Growth and physiological responses of four Rose rootstocks to drought stress
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Drought-tolerant garden roses (Rosa spp.) are needed in arid and semiarid regions where irrigation water is scarce. The vast majority of garden rose cultivars are budded or grafted onto rootstocks and are seldom grown on their own roots. The objective of this study was to compare the growth and physiological responses of four rose rootstocks to drought stress. Rosa hybrida 'Dr. Huey', R. fortuniana, R. multiflora, and R. odorata grown in 12-L containers were well-irrigated throughout the experiment or were subjected to mild drought stress for five or six cycles, depending on rootstocks, over 10 weeks. Following the mild drought stress cycles, plants that received the mild drought treatment were subjected to a severe dry-down before termination of the experiment. In R. fortuniana, drought stress did not affect any growth parameter. Compared with the well-irrigated plants, shoot dry weight of 'Dr. Huey', R. multiflora, and R. odorata were reduced by 22%, 33%, and 38%, respectively, by the drought treatment. The final leaf area of R. multiflora and R. odorata was reduced by 42% and 59%, respectively, compared with the control plants. The final leaf area of 'Dr. Huey' was not influenced by the drought treatment. Root to shoot ratio in 'Dr. Huey' was unaffected, while that of R. multiflora and R. odorata increased as a result of the drought treatment. As substrate moisture content decreased, leaf relative water content (RWC) of all rootstocks decreased linearly, with differences in decreasing slope or intercept of the linear regression lines among rootstocks. Predawn leaf water potential during the dry-down began to decrease rapidly when substrate moisture content decreased to below 20% (25% in R. odorata) in 'Dr. Huey', R. fortuniana, and R. multiflora. Leaf net photosynthetic rate (P n), transpiration rate (E) and stomatal conductance (g s) of all rootstocks decreased rapidly during the dry-down as substrate moisture content decreased from 25%. In 'Dr. Huey' and R. fortuniana, Pn, E, and gs were closely correlated with leaf RWC, while E and gs of R. odorata were not. Correlations between gas exchange rates (P n, E, and g s) and leaf RWC in R. multiflora were weaker than those in 'Dr. Huey' and R. fortuniana. At low substrate moisture content (below 15%), P n, E, and g s in R. odorata were lower than those in other rootstocks. Growth reduction was greatest in R. odorata, regardless of the least water deficit of the substrate, followed by R. multiflora and 'Dr. Huey'. The results of this study suggest that R. fortuniana was the most tolerant and R. odorata was the least tolerant to drought stress.
