Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies

Abstract

The effects of abscisic acid (ABA) on photosynthesis in leaves of Helianthus annuus L. were compared with those in leaves of Vicia faba L. After the ABA treatment, the response of photosynthetic CO2 assimilation rate, A, to calculated intercellular partial pressure of CO2, Pi, (A(pi) relationship) was markedly depressed in H. annuus. A less marked depression was also observed in V.faba. However, when the abaxial epidermes were removed from these leaves, neither the maximum rate nor the CO2 response of photosynthetic oxygen evolution was affected by the application of ABA. Starch-iodine tests revealed that photosynthesis was not uniform over the leaves of H. annuus treated with ABA. The starch content was diffferent in each bundle sheath extension compartment (the smallest subdivision of mesophyll by veins with bundle sheath extensions, having an area of ca. 0.25 mm2 and ca. 50 stomata). In some compartments, no starch was detected. The distribution of open stomata, examined using the silicone rubber impression techniques, was similar to the pattern of starch accumulation. In V.faba leaves, which lack bundle sheath extensions, distribution of starch was more homogeneous. These results indicate that the apparent non-stomatal inhibition of photosynthesis by ABA deduced from the depression of A(pi) relationship is an artifact which can be attributed to the non-uniform distribution of transpiration and photosynthesis over the leaf. Intercellular gaseous environment in the ABA-treated leaves is discussed in relation to mesophyll anatomy.