity and water stress on growth and leaf water relations of Asteriscus maritimus plants

Potted plants of Asteriscus maritimus (L.) Less were submitted to water stress (during two consecutive cycles, irrigation water was withheld for 5 days followed by a recovery period of 25 days) and saline stress (150 days of exposure to 0, 70 and 140 mM NaCl daily irrigation) in order to assess the effect on leaf water relations and growth parameters. Plants under saline and water stress conditions showed lower biomass and an early reduction in leaf expansion growth. Both stresses promoted a substantial degree of stomatal regulation; but, in spite of this, the plants showed signs of leaf tissue dehydration, decreases in RWC and Ψ_pd values. However, salt-treated plants, developed a NaCl inclusion mechanisms, underwent osmotic adjustment, which was able to maintain leaf turgor. Under both stress conditions g_l was independent to plant water status in the range between –0.8 and 1.0 MPa. Under water stress conditions, midday leaf water potential showed a threshold value (around −1.1 MPa), below which leaf conductance remained constant. In the salt-treated plants, the gradual closure of the stomata over a wide range of Ψ_md may be important in maintaining some level of photosynthesis.     

Disentangling the contributions of ontogeny and water stress to photosynthetic limitations in almond trees

Very few studies have attempted to disentangle the respective role of ontogeny and water stress on leaf photosynthetic attributes. The relative significance of both effects on photosynthetic attributes has been investigated in leaves of field‐grown almond trees [Prunus dulcis (Mill.) D. A. Webb] during four growth cycles. Leaf ontogeny resulted in enhanced leaf dry weight per unit area (W_a), greater leaf dry‐to‐fresh weight ratio and lower N content per unit of leaf dry weight (N_w). Concomitantly, area‐based maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), mesophyll conductance to CO₂ diffusion (gm)′ and light‐saturated net photosynthesis (A_max) declined in both well‐watered and water‐stressed almond leaves. Although g_m and stomatal conductance (g_s) seemed to be co‐ordinated, a much stronger coordination in response to ontogeny and prolonged water stress was observed between g_m and the leaf photosynthetic capacity. Under unrestricted water supply, the leaf age‐related decline of A_max was equally driven by diffusional and biochemical limitations. Under restricted soil water availability, A_max was mainly limited by g_s and, to a lesser extent, by photosynthetic capacity and g_m. When both ontogeny and water stress effects were combined, diffusional limitations was the main determinant of photosynthesis limitation, while stomatal and biochemical limitations contributed similarly.     

Does elevated CO2 protect photosynthesis from damage by high temperature via modifying leaf water status in maize seedlings?

We hypothesized that decreased stomatal conductance (g _s) at elevated CO₂ might decrease transpiration (E), increase leaf water potential (Ψ_W), and thereby protect net photosynthesis rate (P _N) from heat damage in maize (Zea mays L) seedlings. To separate long-term effects of elevated CO₂, plants grew at either ambient CO₂ or elevated CO₂. During high-temperature treatment (HT) at 45°C for 15 min, leaves were exposed either to ambient CO₂ (380 μmol mol^?1) or to elevated CO₂ (560 μmol mol^?1). HT reduced P _N by 25 to 38% across four CO₂ combinations. However, the g _s and E did not differ among all CO₂ treatments during HT. After returning the leaf temperature to 35°C within 30 min, g _s and E were the same or higher than the initial values. Leaf water potential (Ψ_W) was slightly lower at ambient CO₂, but not at elevated CO₂. This study highlighted that elevated CO₂ failed in protecting P _N from 45°C via decreasing g _s and Ψ_W.     

Differential response of two almond rootstocks to chloride salt mixtures in the growing medium

