My, what big leaves you’ve got

A plant’s gotta breathe. Carbon dioxide in, oxygen out – at least, while the sun shines. Trouble is, water out too, and that is a problem. So plants have sophisticated mechanisms of controlling gas transfer and water loss though controllable pores on the leaves called stomata.

In large leaves, a considerable amount of the leaf area will be quite distant from the point of insertion of the petiole that represents the source of water. For a large and entire (round) leaf, the margins and tip should therefore be more prone to water stress than basal or more central leaf regions since these sites are located at the ‘end’ of the pressure drop that extends over the leaf venation during transpiration. In fact, size is one leaf trait that shows significant negative correlation with decreasing humidity. Spatial differences in stomatal regulation preventing leaf zonal water stress might therefore be anticipated.

A recent paper in Annals of Botany examines the large-leaved temperate vine Aristolochia macrophylla. The study was performed on an adult specimen under natural conditions. This species has a dense crown with large, thin and entire leaves – the leaves show almost identical length and width. This shape leads to a high ‘accumulation’ of leaf area that should particularly promote two-dimensional gradients of water supply. It finds that the diurnal course of gas exchange regulation in A. macrophylla leaves depends on the leaf zone. Positional co-ordination of gas exchange is commonly found for the different insertion heights within a plant. These intercanopy gradients reflect not only differences in microclimate but also local water deficits and/or hydraulic constraints. The entire plant water system with its local differentiations is integrated by stomatal function modulating supply and demand.

Diurnal pattern of stomatal conductance in the large-leaved temperate liana Aristolochia macrophylla depends on spatial position within the leaf lamina. (2013) Annals of Botany 111 (5): 905-915. doi: 10.1093/aob/mct061
The large distance between peripheral leaf regions and the petiole in large leaves is expected to cause stronger negative water potentials at the leaf apex and marginal zones compared with more central or basal leaf regions. Leaf zone-specific differences in water supply and/or gas exchange may therefore be anticipated. In this study, an investigation was made to see whether zonal differences in gas exchange regulation can be detected in large leaves. The diurnal course of stomatal conductance, gs, was monitored at defined lamina zones during two consecutive vegetation periods in the liana Aristolochia macrophylla that has large leaves. Local climate and stem water potential were also monitored to include parameters involved in stomatal response. Additionally, leaf zonal vein densities were measured to assess possible trends in local hydraulic supply. It was found that the diurnal pattern of gs depends on the position within a leaf in A. macrophylla. The highest values during the early morning were shown by the apical region, with subsequent decline later in the morning and a further gradual decline towards the evening. The diurnal pattern of gs at the marginal regions was similar to that of the leaf tip but showed a time lag of about 1 h. At the leaf base, the diurnal pattern of gs was similar to that of the margins but with lower maximum gs. At the the leaf centre regions, gs tended to show quite constant moderate values during most of the day. Densities of minor veins were lower at the margin and tip compared with the centre and base. Gas exchange regulation appears to be zone specific in A. macrophylla leaves. It is suggested that the spatial–diurnal pattern of gs expressed by A. macrophylla leaves represents a strategy to prevent leaf zonal water stress and subsequent vein embolism.