S11 - Session O3 - The wild almond P. arabica utilizes stems as a major source tissue for photosynthesis

Authors: Tamar Azoulay-Shemer *

Leaves are the major plant tissue for transpiration and carbon fixation in deciduous trees. In harsh habitats, stem photosynthesis is more common, providing extra carbon gain to cope with the detrimental conditions. We studied two almond species, the commercial cultivar "Um-el-Fahem" ( P.dulcis) and the rare wild P.arabica . Physiological, anatomical, and chlorophyll-fluorescence-based analyses revealed two distinctive strategies for carbon gain in these almond species. We found significant differences between leaf, barked-stem and green-stem relative surface-area in P.dulcis (94%, 6%, 0.1%) vs. P.arabica (32%, 7%, 60%). P.dulcis and P.arabica leaves showed comparable chlorophyll levels. However, transpiration and CO 2 assimilation rates were significantly higher in P.dulcis leaves. Leaf stomata in P.arabica responded to a lower threshold of elevated-temperatures than those of P.dulcis , suggesting differences in adaptation to harsh habitats. Interestingly, P.arabica stems were distinctively different than those of P.dulcis . Stomata imprint and scanning-electron-microscopy revealed that 1st-year green stems of P.arabica possessed 4x more sunken stomata than P.dulcis . Gas-exchange analyses revealed that in the green stems of P.arabica and P.dulcis, the stomata were functional, yet, transpiration and assimilation were significantly higher in P.arabica 1 st year stems. While P.arabica 2 nd year stems remained green with functional stomata for CO 2 assimilation rates, P.dulcis stems developed a cork-layer and lost their negligible CO 2 assimilation rate ability. The significantly higher chlorophyll levels in P.arabica , distinctively concentrated in stems parenchyma-layer and it's fundamental histological unique infrastructures, further support the high-photosynthetic functionality of P.arabica stems, as compared to P.dulcis . Furthermore, PAM analysis showed differences between P.arabic and P.dulcis chlorophyll based fluorescence parameters in both leaves and stems, suggesting differences in the light reactions between the two species. The data suggest a distinctive strategy for coping with high-temperature and detrimental conditions in P.arabica and are of high importance for developing new almond cultivars with agriculturally-important traits.