Relationships between xylem embolism and tree functioning during drought, recovery, and recurring drought in Aleppo pine

Recent findings suggest that trees can survive high levels of drought-induced xylem embolism. In many cases, the embolism is irreversible and, therefore, can potentially affect post-drought recovery and tree function under recurring droughts. We examined the development of embolism in potted Aleppo pines, a common species in hot, dry Mediterranean habitats. We asked (1) how post-drought recovery is affected by different levels of embolism and (2) what consequences this drought-induced damage has under a recurring drought scenario. Young trees were dehydrated to target water potential (Ψx) values of −3.5, −5.2 and −9.5 MPa (which corresponded to ~6%, ~41% and ~76% embolism), and recovery of the surviving trees was measured over an 8-months period (i.e., embolism, leaf gas-exchange, Ψx). An additional group of trees was exposed to Ψx of −6.0 MPa, either with or without preceding drought (Ψx of −5.2 MPa) to test the effect of hydraulic damage during repeated drought. Trees that reached −9.5 MPa died, but none from the other groups. Embolism levels in dying trees were on average 76% of conductive xylem and no tree was dying below 62% embolism. Stomatal recovery was negatively proportional to the level of hydraulic damage sustained during drought, for at least a month after drought relief. Trees that experienced drought for the second time took longer to reach fatal Ψx levels than first-time dehydrating trees. Decreased stomatal conductance following drought can be seen as “drought legacy,” impeding recovery of tree functioning, but also as a safety mechanism during a consecutive drought

Leaf Carbon Export and Nonstructural Carbohydrates in Relation to Diurnal Water Dynamics in Mature Oak Trees

Trees typically experience large diurnal depressions in water potential, which may impede carbon export from leaves during the day because the xylem is the source of water for the phloem. As water potential becomes more negative, higher phloem osmotic concentrations are needed to draw water in from the xylem. Generating this high concentration of sugar in the phloem is particularly an issue for the ∼50% of trees that exhibit passive loading. These ideas motivate the hypothesis that carbon export in woody plants occurs predominantly at night, with sugars that accumulate during the day assisting in mesophyll turgor maintenance or being converted to starch. To test this, diurnal and seasonal patterns of leaf nonstructural carbohydrates, photosynthesis, solute, and water potential were measured, and carbon export was estimated in leaves of five mature (\u3e20 m tall) red oak (Quercus rubra) trees, a species characterized as a passive loader. Export occurred throughout the day at equal or higher rates than at night despite a decrease in water potential to −1.8 MPa at midday. Suc and starch accumulated over the course of the day, with Suc contributing ∼50% of the 0.4 MPa diurnal osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always \u3e0.7 MPa. These findings illustrate the robustness of phloem functioning despite diurnal fluctuations in leaf water potential and the role of nonstructural carbohydrates in leaf turgor maintenance

Metabolic and physiological responses of shiraz and cabernet sauvignon (Vitis vinifera L.) to near optimal temperatures of 25 and 35 \ub0C

Shiraz and Cabernet Sauvignon (Cs) grapevines were grown at near optimal temperatures (25 or 35 \ub0C). Gas exchange, fluorescence, metabolic profiling and correlation based network analysis were used to characterize leaf physiology. When grown at 25 \ub0C, the growth rate and photosynthesis of both cultivars were similar. At 35 \ub0C Shiraz showed increased respiration, non-photochemical quenching and reductions of photosynthesis and growth. In contrast, Cs maintained relatively stable photosynthetic activity and growth regardless of the condition. In both cultivars, growth at 35 \ub0C resulted in accumulations of secondary sugars (raffinose, fucose and ribulose) and reduction of primary sugars concentration (glucose, fructose and sucrose), more noticeably in Shiraz than Cs. In spite of similar patterns of metabolic changes in response to growth at 35 \ub0C, significant differences in important leaf antioxidants and antioxidant precursors (DHA/ascorbate, quinates, cathechins) characterized the cultivar response. Correlation analysis reinforced Shiraz sensitivity to the 35 \ub0C, showing higher number of newly formed edges at 35 \ub0C and higher modularity in Shiraz as compared to Cs. The results suggest that the optimal growth temperatures of grapevines are cultivar dependent, and allow a first insight into the variability of the metabolic responses of grapevines under varied temperatures

An extended Simes' test

Simes' (1986) procedure for testing an intersection null hypothesis in terms of the individual p-values corresponding to the component hypotheses is extended to allow the allotment of different weights to the different component hypotheses. Preliminary necessary results on distributions of statistics based on the ordered values of independent non-identically distributed uniform random variables are obtained and used to prove (by induction) that the extended procedure controls the type-I error probability exactly at [alpha].Intersection null hypothesis Multiple comparisons Weights for component hypotheses Order statistics Non-identical uniform variables

Advancements in the study of the grapevine hydraulic strategy under water deficit in relation to the formation of embolism

