ALTERATIONS IN PHOTOSYNTHESIS AND PIGMENT DISTRIBUTIONS IN PEA LEAVESFOLLOWING UV-B EXPOSURE

We compared photosynthetic and UV-B-absorbing pigment concentrations, gas-ex change rates and photosystem II (PSII) electron transport rates in leaves of pea (Pisum sativum mutant Argenteum) grown without UV-B or under an enhanced UV-B treatment (18 kJ m(-2) biologically effectiv e daily dose) in a greenhouse. We also compared the distribution of ch lorophyll by depth within leaves of each treatment by using image anal ysis of chlorophyll autofluorescence. Ultraviolet-B treatment elicited putative protective responses such as an 80% increase in UV-B-absorbi ng compound concentrations (leaf-area basis), and a slight increase in mesophyll thickness (178 in controls compared to 191 mu m in UV-B-tre ated leaves). However, photosynthetic rates of UV-B-treated leaves wer e only 80% of those of controls. This was paralleled by reductions in leaf conductance to water vapor (50% of controls) and intercellular CO 2 concentrations, suggesting that stomatal limitations were at least p artly responsible for lower photosynthetic rates under the UV-B treatm ent. Total chlorophyll concentrations (leaf-area basis) in UV-B-treate d leaves were only 70% of controls, and there was a shift in the relat ive distribution of chlorophyll with depth in UV-B-treated leaves. In control leaves chlorophyll concentrations were highest near the adaxia l surface of the upper palisade, dropped with depth and then increased slightly in the bottom of the spongy mesophyll nearest the abaxial su rface. In contrast, in UV-B-treated leaves chlorophyll concentrations were lowest at the adaxial surface of the upper palisade and increased with depth through the leaf. The most notable treatment difference in chlorophyll concentrations was in the upper palisade near the adaxial surface of leaves, where we estimate that chlorophyll concentrations in each 1-mu m-thick paradermal layer were about 50% lower in UV-B-tre ated leaves than in controls. We found reduced electron transport capa city in UV-B-heated leaves, based on lower maximum fluorescence (F-m), variable to maximum fluorescence ratios (F-v/F-m) and quantum yield o f PSII electron transport (Y). However. the above were assessed from f luorometer measurements on the adaxial leaf surface and may reflect th e markedly lower chlorophyll concentrations in the upper palisade of U V-B-treated leaves.