Physiological characterization of the wild almond P. arabica stem photosynthetic capability

Leaves are the major plant tissue for transpiration and carbon fixation in deciduous trees. In harsh habitats, atmospheric CO2 assimilation via stem photosynthesis is common, providing extra carbon gain to cope with the detrimental conditions. We studied two almond species, the commercial P. dulcis cultivar and the rare wild P. arabica. Our study revealed two distinctive strategies for carbon-gain. While in P. dulcis, leaves possess the major photosynthetic surface area, in P. arabica, green stems play this function in particular during the winter after leaf drop. Anatomical and physiological comparisons show that P. arabica carries unique features that support stem gas-exchange and high gross photosynthetic rates via stem photosynthetic capabilities (SPC). On the other hand, P. dulcis stems contribute extremely low gross photosynthesic levels via stem recycling photosynthesis (SRP). Results show that a) P. arabica stems are covered with a high density of sunken stomata, while the stomata on P. dulcis stems disappear under a thick bark layer. b) P. arabica stems contain significantly higher levels of chlorophyll compartmentalized to a mesophyll-like, chloroplast-rich, parenchyma layer, in contrast to rounded-shape cells of P. dulcis’s stem parenchyma. c) Pulse amplitude modulated fluorometry of P. arabica and P. dulcis stems revealed differences in their chlorophyll fluorescence and quenching parameters. d) gas-exchange analysis showed that guard cells of P. arabica stems tightly regulate water-loss under elevated temperatures, maintaining constant and high assimilation rates throughout the stem. These findings are highly important and can be used to develop new almond cultivars with agriculturally essential traits.