Growth and physiological responses to drought stress in four oleander clones

Oleander (Nerium oleander L.), native to southern Asia and the Mediterranean region, is a fast-growing evergreen shrub planted widely in the southern United States. A greenhouse study was conducted to quantify the growth and physiological responses of two cultivars, Hardy Pink and Hardy Red, and two breeding lines, EP1 and EP2, of oleander to a 12-week cyclic drought stress. Drought stress was imposed by irrigating the plants to near container capacity and then withholding irrigation until predetermined container weights were reached. Compared with the control where plants were well-irrigated throughout the experiment, shoot dry weight (DW) was reduced by 52%, 41%, 34%, and 11 % in EP1, EP2, 'Hardy Red', and 'Hardy Pink', respectively. Root-to-shoot DW ratio was higher for the drought-treated plants than the control, regardless of cultivar or breeding line (hereafter, clone). The increase in root-to-shoot DW ratio from the drought treatment was highest in EP1, followed by EP2, 'Hardy Pink', and 'Hardy Red.' New shoot growth was greatest in 'Hardy Pink', followed by 'Hardy Red', EP1, and EP2. The number of newly developed shoots during the drought treatment period was 6.8, 3.0, 0.7, and 0.0 in 'Hardy Pink', 'Hardy Red', EP1, and EP2, respectively. As substrate volumetric moisture content decreased from 30%, leaf net photosynthetic rate (P-n), evapotranspiration rate (E), and stomatal conductance (g(s)) decreased in all clones. A curvilinear relationship between P-n and g(s) was found in all clones. EPI had a lower maximum P-n (P-m) than those of 'Hardy Pink' and EP2 but was not different from that of 'Hardy Red'. Predawn leafwater potential began to decrease rapidly when substrate moisture content dropped below 15% in all clones. During the dry-down, compared with the control, increases in minimal fluorescence (F-0) or decreases in maximal fluorescence (F-m) and F-v/F-m (F-v = F-m - F-0) in drought-stressed plants were observed in all clones, indicating some damage in photosystem 11 from the drought treatment. However, compared with growth parameters, the differences in physiological responses to drought stress among the clones were much smaller. 'Hardy Pink' was more tolerant to drought stress than 'Hardy Red' and the other two clones in terms of productivity because it maintained greatest growth during the drought-stress period. However, EP2 and EP1 may be more tolerant if survival is concerned because they had a higher root-to-shoot DW ratio with minimal new growth.