THE NATURE OF HETEROGENEITY IN THE STOMATAL BEHAVIOR OF PHASEOLUS-VULGARIS L. PRIMARY LEAVES

The aim of this research was to investigate the nature of heterogeneit y in stomatal conductance and, in particular, to determine whether the characteristic 'patchy' pattern of water infiltration is reflected in measurements on individual stomata. Silicone rubber replicas were mad e of primary leaves of glasshouse-grown Phaseolus vulgaris L, plants, and the leaves were then infiltrated with water at controlled subatmos pheric gas pressures according to their estimated or measured stomatal conductance. Seven leaves examined in detail all showed patchy infilt ration, and the mean size of infiltrated areas was negatively correlat ed with the prevailing stomatal conductance. in four of the leaves, a one millimetre wide transect across the leaf was selected for further detailed study. Measurements of mean peristomatal groove distance (PGD ) and stomatal frequency were made along the transect and related to t he state of infiltration. Analysis of variance indicated that, in all four cases, variation in PGD among patches was highly significant, but there was no significant difference between patches of different infi ltration categories. Thus, local (patch-level) variation in stomatal a perture appeared to bear no relation to the infiltration status of the patches. The dominant source of stomatal variability was between indi vidual pores in the same locality, which accounted for 82% or more of the total variability. Taking into account Variation in stomatal frequ ency, correlations between predicted stomatal conductance and the exte nt of infiltration were significant in only one out of the seven leave s studied. Possible reasons for these results are discussed. It is sug gested that the infiltration method misrepresents the underlying state of the stomata as being either open or closed, when there is little e vidence for this from measurements of stomatal dimensions. For these u nstressed plants under relatively stable conditions, it is concluded t hat the 'unit of variability' in stomatal heterogeneity may rest at th e individual pore ('micro') scale, rather than at the areolar patch (' macro') scale, or above.