Elevated CO2 and temperature have different effects on leaf anatomy of perennial ryegrass in spring and summer

Mature second leaves of Lolium perenne L. cv. Vigor, were sampled in a spring and summer regrowth period. Effects of CO2 enrichment and increased air temperature on stomatal density, stomatal index, guard cell length, epidermal cell density, epidermal cell length and mesophyll cell area were examined for different positions on the leaf and seasons of growth. Leaf stomatal density was smaller in spring but greater in summer in elevated CO2 and higher in both seasons in elevated temperature and in elevated CO2 x temperature relative to the respective controls. In spring, leaf stomatal index was reduced in elevated CO2 but in summer it varied with position on the leaf. In elevated temperature, stomatal index in both seasons was lower at the tip/middle of the leaf but slightly higher at the base. In elevated CO2 x temperature, stomatal index varied with position on the leaf and between seasons. Leaf epidermal cell density was higher in all treatments relative to controls except in elevated CO2 (spring) and elevated CO2 x temperature (summer), it was reduced at the leaf base. In all treatments, stomatal density and epidermal cell density declined from leaf tip to base, whilst guard cell length showed an inverse relationship, increasing towards the base. Leaf epidermal cell length and mesophyll cell area increased in elevated CO2 in spring and decreased in summer. In elevated CO2 x temperature leaf epidermal cell length remained unaltered in spring compared to the control but decreased in summer. Stomatal conductance was lower in all treatments except in summer in elevated CO2 it was higher than in the ambient CO2. These contrasting responses in anatomy to elevated CO2 and temperature provide information that might account for differences in seasonal leaf area development observed in L. perenne under the same conditions.