Carbon is the currency that plants use to build and maintain tissues. Various strategies for maximizing carbon acquisition and investment are major determinants for overall plant success. Plant carbon economics describes the relationships between construction cost, photosynthetic input, maintenance costs and longevity of plant tissue. How these relationships vary over the lifetime of an individual and the processes that may govern any differences is not well understood. Plants transition through distinct developmental phases as they age. Vegetative phase change (VPC), the developmental transition from juvenile to adult vegetative growth, is regulated by an evolutionarily conserved microRNA, miR156. miR156 is expressed at high levels during the juvenile phase and declines later in development. In addition to differences in gene expression, juvenile and adult leaves vary in photosynthetic traits and morphology suggesting that plant carbon economic strategies may differ through development. We examined leaf carbon economics through VPC by measuring construction cost, photosynthetic rates, respiration rates and lifespan of juvenile and adult leaves in both wild-type and miR156 overexpressor mutants that have a prolonged juvenile phase in Arabidopsis thaliana, Populus tremula x alba and Zea mays. We also modeled carbon economics under variable light environments inherently experienced by plants in natural settings by integrating light response properties for juvenile and adult leaves of each species.
Results/Conclusions
In all three species, juvenile leaves had lower construction costs than their adult counterparts. Despite their often-lower photosynthetic rates, the payback time for juvenile leaves (time required to meet its own construction cost) was significantly shorter under all light conditions. This advantage in payback time was higher under low light, suggesting an importance for the juvenile phase in shaded conditions when young plants are establishing with little stored resources. While adult leaves had higher costs and longer payback times, they had a longer lifespan than juvenile leaves. This increased lifespan lead to a greater return on investment for the plant, especially in high-light environments. These differences in carbon economics can be attributed to VPC because juvenilized leaves of miR156 overexpressor mutants had the same characteristics as the wild-type juvenile leaves on younger plants. These results suggest that plants employ different carbon economic strategies during development and that these differences are likely important contributors to how plants respond and adapt to environmental conditions.