The water-related traits of flowers are more conservative than those of leaves for epiphytic and terrestrial species in Cymbidium, Orchidaceae

Abstract

Epiphytes occupy arboreal niches in forest ecosystems, which are particularly vulnerable to drought stress due to the absence of a buffered substrate for water retention in epiphytic habitats. Characterizing the differences and relationships among plant morphological and physiological traits is critical for elucidating different adaptive strategies. However, it is still unclear whether there are differences in floral and leaf morphological and physiological traits between epiphytic and terrestrial plants, and whether there is a correlation between flower and leaf traits in epiphytes. Here, we measured 13 floral traits and 8 leaf traits from 7 terrestrial and 12 epiphytic Cymbidium species. We found that, compared with these terrestrial Cymbidium species, epiphytic species had a higher leaf mass per unit area, greater leaf thickness, a longer time required to dry saturated leaves to 70% relative water content, and greater epidermal thickness. However, no significant differences in floral traits were found between the epiphytic and the terrestrial species, which suggest that the water-related traits of flowers in Cymbidium are not influenced by the plant's life forms. Moreover, there were no strong associations between floral and leaf morphological and physiological traits floral traits, implying that they may be developmentally modular. These findings provide novel insights into the decoupled evolution of vegetative and reproductive traits in response to environmental pressures. By shedding light on this pattern, our study advances the understanding of plant adaptation strategies in heterogeneous habitats within the genus Cymbidium, providing a more comprehensive view of how plants evolve to flourish in diverse ecological conditions.

Keywords: adaptation strategy; epiphytic orchids; floral traits; life form; morphological and physiological traits.

Figure 1.

Traditional and PICs correlations between the time required for drying of saturated flowers to 70% RWC (T_flower70) with FMA (a), FL (b), and the relationship of FMA and FL (c) in epiphytic and terrestrial Cymbidium species; the correlations between the time required for drying of saturated leaves to 70% RWC (T_leaf70) with LMA (d), LT (e), LMT (f); the relationship between LMA and LT (g); the correlation between LSD and LVD (h). The dashed line represents epiphytic species, the bold solid line represents all species, the squares represent epiphytic species, and the circles represent terrestrial. *P < .05, **P < .01, ***P < .001.

Figure 2.

Arrangement of the 13 floral and 8 leaf morphological and physiological traits along the first 2 principal component axes constructed based on (a) Cymbidium species traits means and (b) PICs and the contribution of each variable to the first (c) and second (d) PCs based on traditional phylogenetic PCA. The red horizontal dashed lines in (c) and (d) indicate the average contribution of each variable to the corresponding PC, FMA, FL, time required for drying of saturated flowers to 70% RWC (T_{flower  70}), PT, PUET, PLET, PMT, PVD, ST, SUET, SLET, SMT, SVD, LMA, time required for drying of saturated leaves to 70% RWC (T_{leaf 70}), LMT, LT, LUET, LLET, LSD, and LVD.

Figure 3.

Correlations between flower PC1 and leaf PC1 with respect to epiphytic Cymbidium species, terrestrial species, and all species. Pearson correlation coefficients (r) were −0.49 (P = .11), 0.18 (P = .69) and −0.36 (P = .13), respectively. The squares represent epiphytic species, and the circles represent terrestrial.