Physiological characterization of the wild almond stem photosynthetic capability.

Leaves are the major plant tissue for transpiration and carbon fixation in deciduous trees. In harsh habitats, atmospheric CO assimilation stem photosynthesis is common, providing extra carbon gain to cope with the detrimental conditions. We studied two almond species, the commercial cultivar "Um-el-Fahem" and the rare wild . Our study revealed two distinctive strategies for carbon gain in these almond species. While, in , leaves possess the major photosynthetic surface area, in , green stems perform this function, in particular during the winter after leaf drop. These two species' anatomical and physiological comparisons show that carries unique features that support stem gas exchange and high-gross photosynthetic rates stem photosynthetic capabilities (SPC). On the other hand, stems contribute low gross photosynthesis levels, as they are designed solely for reassimilation of CO from respiration, which is termed stem recycling photosynthesis (SRP). Results show that (a) stems are covered with a high density of sunken stomata, in contrast to the stomata on stems, which disappear under a thick peridermal (bark) layer by their second year of development. (b) stems contain significantly higher levels of chlorophyll compartmentalized to a mesophyll-like, chloroplast-rich, parenchyma layer, in contrast to rounded-shape cells of stem parenchyma. (c) Pulse amplitude-modulated (PAM) fluorometry of and stems revealed differences in the chlorophyll fluorescence and quenching parameters between the two species. (d) Gas exchange analysis showed that guard cells of stems tightly regulate water loss under elevated temperatures while maintaining constant and high assimilation rates throughout the stem. Our data show that uses a distinctive strategy for tree carbon gain stem photosynthetic capability, which is regulated efficiently under harsh environmental conditions, such as elevated temperatures. These findings are highly important and can be used to develop new almond cultivars with agriculturally essential traits.