Acquired changes in stomatal characteristics in response to ozone during plant growth and leaf development of bush beans (Phaseolus vulgaris L.) indicate phenotypic plasticity

Bush bean (Phaseolus vulgaris L.) lines ‘S156’ (O3-sensitive)/‘R123’ (O3-tolerant) and cultivars ‘BBL 290’ (O3-sensitive)/‘BBL 274’ (O3-tolerant) were used to study the effects of O3 on stomatal conductance (gs), density, and aperture size on leaf and pod surfaces with the objective of establishing links between the degree of plant sensitivity to O3 and plasticity of stomatal properties in response to O3. Studies in open-top chambers (OTCs) and in continuously stirred tank reactors (CSTRs) established a clear relationship between plant developmental stages, degrees of O3 sensitivity and gs: while ‘S156’ had higher gs rates than ‘R123’ earlier in development, similar differences between ‘BBL 290’ and ‘BBL 274’ were observed at later stages. Gs rates on the abaxial leaf surfaces of ‘S156’ and ‘BBL 290’, accompanied by low leaf temperatures, were significantly higher than their O3-tolerant counterparts. Exposure to O3 in CSTRs had greater and more consistent impacts on both stomatal densities and aperture sizes of O3-sensitive cultivars. Stomatal densities were highest on the abaxial leaf surfaces of ‘S156’ and ‘BBL 290’ at higher O3 concentrations (60 ppb), but the largest aperture sizes were recorded on the adaxial leaf surfaces at moderate O3 concentrations (30 ppb). Exposure to O3 eliminated aperture size differences on the adaxial leaf surfaces between sensitive and tolerant cultivars. Regardless of sensitivity to O3 and treatment regimes, the smallest aperture sizes and highest stomatal densities were found on the abaxial leaf surface. Our studies showed that O3 has the potential to affect stomatal plasticity and confirmed the presence of different control mechanisms for stomatal development on each leaf surface. This appeared to be more evident in O3-sensitive cultivars.