Cycads, considered as living fossils, are the most ancient living seed plant lineage because of the morphological similarity between fossil and extant species. It is little-known whether cycad leaves show strong variation in physiology, or whether that variation would be constrained by similar principles as in angiosperms.

Dr. ZHANG Yongjiang from Xishuangbanna Tropical Botanical Garden (XTBG) of Chinese Academy of Sciences and his international colleagues conducted a study to test whether cycad leaves are governed by the same fundamental design principles previously established for ferns, conifers and angiosperms. They characterized the uniqueness of this relict lineage in foliar trait relationship and aimed to assess the association of traits (including nutrient composition, leaf structure, stomatal conductance and leaf hydraulics) with photosynthetic rate, based on studies of angiosperms, conifers, ferns and mosses.

The research was carried out in the Cycad Garden at XTBG and the National Cycad Germplasm Conservation at Fairylake Botanical Garden. 33 cycad species from nine genera and three families were selected. Based on their native habitats, the species were classified into three classes: native to dry habitats, moist habitats, or both dry and moist habitats. Leaf structure, photosynthesis, hydraulics and nutrient composition were studied.

Using linear or nonlinear regression, the researchers analyzed the relationship among functional traits (including leaf thickness and density, gas exchange, nutrient and chlorophyll concentrations, and leaf hydraulic conductance). They tested the equal variances of the variables and used one-way analysis of variance to test trait differences among cycad genera.

The study found that the cycad species varied strongly in leaf gross morphology and shape, photosynthetic rate, stomata as well as in hydraulic conductance. They also varied strongly in nutrient concentrations and leaf mass per area. The leaf economic trait relationships that were hypothesized based on those established for angiosperms were strong across the cycad species. There were stronger trait relationships for mass-based than area-based light-saturated CO₂ assimilation rate. 

The researchers concluded that the relationships among cycad leaf functional traits generally agreed with the global economic spectrum a strikingly consistent pattern (i.e. a strikingly consistent pattern of correlations among leaf traits). High hydraulic conductance and photosynthetic rate, as well as high diversification in leaf structure and function would have facilitated their dominance in a variety of ecosystems. Nutrients other than nitrogen (e.g. phosphorus and iron) explained a significant amount of variation in photosynthetic rate per mass of cycads as well. This suggests the importance of introducing nutrients other than nitrogen and phosphorus into a higher resolution understanding of the leaf economic spectrum. There was no coordination between leaf hydraulic conductance and photosynthesis among cycads. 

The study entitled "Extending the generality of leaf economic design principles in the cycads, an ancient lineage” has been published on New Phytologist. 

Figure: Cycad (Image by DUAN Qiwu)