It was examined how essential cations, Ca²⁺ and K⁺, can mitigate the toxic effects of NaCl on two different almond species (Prunus amygdalus Batsch) rootstocks, Garnem (GN15) and Bitter Almond. The tree growth parameters (water potential (Ψ_w), gas exchange, nutrient uptake) and leaf chlorophyll (Chl) content were measured in control and NaCl-treated plants with or without KCl or CaCl₂ supplements. The addition of CaCl₂ and KCl to Bitter Almond trees reduced their dry weight, shoot growth and leaf number although net photosynthetic assimilation rate (A) was not affected. These results indicated that changing of photo-assimilates flux to proline and/or soluble sugars synthesis may help to increase leaf Ψ_w. The Garnem trees also did not respond to the CaCl₂ and KCl addition indicating that the plants are already getting enough of these two cations (C^a2+ and K⁺). In both rootstocks, NaCl in the medium reduced growth attributes, Ψ_w, A, stomatal conductance (g_s), and leaf Chl content. When CaCl₂ and KCl fertilizers were added together with NaCl to Bitter Almond trees, leaf K⁺ and Ca²⁺ contents increased while Na⁺ and Cl^– decreased leading to higher Ca/Na and K/Na ratios, but shoot growth was not improved and even declined compared to NaCl-treated trees. It appears that the addition of salts further aggravated osmotic stress as indicated by the accumulation of proline and soluble sugars in leaf tissues. The addition of KCl or CaCl₂ to NaCl-treated GN15 trees did not increase A, leaf Ψ_w, and shoot growth but improved ionic balances as indicated by higher Ca/Na and K/Na ratios. The reduction in A was mainly due to non-stomatal limitations in GN15, possibly due to the degradation of Chl a, unlike Bitter Almond, for which the reduction of A was due to stomata closure. The improvement in ionic balances and water status of Bitter Almond trees in response to addition of KCl or CaCl₂ was apparently offset by a high sensitivity to Cl^–; therefore, no-chloride salts should be the preferred forms of fertilizers for this rootstock. Both rootstocks were sensitive to soil salinity and cation supplements were of limited value in mitigating the effect of excessive salt concentrations.     

Improvement of growth conditions and gas exchange of<Emphasis Type="Italic"> Fagus sylvatica</Emphasis> L. seedlings planted below a recently thinned<Emphasis Type="Italic"> Pinus sylvestris</Emphasis> L. stand

Abstract Gas exchange and growth of beech seedlings planted in the understory of a recently thinned pinewood were recorded for 2 years. Relative irradiance was assessed by hemispherical photographs taken just after the thinning. Predawn water potential (_pd), daily gas exchange and chlorophyll fluorescence were measured several times during the two growing seasons. Maximum values of photosynthesis (A _max) and stomatal conductance to water vapour (g _wvmax) were established from daily data. Maximum quantum efficiency of PS II was recorded at dawn by taking the variable to maximum chlorophyll fluorescence ratio on dark adapted leaves (F _v/F _m). In the middle of each summer, leaf nitrogen content and leaf mass per area were evaluated, and height growth and basal area increment were recorded at the end of the season. The thinning treatment removed half the trees and generated around 10% more available relative irradiance (GLF). This was followed by an increase in net photosynthesis at saturating PPFD (A _sat) and in maximum stomatal conductance to water vapour (g _wvmax). Moreover, specific leaf mass (SLM) and mass based nitrogen content (N_m) showed higher values for seedlings in the thinned stand. In both years, a positive relationship was established between the area based nitrogen content (N_a) and maximum net photosynthesis (A _max). In 1998, a year with a dry summer, seedlings suffered a significant drop in daily A _max irrespective of the thinning regime. This was a response to an increase in stomatal limitation to net photosynthesis, g _wvmax reaching the lowest value on dates with the highest drought. A lack of decrease of Fv/Fm confirmed the absence of significant non-stomatal limitation to A as a consequence of photoinhibition after opening the pinewood. A higher maximum quantum efficiency of open PS II centres (Fv/Fm) was registered in seedlings in the thinned stand. The significance of the differences between the treatments was stronger in the second year after thinning. In 1999, a year with frequent summer storms, water availability increased for seedlings growing under the thinned pinewood. Overall, the reduced pine overstory had a positive effect on physiological responses of beech seedlings, which was translated into improved seedling growth.     

Effect of drought on ear and flag leaf photosynthesis of two wheat cultivars differing in drought resistance

We investigated net photosynthetic rate (P_N) of ear and two uppermost (flag and penultimate) leaves of wheat cultivars Hongmangmai (drought resistant) and Haruhikari (drought sensitive) during post-anthesis under irrigated and non-irrigated field conditions. The P_Nof ear and flag leaf were significantly higher and less affected by drought in Hongmangmai than in Haruhikari. The rate of reduction in stomatal conductance (g_s) was similar for the two cultivars, but intercellular CO₂concentration (C_i) in the flag leaf of Hongmangmai was lower than that of Haruhikari in non-irrigated treatment. No differences were observed in leaf water potential (ψ₁) and osmotic adjustment of the flag leaf of the cultivars. These results imply that differences in photosynthetic inhibition on the flag leaf at low leaf ψ₁between the cultivars were primarily due to non-stomatal effects. Hence the main physiological factor associated with yield stability of Hongmangmai under drought stress may be attributed to the capacity for chloroplast activity in the flag leaf, which apparently allows sustained P_Nof flag leaf during grain filling under drought stress. The higher P_Nof ear in Hongmangmai under drought could also be related to its drought resistance.     

