Influence of tree water potential in inducing flowering in Rhododendron arboreum in the central Himalayan region

Abstract

Rise in temperature has been reported as the principal cause of variation in flowering phenology in several tree species around the globe. In this study, we hypothesized that not only temperature but also rainfall periodicity, soil moisture and the related changes of twig water potential (ψ) in winter and early spring are important drivers of bud expansion and flowering in Rhododendron arboreum in central Himalayas. To this purpose, phenological and physiological variables (flowering time, flower bud size and twig water potential) were monitored over two years in a wild population of R. arboreum (Uttarakhand, India) and related with environmental variables (rainfall, soil moisture and temperature). Results showed that a rise in twig ψ to -0.7MPa, one week after moderate winter precipitation resulted in flower bud enlargement. In both years flowering was triggered after twig ψ reached the threshold of -0.5 Mpa, though the starting date differed between years. Floral bud size was correlated positively with twig ψ (r = 0.43, df =162, p < 0.001) and soil moisture (r = 0.61, df = 71, p < 0.001), while temperature did not influence flower bud size, soil moisture and twig ψ. Flower bud size increment was related with increase in twig ψ and soil moisture. Based on our results, we concluded that water availability plays an important role in inducing flowering in R. arboreum.