Effects of the velamen water saturation on the photosynthetic performance of Vanda hybrid (Orchidaceae), a species with conspicuous pneumathodes

Abstract

Anatomical and physiological characteristics in orchids efficiently optimize the acquisition and use of water, such as the velamen in the roots, and the expression of crassulacean acid metabolism (CAM). However, the water saturation of the velamen may generate hypoxic conditions, which the presence of pneumathodes may reduce. The present study aimed to analyze the tolerance of the epiphyte Vanda hybrid subjected to water saturation, determining whether the velamen water saturation leads to root oxidative stress with a systemic effect on plant metabolism, and discussing the role of pneumathodes in root aeration. Five individuals were used as controls, and five were subjected to root system submersion for 120 h (time intervals: 0, 24, 72, and 120 h after submersion). Fragments of leaves and roots were used to obtain the specific mass and the relative water content, and to quantify the diurnal organic acid changes, the contents of chloroplast pigments, and chlorophyll a fluorescence. Histochemical analyses were performed to detect oxidative stress in the roots. The initial submersion of the roots led to plant hydration and increased CAM expression and photochemical quantum yield by leaves. Pneumathodes remained white throughout the experiment, but the submerged roots exhibited higher oxidative stress than those of control plants after 120 h of submersion. In addition to the connection between the root cortex and the external environment, the air pockets formed by pneumathode cells and the O₂ product of photosynthetic metabolism can reduce or at least delay the effects generated by water saturation conditions at the root.