Photosynthetic characteristics, stomatal responses and water relations of Fagus sylvatica: impact of air quality at a site in southern Britain.

Transplants of beech (Fagus sylvatcia L.) were grown in open-top chambers ventilated with either charcoal-filtered or unfiltered air. From May until September measurements of stomatal conductance to water vapour (gs ) were made on trees in the chambers. On six out of seven occasions, leaves which had expanded during the first flush (initial shoot extension, during May) in unfiltered air, had lower stomatal conductances, compared with those grown in filtered air; this was significant on three occasions (P < 0.05). Leaves which expanded later in the season (second Hush or lammas growth) responded differently to the air quality treatments, with greater stomatal conductances recorded for trees exposed to unfiltered air. Measurements of leaf (psychrometry) and shoot (pressure bomb) water relations indicated that air quality had little effect on turgor pressure (P). Values of solute (ψs ) and water (ψw ) potential were higher for trees grown in unfiltered air. Trees were transported to the laboralory for measurement of photosynthesis (A), transpiration (E) and stomatal conductance (gs ). Measurements of CO, uptake were also made whilst the CO, concentration within the leaf (Cf ) was experimentally controlled (A/Ci analysis). This revealed that higher rates of photosynthesis for lammas leaves grown in unfiltered air were due to enhanced regeneration of RuBP (increased Jmax , P < 0.08). Carboxylation efficiency (d.A/dCf ) and percentage stomatal limitation were not significantly altered by air quality. Jmax and dA/dCi were similarly reduced following exposure to drought regardless of the air quality treatment.