In natural populations of the desert chamaephyte Cryptantha flava, leaf level gas exchange properties vary with plant size, but in some ways inconsistent with whole plant performance. Smaller, usually younger plants often exhibit lower maximum photosynthesis (Amax), lower midday leaf water potential (Ψ), higher stomatal conductance (gmax), and lower instantaneous water use efficiency (A/E), despite having higher relative growth rates. We explored whether these size specific differences in gas exchange hold for other species and whether any differences are strictly size specific or related to developmental stage. The juvenile to adult developmental transition, vegetative phase change, is controlled by a microRNA, miR156. Juvenile leaves, produced when plants are younger and smaller, have higher expression of miR156 and differ from adult leaves in a number of anatomical traits. We measured within plant variation in leaf level gas exchange and leaf anatomy in Arabidopsis thaliana and Populus tremula x alba and separated variation due to size or age (leaf position) and developmental stage by including mutants overexpressing miR156, prolonging the juvenile stage to later leaf positions.
Results/Conclusions
In both species, Amax was lower in juvenile stages as were leaf N, stomatal density, and leaf vein density, while specific leaf area (SLA) was greater. In contrast, A/E was primarily explained by leaf position, increasing in later leaf positions in P. tremula x alba. Conductance (gs) did not differ with development or age in either species. These patterns are roughly consistent with leaf gas exchange in C. flava where less carbon is obtained in smaller plants for the same or greater water expenditure. In C. flava, drought affects gas exchange equally across size categories but still has the demographic consequence of reshuffling the population size hierarchy. Most small plants maintain positive growth during drought while larger plants can show considerable dieback, where a portion of the shoot and its associated root die between growing seasons. We rely on both data sets when concluding that leaf level measurements must be viewed within the context of the whole plant, including changes in root architecture with age. Younger, smaller plants may exhibit lower photosynthetic competence but access at least as much soil water per leaf surface area. Maintenance costs and N budgets must also factor in explaining size specific differences in plant growth. Understanding the physiology of the whole plant at different developmental stages is important for forecasting population responses to a changing climate.