Abstract
Epiphytes are an important component of tropical and subtropical flora, and serve vital ecological functions in forest hydrology and nutrient fluxes. However, they often encounter water deficits because there is no direct contact between their roots and the soil. The strategies employed by epiphytes for maintaining water balance in relatively water-limited habitats are not completely understood. In the present study, we investigated the anatomical traits, water loss rates, and physiology of leaves and pseudobulbs of four Dendrobium species with different pseudobulb morphologies to understand the roles of leaf and pseudobulb in maintaining water balance of epiphytic orchids. Our results showed that two species (D. chrysotoxum and D. officinale), with lower rates of water loss, have thicker leaves and upper cuticles, but lower epidermal thickness and leaf dry mass per area. In contrast, the other two species (D. chrysanthum and D. crystallinum) with thinner cuticles and higher rates of water loss, have less tissue density and greater saturated water contents in their pseudobulbs. Therefore, our results indicate that these latter two species may resist drought by storing water in the pseudobulbs to compensate for their thin cuticles and rapid water loss through the leaves. Under the same laboratory conditions, excised pseudobulbs with attached leaves had lower rates of water loss when compared with samples comprising only excised leaves. This implies that epiphytic orchids utilize two different strategies for sustaining water balance: thick cuticles to conserve water in leaves and water storage in pseudobulbs. Our results also show that Dendrobium species with thin cuticles tend to have pseudobulbs with high water storage capacity that compensates for their faster rates of water loss. These outcomes contribute to our understanding of the adaptive water-use strategies in Dendrobium species, which is beneficial for the conservation and cultivation of epiphytic orchids.
Highlights
Vascular epiphytes live non-parasitically on other living plants for support, and are often rich in tropical forests (Benzing 1990)
According to the photosynthetic pathway classification by Perez-Harguindeguy et al (2013), the d13C values of the four tested species indicated that the photosynthetic modes of D. chrysanthum, D. chrysotoxum and D. crystallinum are C3 patterns (d13C values ranged from À21.0& to À35.0&), while D. officinale (–20.9&) shows facultative crassulacean acid metabolism (CAM) photosynthetic pathway (Fig. 5C, the details were given in the Discussion section)
Rates of water loss and relative water content of pseudobulbs and leaves Under the same laboratory conditions, the rates of water loss from the excised leafless pseudobulbs and the detached leaves differed among the four Dendrobium species, following the order of D. chrysotoxum < D. officinale < D. crystallinum < D. chrysanthum
Summary
Vascular epiphytes live non-parasitically on other living plants for support, and are often rich in tropical forests (Benzing 1990). They play critical roles in the hydrological and nutrient cycling of forest ecosystems. Many vascular epiphytes found in tropical and subtropical regions normally grow on tree trunks in forests and/or on rock surfaces in valleys (Benzing 1990). Unlike terrestrial plants, their roots have no contact with the soil. Vascular epiphytes exhibit more drought tolerance characteristics than do terrestrial species (Rada and Jaimez 1992; Watkins and Cardelus 2012; Zhang et al 2015), even though the water-adaptive strategies may differ among epiphytic species
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