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 concentra tion near a natural CO2 vent in Orciatico, Italy. Diurnal courses of net ph otosynthetic rate (P-N), ratio of variable to maximum chlorophyll fluoresce nce (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 cat egories were also determined. A morning peak and midday depression of P-N w ere found for both AC and EC sun leaves. Long-term EC caused little or no d own-acclimation of P-N in sun leaves. The estimate of total daily CO2 uptak e 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 tre nd in the morning, reached a minimum at midday (90 % of dawn value), and th en increased in the afternoon. The EC had no effect on F-v/F-m, either in s un 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.