Effects of water stress and high temperature on photosynthetic rates of two species of Prosopis

Effects of Water Deficits on Prosopis tamarugo Growth, Water Status and Stomata Functioning

The effect of water deficit on growth, water status and stomatal functioning of Prosopis tamarugo was investigated under controlled water conditions. The study was done at the Antumapu Experiment Station of the University of Chile. Three levels of water stress were tested: (i) well-watered (WW), (ii) medium stress intensity (low-watered (LW)) and (iii) intense stress (non-watered (NW)), with 10 replicates each level. All growth parameters evaluated, such as twig growth, specific leaf area and apical dominance index, were significantly decreased under water deficit. Tamarugo twig growth decreased along with twig water potential. The stomatal conductance and CO₂ assimilation decreased significantly under the water deficit condition. Tamarugo maintained a high stomatal conductance at low leaf water potential. In addition, tamarugo reduced its leaf area as a strategy to diminish the water demand. These results suggest that, despite a significant decrease in water status, tamarugo can maintain its growth at low leaf water potential and can tolerate intense water deficit due to a partial stomatal closing strategy that allows the sustaining of CO₂ assimilation in the condition of reduced water availability.

Surface Canopy Position Determines the Photosystem II Photochemistry in Invasive and Native Prosopis Congeners at Sharjah Desert, UAE

Plants have evolved photoprotective mechanisms in order to counteract the damaging effects of excess light in hyper-arid desert environments. We evaluated the impact of surface canopy positions on the photosynthetic adjustments and chlorophyll fluorescence attributes (photosystem II photochemistry, quantum yield, fluorescence quenching, and photon energy dissipation), leaf biomass and nutrient content of sun-exposed leaves at the south east (SE canopy position) and shaded-leaves at the north west (NW canopy position) in the invasive Prosopis juliflora and native Prosopis cineraria in the extreme environment (hyper-arid desert area, United Arab Emirates (UAE)). The main aim of this research was to study the photoprotection mechanism in invasive and native Prosopis congeners via the safe removal—as thermal energy—of excess solar energy absorbed by the light collecting system, which counteracts the formation of reactive oxygen species. Maximum photosynthetic efficiency (Fv/Fm) from dark-adapted leaves in P. juliflora and P. cineraria was higher on NW than SE canopy position while insignificant difference was observed within the two Prosopis congeners. Greater quantum yield was observed in P. juliflora than P. cineraria on the NW canopy position than SE. With the change of canopy positions from NW to SE, the reduction of the PSII reaction center activity in the leaves of both Prosopis congeners was accelerated. On the SE canopy position, a significant decline in the electron transport rate (ETR) of in the leaves of both Prosopis congeners occurred, which might be due to the blockage of electron transfer from QA to QB on the PSII acceptor side. On the SE canopy position; Prosopis leaves dissipated excess light energy by increasing non-photochemical quenching (NPQ). However, in P. cineraria, the protective ability of NPQ decreased, which led to the accumulation of excess excitation energy (1 − qP)/NPQ and the aggravation of photoinhibition. The results also explain the role of different physiological attributes contributing to invasiveness of P. juliflora and to evaluate its liaison between plasticity of these characters and invasiveness.

Controversies in Midday Water Potential Regulation and Stomatal Behavior Might Result From the Environment, Genotype, and/or Rootstock: Evidence From Carménère and Syrah Grapevine Varieties

