Seedlings from two Agave species differing in microhabitat evolve different tolerance mechanisms to drought and shade under nurse plants

摘要

We studied two Agave species differing in microhabitat; when adult, A. striata is light-tolerant and A. salmiana is drought-tolerant. However, both species establish under nurse plants. The aim of this work was to investigate the ecophysiological mechanisms of these two species to tolerate different shade and water conditions at the seedling stage. Seedlings were grown for six months under two water treatments (well-irrigated and drought) and three shade treatments, denominated high shade (HS), medium shade (MS) and low shade (LS) (241, 563 and 804 mu mol m(-2) s(-1) of photosynthetic photon flux density, respectively). Concentrations of chlorophylls a and b, total chlorophylls, carotenoids and water potential (Psi) were analyzed. The decreasing shade levels in combination with the regime "well-irrigated" did promote increased concentration of carotenoids in A. salmiana but not in A. striata. Besides, the well-irrigated plants produced high concentration levels of chlorophyll b and total chlorophyll in both species, showing no changes due to shade treatments. Conversely, drought generated significantly low concentrations of the pigments in A. striata with no variations due to shade treatments. Furthermore, concentrations of the pigments of A. salmiana under drought at HS and MS were similar to those for well-irrigated plants, but diminished at LS. Likewise, drought reduced Psi in A. striata but not in A. salmiana, regardless of the shade effect. These results indicate that the tolerance of seedlings to drought stress and decrease in shade in A. salmiana relates to its mechanisms for maintaining hydration, whereas in A. striata this depends on mechanisms that keep the concentration of pigments stable. Thus, Agave species evolve different mechanisms for tolerating drought and low solar radiation stress.