In a greenhouse study, 12 common bean cultivars from a wide geographical range
were compared for their morphological, gas exchange and hydraulic architecture
characters. Cultivars bred for cultivation in hot and dry regions had
significantly smaller leaves and crowns, but higher stomatal conductances and
transpiration rates per unit of leaf area. Short-term variability in gas
exchange rates was confirmed using leaf carbon isotope discrimination. A
literature survey showed that, although previously unnoticed, the strong
inverse coupling between leaf size and gas exchange rates was present in three
other studies using the same set of cultivars. Several measures of
‘leaf-specific hydraulic conductance’ (i.e. for the whole plant
and for different parts of the xylem pathway) were also linearly related to
rates of water loss, suggesting that the coupling between leaf size and gas
exchange was mediated by a hydraulic mechanism. It is possible that breeding
for high production in hot regions has exerted a selection pressure to
increase leaf-level gas exchange rates and leaf cooling. The associated
reductions in leaf size may be explained by the need to maintain equilibrium
between whole-plant water loss and liquid-phase hydraulic conductance.
Heterogeneity of gas exchange rates over the leaf surface in tobacco: an effect of hydraulic architecture?
