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Seedlings of three selected Juglans rootstocks (Juglans mandshurica Maxim, Juglans nigra L. and Juglans regia L. cv. Jizhaomian) were analyzed in terms of growth, water use efficiency (WUE), chlorophyll fluorescence and photosynthetic response to drought stress. Significant differences among seedlings of the three analyzed Juglans rootstocks were observed on performance in response to water deficit. Juglans mandshurica Maxim. and Juglans regia L. cv. Jizhaomian showed larger increase in WUE and smaller reduction in growth rate, chlorophyll fluorescence and gas exchange parameters than Juglans nigra L. under drought-stressed condition. It suggests that Juglans mandshurica Maxim. and Juglans regia L. cv. Jizhaomian are more tolerant to drought stress and more suitable as walnut rootstock in water deficit regions as compared with Juglans nigra L.

Seedlings of six selected walnut rootstock species were analyzed in terms of seed germination rate (GR) and emergence rate (ER) as well as the seedlings’ morphological and physiological performance. A wide variation, with coefficient of variation (CV) range from 6.47 to 38.45 %, was detected for sixteen traits among the studied species. Plant height (PH), total dry biomass, root/shoot ratio, axial root length, number of lateral root, total leaf area, leaf area ratio and leaf chlorophyll content showed higher variation than the other eight indictors, as manifested by the higher CV that was more than 20.00 %. In comparison with other species, Juglans regia L. cv. Jizhaomian (JRJ) and Juglans regia L. cv. Xiangling (JRX) exhibited significantly higher value in GR, ER, morphological and physiological variables. Positive correlations among most variables representing seedling growth suggested that these parameters can be used to predict each other. According to principal components analysis (PCA), three principal components accounting for 75.905 % of the total variance was extracted. Variables with high discriminating values including GR, ER, PH, specific leaf area and water-holding capacity were identified as the most useful variables for evaluation. Bi-plot and cluster analysis divided the studied species into three groups. The first group including JRJ and JRX that were common walnut (J. regia). species and characterized by high GR, ER, well growth performance and high photosynthetic capacity suggesting strong growth vigour. The second group was consisted of three species (J. hopeiensis Hu., J. mandshurica Maxim. and J. sigllata Dode.) which were characterized by moderate value in the studied variables. The third group included Pterocarya stenoptera C. DC. which characterized by low value in the studied variables suggesting weak growth vigour. It suggested a considerable phenotypical diversity in the six walnut rootstock species that would available to rootstock development programs. Our results would have important implications for the production use of these species as walnut rootstocks that are most appropriate for a certain particular walnut planting regions in China. Also, it is of great importance to exploiting the genetic variability and identifying of superior genotypes for conservation action as well as breeding programs.