Screening for water stress tolerance in eleven plum (Prunussalicina L.) Cultivars using agronomic and physiological traits

The purpose of this study was to evaluate eleven plum cultivars (Prunus domestica L.) for drought tolerance based on agronomic performance and various physiological traits. The experiment was carried out under field conditions in Sais Plain (NW Morocco) on eight-year-old plum trees under drip irrigation. Two water treatments were applied during the fruit set (late March) to harvest (June): a control treatment fully irrigated at 100 % of seasonal ETc (FI), and a continuous deficit treatment irrigated at 50 % of ETc (CDI). The plasticity to drought was assessed through the following traits: yield, fruit weight, number of leaves per 10 cm of shoot, wood density, leaf area, stomatal density, stomatal area, stomatal area index, chlorophyll pigments content (Chₐ and Chb), stomatal conductance and leaf content of cuticular waxes and proline. Results showed significant differences among cultivars in response to water stress regarding all the above mentioned traits. The cluster analysis based on mean ratios of CDI and FI treatments for all traits highlighted three distinct clusters within the studied cultivars, with regard to drought tolerance level. The PCA analysis using the above-named ratios revealed that water stress effects on fruit weight, leaf area, stomatal density, stomatal area and leaf proline content had the highest impact on discrimination among the cultivars for drought tolerance. Therefore, retained cultivars for their remarkable tolerance to drought were, in order of importance, "INRA-PR34″, "INRA-PR35″, "INRA-PR41″ and "Santa Rosa", while the cultivar "Timhdit" was the most sensitive, with a strong yield decrease of 95 %. The results herein reported identified cultivars, with high degree of plasticity in response to water deficit, suggesting their use in breeding programs aiming at improving the species drought tolerance.