RESUMEN
BACKGROUND AND AIMS: A positive correlation between tissue thickness and crassulacean acid metabolism (CAM) expression has been frequently suggested. Therefore, this study addressed the question of whether water availability modulates photosynthetic plasticity in different organs of two epiphytic orchids with distinct leaf thickness. METHODS: Tissue morphology and photosynthetic mode (C3 and/or CAM) were examined in leaves, pseudobulbs and roots of a thick-leaved (Cattleya walkeriana) and a thin-leaved (Oncidium 'Aloha') epiphytic orchid. Morphological features were studied comparing the drought-induced physiological responses observed in each organ after 30 d of either drought or well-watered treatments. KEY RESULTS: Cattleya walkeriana, which is considered a constitutive CAM orchid, displayed a clear drought-induced up-regulation of CAM in its thick leaves but not in its non-leaf organs (pseudobulbs and roots). The set of morphological traits of Cattleya leaves suggested the drought-inducible CAM up-regulation as a possible mechanism of increasing water-use efficiency and carbon economy. Conversely, although belonging to an orchid genus classically considered as performing C3 photosynthesis, Oncidium 'Aloha' under drought seemed to express facultative CAM in its roots and pseudobulbs but not in its leaves, indicating that such photosynthetic responses might compensate for the lack of capacity to perform CAM in its thin leaves. Morphological features of Oncidium leaves also indicated lower efficiency in preventing water and CO2 losses, while aerenchyma ducts connecting pseudobulbs and leaves suggested a compartmentalized mechanism of nighttime carboxylation via phosphoenolpyruvate carboxylase (PEPC) (pseudobulbs) and daytime carboxylation via Rubisco (leaves) in drought-exposed Oncidium plants. CONCLUSIONS: Water availability modulated CAM expression in an organ-compartmented manner in both orchids studied. As distinct regions of the same orchid could perform different photosynthetic pathways and variable degrees of CAM expression depending on the water availability, more attention should be addressed to this in future studies concerning the abundance of CAM plants.
Asunto(s)
Orchidaceae/anatomía & histología , Orchidaceae/metabolismo , Fotosíntesis/fisiología , Hojas de la Planta/anatomía & histología , Carbono/metabolismo , Sequías , Malato Deshidrogenasa/metabolismo , Orchidaceae/fisiología , Fosfoenolpiruvato Carboxilasa/metabolismo , Hojas de la Planta/metabolismo , Raíces de Plantas/anatomía & histología , Raíces de Plantas/metabolismo , Raíces de Plantas/fisiología , Agua/metabolismoRESUMEN
Two year-old cloned plants of Dendrobium Second Love were submitted to 25 degrees C (light) and 10 degrees C (dark) under a 12h photoperiod (60 micromol m(-1)s(-1)) for 30 days. The endogenous levels of IAA, ABA, and the cytokinins Z, [9R]Z, iP, and [9R]iP were measured 15, 22, and 30 days after the start of the thermoperiodic treatment in lateral buds and leaves. The endogenous levels of IAA and cytokinins, especially the zeatin-derived forms, increased significantly in buds after 15 days of treatment. On the other hand, the amount of ABA decreased progressively and significantly throughout the treatment. The treatment conspicuously accelerated flower-bud development. The found correlation suggests that hormones are involved in the signal transduction pathway of thermoperiodic flowering control.