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Summary: Xylem hydraulic failure (HF) has been identified as a ubiquitous factor in triggering drought‐induced tree mortality through the damage induced by the progressive dehydration of plant living cells. However, fundamental evidence of the mechanistic link connecting xylem HF to cell death has not been identified yet. The main aim of this study was to evaluate, at the leaf level, the relationship between loss of hydraulic function due to cavitation and cell death under drought conditions and discern how this relationship varied across species with contrasting resistances to cavitation.Drought was induced by withholding water from potted seedlings, and their leaves were sampled to measure their relative water content (RWC) and cell mortality. Vulnerability curves to cavitation at the leaf level were constructed for each species.An increment in cavitation events occurrence precedes the onset of cell mortality. A variation in cells tolerance to dehydration was observed along with the resistance to cavitation.Overall, our results indicate that the onset of cellular mortality occurs at lower RWC than the one for cavitation indicating the role of cavitation in triggering cellular death. They also evidenced a critical RWC for cellular death varying across species with different cavitation resistance. [ABSTRACT FROM AUTHOR]

Summary: Catastrophic failure of the water transport pathway in trees is a principal mechanism of mortality during extreme drought. To be able to predict the probability of mortality at an individual and landscape scale we need knowledge of the time for plants to reach critical levels of hydraulic failure.We grew plants of eight species of Eucalyptus originating from contrasting climates before allowing a subset to dehydrate. We tested whether a trait‐based model of time to plant desiccation tcrit, from stomatal closure gs90 to a critical level of hydraulic dysfunction Ψcrit is consistent with observed dry‐down times.Plant desiccation time varied among species, ranging from 96.2 to 332 h at a vapour‐pressure deficit of 1 kPa, and was highly predictable using the tcrit model in conjunction with a leaf shedding function. Plant desiccation time was longest in species with high cavitation resistance, strong vulnerability segmentation, wide stomatal‐hydraulic safety, and a high ratio of total plant water content to leaf area.Knowledge of tcrit in combination with water‐use traits that influence stomatal closure could significantly increase our ability to predict the timing of drought‐induced mortality at tree and forest scales. [ABSTRACT FROM AUTHOR]

Summary: Although primarily valued for their suitability for oceanographic applications and soil moisture estimation, microwave remote sensing observations are also sensitive to plant water content (Mw). Since Mw depends on both plant water status and biomass, these observations have the potential to be useful for a range of plant drought response studies. In this paper, we introduce the principles behind microwave remote sensing observations to illustrate how they are sensitive to plant water content and discuss the relationship between landscape‐scale Mw and common stand‐scale metrics, including plant‐scale relative water content, live fuel moisture content and leaf water potential. Lastly, we discuss how various sensor types can be leveraged for specific applications depending on the spatio‐temporal resolution needed. [ABSTRACT FROM AUTHOR]

Summary: Drought‐induced tree mortality has major impacts on ecosystem carbon and water cycles, and is expected to increase in forests across the globe with climate change. A large body of research in the past decade has advanced our understanding of plant water and carbon relations under drought. However, despite intense research, we still lack generalizable, cross‐scale indicators of mortality risk. In this Viewpoint, we propose that a more explicit consideration of water pools could improve our ability to monitor and anticipate mortality risk. Specifically, we focus on the relative water content (RWC), a classic metric in plant water relations, as a potential indicator of mortality risk that is physiologically relevant and integrates different aspects related to hydraulics, stomatal responses and carbon economy under drought. Measures of plant water content are likely to have a strong mechanistic link with mortality and to be integrative, threshold‐prone and relatively easy to measure and monitor at large spatial scales, and may complement current mortality metrics based on water potential, loss of hydraulic conductivity and nonstructural carbohydrates. We discuss some of the potential advantages and limitations of these metrics to improve our capacity to monitor and predict drought‐induced tree mortality. [ABSTRACT FROM AUTHOR]

The extent of an external water layer around moss tissue influences CO2 assimilation. Experiments on the desiccation-tolerant moss Syntrichia ruralis assessed the real-time dependence of the carbon and oxygen isotopic compositions of CO2 and H2 O in terms of moss water status and integrated isotope signals in cellulose., As external (capillary) water, and then mesophyll water, evaporated from moss tissue, assimilation rate, relative water content and the stable isotope composition of tissue water (δ18 OTW), and the CO2 and H2 O fluxes, were analysed. After drying, carbon (δ13 CC) and oxygen (δ18 OC) cellulose compositions were determined., During desiccation, assimilation and 13 CO2 discrimination increased to a maximum and then declined; δ18 OTW increased progressively by 8‰, indicative of evaporative isotopic enrichment. Experimental and meteorological data were combined to predict tissue hydration dynamics over one growing season. Nonsteady-state model predictions of δ18 OTW were consistent with instantaneous measurements., δ13 CC values suggest that net assimilation occurs at 25% of maximum relative water content, while δ18 OC data suggests that cellulose is synthesized during much higher relative water content conditions. This implies that carbon assimilation and cellulose synthesis (growth) may be temporally separated, with carbon reserves possibly contributing to desiccation tolerance and resumption of metabolism upon rehydration. [ABSTRACT FROM AUTHOR]