Controversies exist regarding the iso/anisohydric continuum for classifying plant water-use strategies. Isohydricity has been argued to result from plant-environment interaction rather than it being an intrinsic property of the plant itself. Discrepancies remain regarding the degree of isohydricity (σ) of plants and their threshold for physiological responses and resistance to drought. Thus, the aim of this study was to evaluate the isohydricity of the grapevine varieties Syrah and Carménère under a non-lethal water deficit progression from veraison from two different locations, the Cachapoal Valley (CV) and Maipo Valley (MV), in central Chile and with different rootstock only in Syrah. For this purpose, the midday stem water potential (Ψ_mds) regulation and stomatal responses to drought, leaf traits related to pressure-volume curves, stomatal sensitivity to ABA, cavitation threshold, and photosynthetic responses were assessed. A higher atmospheric water demand was observed in the CV compared to the MV, with lower Ψ_mds values in the former for both varieties. Also, the σ values in Carménère were 1.11 ± 0.14 MPa MPa^-1 and 0.68 ± 0.18 MPa MPa^-1 in the CV and MV, respectively, and in Syrah they were 1.10 ± 0.07 MPa MPa^-1 in the CV and 0.60 ± 0.10 MPa MPa^-1 in the MV. Even though similar variations in σ between locations in both varieties were evident, Carménère plants showed a conserved stomatal response to Ψ_mds in both study sites, while those of Syrah resulted in a higher stomatal sensitivity to Ψ_mds in the site of lower σ. Besides the differences in seasonal weather conditions, it is likely that the different rootstock and clonal variability of each season in Syrah were able to induce coordinated changes in σ, Ψ_gs12, and osmotic potential at full turgor (π₀). On the other hand, irrespective of the σ, and given the similarity between the π₀ and Ψ_gs12 in leaves before drought, it seems that π₀ could be a convenient tool for assessing the Ψ_mds threshold values posing a risk to the plants in order to aid the irrigation decision making in grapevines under controlled water deficit. Finally, water deficits in vineyards might irreversibly compromise the photosynthetic capacity of leaves.

Inter- and intrapopulation variation in the response of tree seedlings to drought: physiological adjustments based on geographical origin, water supply and species

Initiatives to restore natural ecosystems have had little success in arid and hyperarid ecosystems. In this context, the natural seedling establishment is particularly affected by drought patterns and climatic variability. Likewise, the effect of plant provenance on forest restoration success remains unclear, although previous studies have concluded that some seed locations might be better able to tolerate water stress. In this study, we examined the physiological mechanisms involved in the drought stress resistance of Prosopis tamarugo and Prosopis alba seedlings from different arid and hyperarid locations of the Atacama Desert in northern Chile. We measured the xylem water potential (Ψ), cuticular transpiration (E_c), specific leaf area (SLA) and pressure-volume curves at the intrapopulation and interpopulation levels of seedlings of both species subjected to three drought-induced treatments. In addition, plant characteristics such as seedling height (Sh), stem diameter (Sd), leaf biomass (Lb), root biomass (Rb) and seedling survival (Ss) were measured during the treatments. Seedlings of most hyperarid habitats had the highest values of Ψ and water content relative to the turgor loss point, as well as decreased SLA, especially during the strongest drought treatment. Ψ was strongly correlated with Sh in both species, and soil humidity was correlated with Sd. This study highlights the high variability of physiological responses to water stress in both species at the interpopulation and intrapopulation levels, which provides us with a powerful seed selection tool for future reforestation programmes aimed at the early selection and genetic improvement of species of the Prosopis genus.

A self-photoprotection mechanism helps Stipa baicalensis adapt to future climate change

We examined the photosynthetic responses of Stipa baicalensis to relative long-term exposure (42 days) to the predicted elevated temperature and water availability changes to determine the mechanisms through which the plant would acclimate to future climate change. Two thermal regimes (ambient and +4 °C) and three irrigation levels (partial, normal and excess) were used in environmental control chambers. The gas exchange parameters, light response curves and A/Ci curves were determined. The elevated temperature and partial irrigation reduced the net photosynthetic rate due to a limitation in the photosynthetic capacity instead of the intercellular CO2 concentration. Partial irrigation decreased Rubisco activation and limited RuBP regeneration. The reduction in Vcmax increased with increasing temperature. Excess irrigation offset the negative effect of drought and led to a partial recovery of the photosynthetic capacity. Although its light use efficiency was restricted, the use of light and dark respiration by Stipa baicalensis was unchanged. We concluded that nonstomatal limitation was the primary reason for photosynthesis regulation in Stipa baicalensis under relative long-term climate change conditions. Although climate change caused reductions in the light use efficiency and photosynthetic rate, a self-photoprotection mechanism in Stipa baicalensis resulted in its high ability to maintain normal live activities.

