The nature of heterogeneity in the stomatal behaviour of Phaseolus vulgaris L. primary leaves

The aim of this research was to investigate the nature of heterogeneity instomatal conductance and, in particular, to determine whether thecharacteristic 'patchy' pattern of water infiltration is reflected inmeasurements on individual stomata. Silicone rubber replicas were made ofprimary leaves of glasshouse-grown Phaseolus vulgarisL. plants, and the leaves were then infiltrated with water at controlledsub-atmospheric gas pressures according to their estimated or measuredstomatal conductance. Seven leaves examined in detail all showed patchyinfiltration, and the mean sire of infiltrated areas was negativelycorrelated with the prevailing stomatal conductance. In four of the leaves,a one millimetre wide transect across the leaf was selected for furtherdetailed study. Measurements of mean peristomatal groove distance(PGD) and stomatal frequency were made along thetransect and related to the state of infiltration. Analysis of varianceindicated that, in all four cases, variation in PGDamong patches was highly significant, but there was no significantdifference between patches of different infiltration categories. Thus,local (patch-level) variation in stomatal aperture appeared to bear norelation to the infiltration status of the patches. The dominant source ofstomatal variability was between individual pores in the same locality,which accounted for 82% or more of the total variability. Taking intoaccount variation in stomatal frequency, correlations between predictedstomatal conductance and the extent of infiltration were significant inonly one out of the seven leaves studied. Possible reasons for theseresults are discussed. It is suggested that the infiltration methodmisrepresents the underlying state of the stomata as being either open orclosed, when there is little evidence for this from measurements ofstomatal dimensions. For these unstressed plants under relatively stableconditions, it is concluded that the 'unit of variability' in stomatalheterogeneity may rest at the individual pore ('micro') scale, rather thanat the areolar patch ('macro') scale, orabove.