Alterations in photosynthesis and pigment distributions in pea leaves following UV‐B exposure

We compared photosynthetic and UV‐B‐absorbing pigment concentrations, gas‐exchange 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 effective daily dose) in a greenhouse. We also compared the distribution of chlorophyll by depth within leaves of each treatment by using image analysis of chlorophyll autofluorescence. Ultraviolet‐B treatment elicited putative protective responses such as an 80% increase in UV‐B‐absorbing compound concentrations (leaf‐area basis), and a slight increase in mesophyll thickness (178 in controls compared to 191 μm in UV‐B‐treated leaves). However, photosynthetic rates of UV‐B‐treated leaves were only 80% of those of controls. This was paralleled by reductions in leaf conductance to water vapor (50% of controls) and intercellular CO₂ concentrations, suggesting that stomatal limitations were at least partly responsible for lower photosynthetic rates under the UV‐B treatment. Total chlorophyll concentrations (leaf‐area basis) in UV‐B‐treated leaves were only 70% of controls, and there was a shift in the relative distribution of chlorophyll with depth in UV‐B‐treated leaves. In control leaves chlorophyll concentrations were highest near the adaxial surface of the upper palisade, dropped with depth and then increased slightly in the bottom of the spongy mesophyll nearest the abaxial surface. 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‐μm‐thick paradermal layer were about 50% lower in UV‐B‐treated leaves than in controls. We found reduced electron transport capacity in UV‐B‐treated leaves, based on lower maximum fluorescence (F_m), variable to maximum fluorescence ratios (F,/F_m) and quantum yield of PSII electron transport (Y). However, the above were assessed from fluorometer measurements on the adaxial leaf surface and may reflect the markedly lower chlorophyll concentrations in the upper palisade of UV‐B‐treated leaves.