Physiological drought resistance mechanisms in wild species vs. rootstocks of almond and plum

Among wild relatives of cultivated almond and plum, a desert almond species, but not a montane plum species, showed higher drought resistance than the common rootstocks routinely used in orchards. Water shortage is a severe environmental factor causing growth disruption and yield-\loss in many agricultural plant species. As fruit trees are likely to suffer from the effects of severe drought in the future, wild relatives of cultivated crops can provide plant breeders a unique material to improve the drought resistance of modern crop varieties. We conducted a drought and rewatering greenhouse experiment along 51 days with young trees of almond (the desert wild species Prunus ramonensis vs. the commonly used rootstock hybrid Prunus dulcis × Prunus persica) and plum (the montane wild species Prunus ursina vs. the rootstock Prunus cerasifera × Prunus persica). To decipher the drought resistance mechanisms in these trees we monitored physiological responses. Expression dynamics of cellular water channels from the plasma intrinsic protein (PIP) aquaporin family were measured in the almond species. Our results indicate a higher drought resistance in wild almond compared to the rootstock, but not in the wild plum species. Under drought, P. ramonensis had ~ ninefold higher photosynthesis activity, ~ 50-fold higher water-use efficiency and lower vulnerability to embolism than the rootstock. In the almond species, PIP downregulation was linked with maintenance of hydraulic conductivity, and vice versa for upregulation. This study implies that there is a link between drought resistance in wild tree species and their native habitat conditions, with an advantage for the desert, but not the montane, species. Finally, our study highlights the need to protect and conserve wild relatives of fruit tree species, partly as potential plant materials to be used by breeders to improve the resilience of orchard tree species to drought.