Evolutionary correlation of water-related traits between different structures of Dendrobium plants

Leaf water conservation and pseudobulb water storage are two of the strategies commonly employed by epiphytic plants to adapt to dry environments. During the flowering period, a great deal of water transpires through the flowers, which then influences water-related processes. However, there is little research on the coordinated relationship between the different structures of epiphytes. Our study explored the phylogenetic conservation and evolutionary correlations between structural traits of 8 species in the genus Dendrobium by using phylogenetic independent contrast (PIC) analysis.

Leaf dry mass, leaf water content, leaf dry matter content, specific leaf area, stomatal density, stomatal area index, pseudobulb length, pseudobulb width, and flower dry mass show strong phylogenetic signals. Pseudobulb length is significantly positively correlated with stomatal volume but significantly negatively correlated with mesophyll thickness according to both species mean values and PIC values. Pseudobulb internode length is also positively correlated with stomatal volume but negatively correlated with stomatal density according to PIC values. Pseudobulb width is significantly positively correlated with leaf dry mass, stomatal density, stomatal area index, flower petal vein number and flower dry mass but negatively correlated with specific leaf area according to species mean values. However, these correlations are insignificant when PIC values are analyzed. Stomatal volume is positively correlated with flower dry mass, and after phylogeny is considered, this correlation is still significant. Leaf dry mass is positively correlated with flower petal vein number according to species values. Flower number per pseudobulb is negatively correlated with upper epidermal cell size according to species values but negatively correlated with stomatal area index according to PIC values. There are no correlations between pseudobulb and flower water-related traits according to PIC values.

A trade-off should exist in epiphytic plants between the two drought-tolerant strategies of pseudobulb storage and leaf water retention. Plants possessing thick blades with a few large stomata tend to use the pseudobulb water storage strategy to adapt to drought. Small flowers and low flower dry mass should be associated with the leaf water retention strategy. In addition, flowers and leaves exhibit an obvious water balance and should share common selection pressures. The present study provides a case with which to understand the coordinated adaptation of different structures in epiphytic plants.