Sunflower seedlings (Helianthus annuus hybrid Select) were grown in a complete nutrient solution in the absence or presence of Cd2+ (10 and 20 μM). Analyses were performed to establish whether there was a differential effect of Cd2+ on mature and young leaves. After 7 d the growth parameters as well as the leaf area had decreased in both mature and young leaves. Accumulation of Cd2+ in the roots exceeded that in the shoots. Seedlings treated with Cd2+ exhibited reduced contents of chlorophyll and CO2 assimilation rate, with a greater decrease in young leaves. The photochemical efficiency of photosystem II (PSII) was not altered by Cd2+ treatment in either mature or young leaves, although during steady-state photosynthesis in young leaves there was a significant alteration in the following parameters: quantum yield of electron transport by PSII (ΦPSII), photochemical quenching (qp), non-photochemical quenching (qNP), and excitation capture efficiency of PSII (Φexc). [ABSTRACT FROM AUTHOR]

summary: We report the changes in CO2 assimilation, potential photochemical activity (as measured by slow fluorescence), photosynthetic pigment concentrations, and dark respiration of two desiccation‐tolerant (DT) lichens (Cladonia convoluta (Lam.) P. Cout. and C. furcata (Huds.) Schrad.), and a DT moss (Tortula ruralis (Hedw.) Gaertn. ssp. ruralis) during slow drying, and on rehydration following a 12 h period of desiccation. Initially there was a two to fourfold increase in net CO., assimilation due to reduction of CO2‐diffusion resistance by elimination of excess water. Optimum water content for photosynthesis was 100–150 % of dry mass (DM) in C. convoluta, c. 100 % DM in C. furcata, and 120–200 % DM in T. ruralis. The intensity of maximum and steady‐state slow fluorescence showed little change above water contents of 56%, DM in the lichens and 73 % DM in T. ruralis (corresponding to c. 30–40 % cell relative water content), but fell sharply at lower water content. The variable duorophyll‐fluorescence decrease ratio (Rfd) at 690 nm peaked at 56 % DM water content in the two lichens, and at 45% DM in T. ruralis. Photochemical activity ceased at the same point in the experiments as CO, assimilation; dark respiration ceased only when desiccation was complete. In all three species, the photosynthetic apparatus remained in a fully and quickly recoverable state. Chlorophyll and carotenoid concentrations remained substantially unaltered throughout. On rehydration, chlorophyll fluorescence parameters returned within 30 min to pre‐desiccation levels, and photosynthesis recovered fully and rapidly (< 1 h). All three species attained a positive carbon balance within 20 min of re‐moistening, in spite of high rates of dark respiration. The results confirm the significance of extracellularly‐stored water to poikilohydric DT lichens and bryophytes. The measurements, in conjunction with published data on the full‐turgor water content of similar mosses and lichens, show that the cell‐physiological response of photosynthesis to water deficit is not greatly different from that of either normal or DT vascular plants. Small plant size and small cell volume in DT lichens and mosses, together with rapid recovery of photosynthesis after desiccation, allow the plants to utilize the small amounts of intermittently available water from brief showers or dew. [ABSTRACT FROM AUTHOR]

We report the changes in CO2 assimilation, potential photochemical activity (as measured by slow fluorescence), photosynthetic pigment concentrations, and dark respiration of two desiccation-tolerant (DT) lichens (Cladonia convoluta (Lam.) P. Cout. and C. furcata (Huds.) Schrad.), and a DT moss (Tortula ruralis (Hedw.) Gaertn. ssp. ruralis) during slow drying, and on rehydration following a 12 h period of desiccation. Initially there was a two to fourfold increase in net CO2 assimilation due to reduction of CO2-diffusion resistance by elimination of excess water. Optimum water content for photosynthesis was 100-150% of dry mass (DM) in C. convoluta, c. 100% DM in C. furcata, and 120-200% DM in T. ruralis. The intensity of maximum and steady-state slow fluorescence showed little change above water contents of 56% DM in the lichens and 73% DM in T. ruralis (corresponding to c. 30-40% cell relative water content), but fell sharply at lower water content. The variable chlorophyll-fluorescence decrease ratio (Rfd) at 690 nm peaked at 56% DM water content in the two lichens, and at 45% DM in T. ruralis. Photochemical activity ceased at the same point in the experiments as CO2 assimilation; dark respiration ceased only when desiccation was complete. In all three species, the photosynthetic apparatus remained in a fully and quickly recoverable state. Chlorophyll and carotenoid concentrations remained substantially unaltered throughout. On rehydration, chlorophyll fluorescence parameters returned within 30 min to pre-desiccation levels, and photosynthesis recovered fully and rapidly (< 1 h). All three species attained a positive carbon balance within 20 min of re-moistening, in spite of high rates of dark respiration. [ABSTRACT FROM AUTHOR]