Leaf Vasculature and the Upper Limit of Photosynthesis

Abstract

The foliar vascular network is responsible for (1) structural support of the lamina as a platform for absorbing photons of light to drive photosynthesis, (2) transfer of information carriers like hormones and other signaling molecules between the leaf and other parts of the plant, (3) distribution of water and nutrients to leaf tissues via the xylem, and (4) movement of photosynthetic products, as well as chemical components remobilized during senescence, from mesophyll tissue into the phloem, and from source leaves to the plant’s many sinks. Foliar venation is thus central to the leaf’s primary role as a photosynthetic organ. Positive relationships between hydraulic conductance of the xylem, foliar vein density, and photosynthesis have been studied, and close links between foliar phloem capacity and intrinsic photosynthetic capacity were identified more recently. In this chapter, the relationship between various features of the foliar vasculature and photosynthetic capacity in mesophytic species with high rates of photosynthesis is explored. These metrics include foliar vein density, numbers and/or cross-sectional areas of xylem, phloem, and companion (including intermediary) cells, tracheary and sieve elements, and expansion of cell membrane area due to cell wall ingrowths in phloem transfer cells. Total xylem conduit volume per leaf area (the product of vein density and xylem cell metrics) of minor foliar veins exhibited a strong positive relationship with photosynthetic capacity per leaf area among multiple summer annuals. In the winter annual Arabidopsis thaliana, acclimation to contrasting growth temperatures involves differential acclimation of photosynthesis versus transpiration and is matched by similar differential acclimation of phloem versus xylem features. Photosynthetic capacity was positively correlated with various phloem metrics among all species and conditions examined, including summer annuals, winter annuals, and biennial species under various temperature and light conditions during growth. Given the essential role of vasculature in leaf functioning, it is not surprising that foliar vascular metrics are adjusted in response to environmental conditions (temperature, light levels, etc.). The vascular grid of the leaf and its xylem and phloem components thus underlies efficient leaf and plant functioning by facilitating the exchange of water, nutrients, and energy and information carriers between photosynthetic and non-photosynthetic parts of the plant. Recognition of this centrality of the foliar vasculature is critical to the effective selection, breeding, and engineering of crop plants to meet the nutritional, energy, fiber, material, and pharmaceutical needs of an expanding human population.