Photosynthetic characteristics of sun and shade leaves in the canopy of Arbutus unedo L. trees exposed to in situ long-term elevated CO2

Photosynthetic parameters were studied in Arbutus unedo L. trees growing at either ambient (AC) or elevated EC (mean 465 mu mol mol(-1)) CO2 concentration near a natural CO2 vent in Orciatico, Italy. Diurnal courses of net photosynthetic rate (P-N), ratio of variable to maximum chlorophyll fluorescence (F-v/F-m), and quantum yield of electron transport through photosystem 2 (Phi(2)) were measured on sun and shade leaves. The contents of N, C, Ca, K, P, and chlorophyll (Chl) and specific leaf area (SLA) in these leaf categories were also determined. A morning peak and midday depression of P-N were found for both AC and EC sun leaves. Long-term EC caused little or no down-acclimation of P-N in sun leaves. The estimate of total daily CO2 uptake was lower in AC leaves than in EC leaves. In shade leaves, it reached up to 70 % of the value of sun leaves. The F-v/F-m ratio showed decreasing trend in the morning, reached a minimum at midday (90 % of dawn value), and then increased in the afternoon. The EC had no effect on F-v/F-m, either in sun or shade leaves. Plants grown near the CO2 spring had lower Chi content, higher SLA, and higher Ca and K contents than plants grown under AC.