Viewing leaf structure and evolution from a hydraulic perspective

Abstract

Abstract. More than 40 000 km 3 year -1 of water flows through the intricate hydraulic pathways inside leaves. This water not only sustains terrestrial productivity, but also constitutes nearly 70% of terrestrial evapotranspiration, thereby influencing both global and local climate (Chapin et al. 2002). Thus, the central role played by leaf vascular systems in terrestrial biology provides an important context for research into the function and evolution of water transport in leaves. Significant progress has been made recently towards understanding the linkages between anatomy and water transport efficiency in leaves, and these discoveries provide a novel perspective to view the evolution of land plants.

Developmentally based scaling of leaf venation architecture explains global ecological patterns

“…The developmental algorithm and scaling relationships we have shown makes this possible for dicotyledons, providing a strong ecological advantage. Dicotyledons can produce large leaves with both large major veins for mechanical support and high vein densities that enable transpirational cooling and high photosynthetic rates 1,[47][48][49] . Large simple leaves with high vein density are an angiosperm innovation, because the ongoing production of minor veins in expanding leaves depends on the plate meristem that is typically found only in angiosperms 18 .…”

“…For example, a large leaf may be selected for a plant of a moist area, for effective light capture and photosynthesis relative to biomass allocation 44 , and a small leaf may be selected in drought-tolerant species 8 ; both may have high minor vein densities, enabling high photosynthetic rates when water is available. The potential of angiosperms to vary strongly in vein density enables dominance in a far greater range of habitats than other plant lineages 3,6,48 , especially as leaf size can adapt independently to optimise other ecological benefits against costs 6,7,44 .…”

Speculations on the application of foliar <sup>13</sup>C discrimination to reveal groundwater dependency of vegetation, provide estimates of root depth and rates of groundwater use

“…Since transpiration is directly linked to availability of water to roots, we hypothesised that LVD will 5 be correlated with depth-to-groundwater (DTGW) in plants for which groundwater is accessible; this correlation should be absent in species with shallow roots which cannot access groundwater. Whilst a number of studies have demonstrated increased LVD with increasing aridity along rainfall gradients (Brodribb et al, 2010;Brodribb and Holbrook, 2003;Sack and Holbrook, 2006), this relationship has not, to our knowledge, been examined in relation to DTGW. Finally, because LVD is strongly correlated with K leaf and rates of leaf-scale gas exchange Sack et al, 2003;Sack and 10 Frole, 2006), we hypothesise that LVD will be significantly correlated with Δ 13 C (and hence WUE i ).…”

“…In order to gain carbon most economically while minimising water loss (i.e., optimization of the ratio A/E), stomata should function such that the marginal water cost of carbon assimilation ( ) remains constant (Cowan and Farquhar, 1977;Farquhar and Sharkey, 1982). This aspect of stomatal 20 control couples the structural traits involved with water-flow with traits associated with primary production (Brodribb and Holbrook, 2007) and explains observed correlations between K leaf and A max in a number of studies Brodribb et al, 2010;Brodribb et al, 2005;Brodribb and Jordan, 2008;Sack and Holbrook, 2006;Sack and Scoffoni, 2013). The length of hydraulic pathway is directly proportional to K leaf and the A/g s ratio determines foliar Δ 13 C (and WUE i ) signatures in leaves.…”

“…LVD provides a direct estimate of K leaf because it correlates with the distance water must traverse from termini of the xylem-network to sites of evaporation (Brodribb et al, 2010). Since transpiration is directly linked to availability of water to roots, we hypothesised that LVD will 5 be correlated with depth-to-groundwater (DTGW) in plants for which groundwater is accessible; this correlation should be absent in species with shallow roots which cannot access groundwater.…”