Abstract:

Aims The Rhododendron as a high ornamental value genus has a great potential for resources development and utilization in China, which provided the most abundant rhododendron resources in the world. The risk of drought stress is rapidly increasing in the context of global warming, but the effects of water stress on growth and physiological and ecological indexes of rhododendrons are still lacking. In this paper, the height and ground diameter growth increment of R. delavayi under the drought stress were studied, and the numerical relationship of the growth increment response to the physiological indicators were established to provide a theoretical basis for the future protection and management of rhododendron.
Methods Pots experiment with 2 years old seedlings of R. delavayi were carried out from March 11th to October 15th, 2022. The water stress gradient was set as 15%, 25%, 35%, 50%, 70%, 90% of the field water holding capacity respectively. The height and ground diameter of each R. delavayi were measured in early and late of March, April, May and in the middle of June, July and October. The physiological index of osmotic system (proline (Pro), soluble sugar (Ss), soluble protein (Sp) contents), antioxidant system (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) activity, malondialdehyde (MDA) content) and photosynthesis system (net photosynthetic rate (P_n), stomatal conductance (g_s), intercellular CO₂ concentration (C_i), transpiration rate (T_r)) were measured in June, July, and August by randomly selecting five seedlings in each treatment.
Important findings 1) The equations of rhododendron height and ground diameter response to the day of year (DOY) were established under each drought gradient with the fitting accuracy between 0.94-0.99. The mortality was the highest under the treatment of 15% of the field water holding capacity. 2) The equations of the relative cumulative tree height response to the soil water content were established with the unimodal curve variation. While that of ground diameter response to the soil water content shows the U shape variation with the increase of soil water content before DOY 161, the linear pattern with that between DOY 161-201, and the unimodal curve variation after DOY 201. 3) With the increase of soil water content, SOD activity, CAT activity, POD activity, P_n, g_s, T_r and Pro content increased firstly and then decreased, the SOD activity, CAT activity, POD activity and Pro content reach the maximum value at field water holding capacity of 20%-30%, P_n, g_s and T_r reach that value at 60%-80%; MDA content gradually decreases; C_i, Ss and Sp contents decrease first and then increase, reaching the minimum value at field water holding capacity of 60%-80%. 4) Based on the regression analysis of the daily increment of ground diameter and the physiological indexes, the significant regression relationship index includes MDA content, P_n, g_s, T_r and Ss content. Except the relationship of daily ground diameter increment response to the MAD was quadratic function, the relationship of that to the other physiological indexes were power equation, the fitting accuracy of the relationships was T_r > P_n > MDA content > g_s > Ss content. The cumulative growth (diameter increment) of R. delavayi was the most when the soil content was 68% of the field water holding capacity. With the soil water varying, the growth was mainly promoted by T_r, P_n and g_s, but inhibited by Ss content. The MDA content promoted the growth when it was lower than 33.53 nmol∙g^-1, but it turned inhibited the growth when it was higher than 33.53 nmol·g^-1. This has further explained the physiological mechanism of R. delavayi seedlings growth.

Key words: Rhododendron delavayi, soil water stress, growth process, physiological mechanism, quantitative relationship