Walnut Tolerance to Abiotic Stresses: Approaches and Prospects

Persian walnut (Juglans regia L.) trees are among the plants sensitive to abiotic stresses. Existing very old Persian walnut trees in Asia, planted hundreds years ago, may possess valuable stress resistance genes that help them cope with unfavourable environmental conditions. We have been collecting drought and salt tolerant cultivars of walnut from around the world, work which is still continuing, and we have identified some walnut seedlings that are very tolerant to drought and salt stresses at the germination stage. Mechanisms of adaptation and tolerance in selected walnut rootstocks were investigated. The results of our work on walnut tolerance to abiotic stress, and that of others, will be discussed in this paper. According to the literature, cavitation avoidance via stomatal regulation and petiole segmentation is a likely physiological response to abiotic stress in walnut, particularly in sensitive genotypes. Hot and dry weather reduces photosynthesis and potential productivity in walnut, even in the absence of soil water deficit, but some promising genotypes show sufficient net assimilation rate and photosynthesis under abiotic stress conditions. Walnut roots are mainly distributed in the upper soil layers, which lose efficiency in water uptake during the dry season. Differences among walnut genotypes in activity of anti-oxidative enzymes could be attributed to differences in the mechanisms underlying oxidative stress injury and subsequent tolerance to abiotic stresses. Higher proline accumulation observed in walnut seedlings tolerant to osmotic stresses was accompanied by higher activity of anti-oxidative enzymes. These results suggest that proline accumulation could activate the anti-oxidative defence mechanism in walnut trees. Biotechnology approaches for resistance to abiotic stresses in walnut are rare. One of the first attempts by our team has been transformation of Persian walnut with a flavodoxin gene (fld) isolated from a cyanobacteria. Plantlets of walnut expressing the fld gene clearly grew better in vitro in the presence of 200 mM NaCl than did non-transgenic controls. Control plants did not produce any callus, turned brown, and died after 10 days, while transgenic lines showed no brown symptoms, produced callus, and continued their growth for up to 45 days on 200 mM NaCl. Walnut transformation with a betaine aldehyde dehydrogenase (badh) gene is also under study.