Water Relations and Foliar Isotopic Composition of Prosopis tamarugo Phil., an Endemic Tree of the Atacama Desert Growing at Three Levels of Water Table Depth

Prosopis tamarugo Phil. is a strict phreatophyte tree species endemic to the "Pampa del Tamarugal", Atacama Desert. The extraction of water for various uses has increased the depth of the water table in the Pampa aquifers threatening its conservation. This study aimed to determine the effect of the groundwater table depth on the water relations of P. tamarugo and to present thresholds of groundwater depth (GWD) that can be used in the groundwater management of the P. tamarugo ecosystem. Three levels of GWD, 11.2 ± 0.3 m, 10.3 ± 0.3 m, and 7.1 ± 0.1 m, (the last GWD being our reference) were selected and groups of four individuals per GWD were studied in the months of January and July of the years 2011 through 2014. When the water table depth exceeded 10 m, P. tamarugo had lower pre-dawn and mid-day water potential but no differences were observed in minimum leaf stomatal resistance when compared to the condition of 7.1 m GWD; the leaf tissue increased its δ(13)C and δ(18)O composition. Furthermore, a smaller green canopy fraction of the trees and increased foliage loss in winter with increasing water table depth was observed. The differences observed in the physiological behavior of P. tamarugo trees, attributable to the ground water depth; show that increasing the depth of the water table from 7 to 11 m significantly affects the water status of P. tamarugo. The results indicate that P. tamarugo has an anisohydric stomatal behavior and that given a reduction in water supply it regulates the water demand via foliage loss. The growth and leaf physiological activities are highly sensitive to GWD. The foliage loss appears to prevent the trees from reaching water potentials leading to complete loss of hydraulic functionality by cavitation. The balance achieved between water supply and demand was reflected in the low variation of the water potential and of the variables related to gas exchange over time for a given GWD. This acclimation capacity of P. tamarugo after experiencing increases in GWD has great value for the implementation of conservation strategies. The thresholds presented in this paper should prove useful for conservation purposes of this unique species.

Assessment of physiological and biochemical responses, metal tolerance and accumulation in two eucalypt hybrid clones for phytoremediation of cadmium-contaminated waters

Eucalyptus is a promising species for ecological restoration but plant performances under environmental constraints need to be better investigated. In particular, the toxic effects of metals on this plant species are poorly described in the literature. In this work, morpho-physiological and biochemical responses to cadmium were analysed in two eucalypt genotypes (hybrid clones of Eucalyptus camaldulensis × Eucalyptus globulus ssp. bicostata J.B. Kirkp named Velino ex 7 and Viglio ex 358) exposed for 3 weeks to 50 μM CdSO4 under hydroponics. The two eucalypt clones showed a different sensitivity to the metal. The growth reduction caused by cadmium was less than 30% in clone Velino and about 50% in clone Viglio. Cadmium mostly accumulated in plant roots and, to a lesser extent, in stem, as highlighted by the low translocation factor (Tf) measured in both clones. Net photosynthesis measurement, chlorophyll fluorescence images, transpiration values and chlorophyll content revealed a cadmium-induced impairment of physiological processes at the leaf level, which was more evident in clone Viglio. Metal binding and antioxidative compound content was differentially affected by cadmium exposure in the two eucalypt clones. Particularly, the content of thiols like cysteine and glutathione, organic acids like oxalate and citrate, and polyamines were markedly modulated in plant organs by metal treatment and highlighted different defence responses between the clones. Cadmium tolerance and accumulation ability of the eucalypt clones were evaluated and the potential of E. camaldulensis for the reclamation of metal polluted-waters is discussed.

Acclimation of foliar respiration and photosynthesis in response to experimental warming in a temperate steppe in northern China

BACKGROUND

Thermal acclimation of foliar respiration and photosynthesis is critical for projection of changes in carbon exchange of terrestrial ecosystems under global warming.

METHODOLOGY/PRINCIPAL FINDINGS

A field manipulative experiment was conducted to elevate foliar temperature (Tleaf) by 2.07°C in a temperate steppe in northern China. Rd/Tleaf curves (responses of dark respiration to Tleaf), An/Tleaf curves (responses of light-saturated net CO2 assimilation rates to Tleaf), responses of biochemical limitations and diffusion limitations in gross CO2 assimilation rates (Ag) to Tleaf, and foliar nitrogen (N) concentration in Stipa krylovii Roshev. were measured in 2010 (a dry year) and 2011 (a wet year). Significant thermal acclimation of Rd to 6-year experimental warming was found. However, An had a limited ability to acclimate to a warmer climate regime. Thermal acclimation of Rd was associated with not only the direct effects of warming, but also the changes in foliar N concentration induced by warming.

CONCLUSIONS/SIGNIFICANCE

Warming decreased the temperature sensitivity (Q10) of the response of Rd/Ag ratio to Tleaf. Our findings may have important implications for improving ecosystem models in simulating carbon cycles and advancing understanding on the interactions between climate change and ecosystem functions.

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.