Manuscript Click here to download Manuscript Choury et al. main text.docx Click here to view linked References 24 We acknowledge the support of the International Atomic Energy Agency (project 25

Plant respiration can acclimate to changing environmental conditions and vary between species as well as biome types, although belowground respiration responses to ongoing climate warming are not well understood. Understanding the thermal acclimation capacity of root respiration (Rroot) in relation to increasing temperatures is therefore critical in elucidating a key uncertainty in plant function in response to warming. However, the degree of temperature acclimation of Rroot in rainforest trees and how root chemical and morphological traits are related to acclimation is unknown. Here we investigated the extent to which respiration of fine roots (≤2 mm) of four tropical and four warm-temperate rainforest tree seedlings differed in response to warmer growth temperatures (control and +6 °C), including temperature sensitivity (Q10) and the degree of acclimation of Rroot. Regardless of biome type, we found no consistent pattern in the short-term temperature responses of Rroot to elevated growth temperature: a significant reduction in the temperature response of Rroot to +6 °C treatment was only observed for a tropical species, Cryptocarya mackinnoniana, whereas the other seven species had either some stimulation or no alteration. Across species, Rroot was positively correlated with root tissue nitrogen concentration (mg g−1), while Q10 was positively correlated with root tissue density (g cm−3). Warming increased root tissue density by 20.8% but did not alter root nitrogen across species. We conclude that thermal acclimation capacity of Rroot to warming is species-specific and suggest that root tissue density is a useful predictor of Rroot and its thermal responses in rainforest tree seedlings.

Abiotic stresses have considerable negative impact on Mediterranean plant ecosystems and better comprehension of the genetic control of response and adaptation of trees to global changes is urgently needed. The single cell gel electrophoresis (SCGE) assay could be considered a good estimator of DNA damage in an individual eukaryotic cell. This method has been mainly employed in animal tissues, because the plant cell wall represents an obstacle for the extraction of nuclei; moreover, in Mediterranean woody species, especially in the sclerophyll plants, this procedure can be quite difficult because of the presence of sclerenchyma and hardened cells. On the other hand, these plants represent an interesting material to be studied because of the ability of these plants to tolerate abiotic stress. For instance, holm oak (Quercus ilex L.) has been selected as the model plant to identify critical levels of O3 for Southern European forests. Consequently, a quantitative method for the evaluation of cell injury of leaf tissues of this species is required. Optimal conditions for high-yield nuclei isolation were obtained by using protoplast technology and a detailed description of the method is provided and discussed. White poplar (Populus alba L.) was used as an internal control for protoplast isolation. Such a method has not been previously reported in newly fully developed leaves of holm oak. This method combined with SCGE assay represents a new tool for testing the DNA integrity of leaf tissues in higher plants under stress conditions.

While trees can acclimate to warming, there is concern that tropical rainforest species may be less able to acclimate because they have adapted to a relatively stable thermal environment. Here we tested whether the physiological adjustments to warming differed among Australian tropical, subtropical and warm-temperate rainforest trees.Photosynthesis and respiration temperature responses were quantified in six Australian rainforest seedlings of tropical, subtropical and warm-temperate climates grown across four growth temperatures in a glasshouse. Temperature-response models were fitted to identify mechanisms underpinning the response to warming.Tropical and subtropical species had higher temperature optima for photosynthesis (T optA ) than temperate species. There was acclimation of T optA to warmer growth temperatures. The rate of acclimation (0.35-0.78°C °C-1 ) was higher in tropical and subtropical than in warm-temperate trees and attributed to differences in underlying biochemical parameters, particularly increased temperature optima of V cmax25 and J max25 . The temperature sensitivity of respiration (Q 10 ) was 24% lower in tropical and subtropical compared with warmtemperate species.Overall, tropical and subtropical species had a similar capacity to acclimate to changes in growth temperature as warm-temperate species, despite being grown at higher temperatures. Quantifying the physiological acclimation in rainforests can improve accuracy of future climate predictions and assess their potential vulnerability to warming.

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