In recent years, the validity of embolism quantification methods has been questioned, especially for long-vesseled plants, as some studies have suggested that cutting xylem while under tension (even under water) might generate artificial cavitation. Accordingly, a rehydration procedure prior to hydraulic measurements has been recommended to avoid this artefact. On the other hand, concerns have been raised that xylem refilling might occur when samples are rehydrated. Recently we have proposed an improved method for reliable quantification of this phenomenon in grapevine petioles in which the inclusion of a short-time (20 min) xylem tension relaxation prior to the hydraulic measurement resulted in reliable quantification of native embolism. Much longer periods of hydration (overnight) were required before xylem refilling was observed to occur. This method yielded xylem vulnerability curves similar to the ones obtained using direct microCT visualization and the values were lower than the ones found in samples harvested under native tensions. The new protocol revealed cavitation in grapevine petioles occurs only after significant stomatal conductance reduction, contrary to what was previously hypothesized. In field-grown vines, over 85% of stomatal closure occurred at less negative xylem tension than that required to induce 12% loss of conductivity (PLC). These findings indicate that grapevines regulate their stomata to prevent xylem embolism and would even shed a large proportion of their leaves in order to avoid cavitation tensions. This pattern of stomatal behavior is more consistent with a strategy of cavitation resistance and avoidance, rather than a paradigm of tolerance featuring diurnal cycles of cavitation accumulation and repair

Protecting biodiversity hotspots with the TiME educational tool

Land transformation and degradation is the major driver of current species extinction, lately recognized as the sixth mass extinction. To address this threat innovative programs that combine both immediate remedies of land protection and comprehensive educational programs are soughed. This is My Earth (TiME) (http://this-is-my-earth.org) is a new voluntary NGO that was established to address these challenges. TiME uses crowd funding to save privately own lands in biodiversity hotspots, providing them protection. Membership fees are very affordable (as low as $1 a year) and 100% of all fees and donations are directed to fund land purchase and protection. TiME welcomes children to become members and use its website to vote for their preferred habitat to be conserved (following vetting by an international committee of experts), and thus offering them the power to decide for the organization where in the world it will purchase lands. Using the PBL [=Problem Based Learning] pedagogical approach we developed a workshop, based on the above inherited powers of the TiME approach, that empowers children to adopt a sense of critical thinking as they become activists for wildlife protection. We began by developing a lesson plan that includes a short presentation of the problem, group discussions about potential solutions, class debates regarding the optimal lands to be saved, and a class-wide democratic vote. Following this workshop we examined the level of awareness and attitude of the children towards activism and democratic position. We report here preliminary results as a preliminary database for developing an international TiME based lesson plan. Participants [N=80] revealed a democratic orientation regarding land reservations and expressed strong affinity for activism and further conservation activities. Responses to the Likert scale-based questions, indicate considerable support for involvement, with endorsement of five actions recommended in the questionnaire: writing letters to governmental authorities, signing petitions, participation in demonstrations, organizing demonstrations, and donations. Interestingly, donations received the highest support level, thus endorsing the TiME approach, while organizing a demonstration the lowest. It appears that the global habitat crisis presented to the students via the TiME platform triggers debate and critical thinking even beyond conservation to such topics as democracy and equity. The TiME website is emerging as an educational tool that offers a novel experience where teachers can harness curiosity and environmental engagement in the learning process about conservation issues. Educators use TiME to explain basic conservation concepts and involve classes in discussions about environmental decision-making and associated ethical dilemmas, prioritization of conservation efforts and practical ways to protect nature.peerReviewe

The effect of differential growth rates across plants on spectral predictions of physiological parameters.

Leaves of various ages and positions in a plant's canopy can present distinct physiological, morphological and anatomical characteristics, leading to complexities in selecting a single leaf for spectral representation of an entire plant. A fortiori, as growth rates between canopies differ, spectral-based comparisons across multiple plants--often based on leaves' position but not age--becomes an even more challenging mission. This study explores the effect of differential growth rates on the reflectance variability between leaves of different canopies, and its implication on physiological predictions made by widely-used spectral indices. Two distinct irrigation treatments were applied for one month, in order to trigger the formation of different growth rates between two groups of grapevines. Throughout the experiment, the plants were physiologically and morphologically monitored, while leaves from every part of their canopies were spectrally and histologically sampled. As the control vines were constantly developing new leaves, the water deficit plants were experiencing growth inhibition, resulting in leaves of different age at similar nodal position across the treatments. This modification of the age-position correlation was characterized by a near infrared reflectance difference between younger and older leaves, which was found to be exponentially correlated (R(2) = 0.98) to the age-dependent area of intercellular air spaces within the spongy parenchyma. Overall, the foliage of the control plant became more spectrally variable, creating complications for intra- and inter-treatment leaf-based comparisons. Of the derived indices, the Structure-Insensitive Pigment Index (SIPI) was found indifferent to the age-position effect, allowing the treatments to be compared at any nodal position, while a Normalized Difference Vegetation Index (NDVI)-based stomatal conductance prediction was substantially affected by differential growth rates. As various biotic and abiotic factors may form distinctions in growth, future precision agriculture studies should consider its spectral effect on physiological predictions