ABSTRACT 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 µmol m−2 s−1 of photosynthetic photon flux density, respectively). Concentrations of chlorophylls a and b, total chlorophylls, carotenoids and water potential (Ψ) 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 Ψ 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.

中文翻译：

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来自微生境不同的两种龙舌兰物种的幼苗在护理植物下对干旱和遮荫的耐受机制不同

摘要 我们研究了两种不同微生境的龙舌兰物种；成年后，A. striata 耐光，A. salmiana 耐旱。然而，这两个物种都在护士植物下建立。这项工作的目的是研究这两个物种在幼苗期耐受不同遮荫和水分条件的生态生理机制。幼苗在两种水处理（良好灌溉和干旱）和三种遮荫处理下生长六个月，称为高遮荫 (HS)、中等遮荫 (MS) 和低遮荫 (LS)（241、563 和 804 µmol m-2 s−1 的光合光子通量密度，分别）。分析了叶绿素 a 和 b、总叶绿素、类胡萝卜素和水势 (Ψ) 的浓度。降低的阴影水平与“良好灌溉”的制度相结合确实促进了 A. salmiana 但不是 A. striata 中类胡萝卜素浓度的增加。此外，灌溉良好的植物在两个物种中产生了高浓度的叶绿素 b 和总叶绿素，没有因遮荫处理而发生变化。相反，干旱在 A. striata 中产生显着低浓度的色素，没有因遮荫处理而产生的变化。此外，在 HS 和 MS 干旱条件下，A. salmiana 的色素浓度与灌溉良好的植物相似，但在 LS 时减少。同样，干旱降低了 A. striata 中的 Ψ 而不是 A. salmiana，无论遮荫效果如何。这些结果表明幼苗对干旱胁迫的耐受性和 A. Salmiana 与其维持水合作用的机制有关，而在 A. striata 中，这取决于保持色素浓度稳定的机制。因此，龙舌兰物种进化出不同的机制来耐受干旱和低太阳辐射压力。