Bud Morphology and Shoot Growth Dynamics in Two Species of Mediterranean Sub-shrubs Co-Existing in Gypsum Outcrops

Abstract

Understanding the effects of the environment on the morphology and shoot growth activities of plants is crucial to identifying plant ecological strategies. This study analysed the bud morphology, bud activity, shoot growth dynamics and shoot water content at full hydration (WC(h)) of two species of Mediterranean sub-shrubs, Lepidium subulatum and Linum suffruticosum, co-existing in gypsum outcrops in north-east Spain. Sampling was conducted monthly over 2 years in one population per species. Buds were dissected under a stereo-microscope. Shoot growth was measured as the mean increase in shoot length of 15 marked individuals between two consecutive samplings. Bud activity was studied following the variations in the number of leaf primordia shorter than 1 mm and longer than 0.025 mm in the buds. Both species bore naked buds and displayed discontinuous seasonal patterns of shoot growth, leaf primordia formation and WC(h). The number of leaf primordia in the bud peaked before the beginning of shoot expansion. In both species, organogenesis and expansion were uncoupled throughout the year. The time lapse between these two processes varied throughout the year, and was greatest for those elements differentiated in autumn. WC(h) was more closely related to shoot expansion than to organogenesis. Both species displayed similar bud morphology and similar seasonal patterns of bud and shoot growth, and WC(h) as a result of the strong seasonality of the Mediterranean climate in gypsum outcrops. The beginning of the spring period of expansion of long branches coincided with maximum values of WC(h), while the rest period of summer matched minimum values. These results support the hypothesis that the growth of long branches is strongly related to WC(h).