Peanut Photosynthesis Under Drought and Re-Watering

The photosynthetic response of three Arachis hypogaea L. cultivars (57-422, 73-30, and GC 8-35) grown for two months was measured under water available conditions, severe water stress, and 24, 72, and 93 h following re-watering. At the end of the drying cycle, all the cultivars reached dehydration, relative water content (RWC) ranging between 40 and 50 %. During dehydration, leaf stomatal conductance (g _s), transpiration rate (E), and net photosynthetic rate (P _N) decreased more in cvs. 57-422 and GC 8-35 than in 73-30. Instantaneous water use efficiency (WUE_i) and photosynthetic capacity (P _max) decreased mostly in cv. GC 8-35. Except in cv. GC 8-35, the activity of photosystem 1 (PS1) was only slightly affected. PS2 and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were the main targets of water stress. After re-watering, cvs. 73-30 and GC 8-35 rapidly regained g _s, E, and P _N activities. Twenty-four hours after re-watering, the electron transport rates and RuBPCO activity strongly increased. P _N and P _max fully recovered later. Considering the different photosynthetic responses of the studied genotype, a general characterisation of the interaction between water stress and this metabolism is presented.     

Physiological responses of tagasaste to a progressive drought in its native environment on the Canary Islands

Diurnal courses of gas exchange, chlorophyll fluorescence, shoot water potential (Ψ) and leaf relative water content (RWC_leaf) were recorded in Chamaecytisus proliferus (L. fil.) Link ssp. proliferus var. palmensis (Christ) (tagasaste) growing in natural conditions in the North-West slope of Tenerife, Canary Islands. During the studied period (April–July 2000), the soil relative water content (RWC_soil) progressively decreased, while the air vapour pressure deficit increased. As a consequence a decrease in Ψ and RWC_leaf took place as well as a decrease in stomatal conductance (g_s) and CO₂ assimilation rate (A). These characteristics, typical of an anisohydric plant, allowed this species to endure a mild drought.Photochemical efficiency of PSII (φ_PSII) followed the inverse pattern to that observed for the photosynthetic photon flux density (PPFD), decreasing at midday and recovering during the late afternoon. This recovering at the end of each day, and the constant values of the maximum quantum yield of PSII photochemistry (F_v/F_m) during the studied period, indicate that there was no permanent damage to the photosynthetic apparatus due to mild water stress.     

Interactions of Elevated CO2 Concentration and Drought Stress on Photosynthesis in Eucalyptus Cladocalyx F. Muell

Response of net photosynthetic rate (P _N), stomatal conductance (g _s), intercellular CO₂ concentration (c _i), and photosynthetic efficiency (F_v/F_m) of photosystem 2 (PS2) was assessed in Eucalyptus cladocalyx grown for long duration at 800 (C₈₀₀) or 380 (C₃₈₀) µmol mol^-1 CO₂ concentration under sufficient water supply or under water stress. The well-watered plants at C₈₀₀ showed a 2.2 fold enhancement of P _N without any change in g _s. Under both C₈₀₀ and C₃₈₀, water stress decreased P _N and g _s significantly without any substantial reduction of c _i, suggesting that both stomatal and non-stomatal factors regulated P _N. However, the photosynthetic efficiency of PS2 was not altered.     

Photosynthesis and the Enzymes of Photosynthetic Carbon Reduction Cycle in Mulberry During Water Stress and Recovery

Net photosynthetic rate (P _N), stomatal conductance (g _S), transpiration rate (E), intercellular CO₂ concentration (C _i), leaf water potential (_w), leaf area, chlorophyll (Chl) content, and the activities of photosynthetic carbon reduction cycle (PCR) enzymes in two mulberry (Morus alba L.) cultivars (drought tolerant Anantha and drought sensitive M-5) were studied during water stress and recovery. During water stress, P _N, g _S, and E declined whereas C _i increased. P _N, g _S, and E were less affected in Anantha than in M-5, which indicates tolerance nature of Anantha over M-5. Activities of ribulose-5-phosphate kinase, NAD- and NADP-glyceraldehyde-3-phosphate dehydrogenases, and 3-phosphoglycerate kinase decreased with increasing stress in both the cultivars. The enzyme activities less affected in tolerant (Anantha) than in sensitive cultivar (M-5) were restored after re-watering to almost initial values in both the cultivars. Re-watering of the plants led to an almost complete recovery of P _N, E, and g _S, indicating that a short-term stress brings about reversible effect in these two cultivars of mulberry.