RESPONSE OF FOLIAR METABOLISM IN MATURE TREES OF QUERCUS-PUBESCENS AND QUERCUS-ILEX TO LONG-TERM ELEVATED CO2

Long-term effects on and adaptations of the carbon physiology of long- lived trees exposed to increasing atmospheric levels of CO2 are unknow n. We compared two indigenous Quercus species, Q. ilex and Q. pubescen s, growing in a natural CO2 spring located in central Italy and at a n earby control site. In May, 1995 photosynthetic rate at least doubled when measured with supplemental CO2 in both species and sites. Dark re spiration was much higher at the CO2 spring site in both species. Foli ar sugar and starch concentrations in Q. ilex exhibited significant si te and diurnal differences (May and September). In July, 1995 there wa s little difference in the water potential values of the measured tree s at the different sites over the diurnal period. Photosynthetic rate was higher for both species in the CO2 spring, particularly in the ear ly morning and late afternoon. Mid-day stomatal closure reduced photos ynthesis to similar levels. In the morning leaf conductance and transp iration were generally lower in the CO2 spring trees, contributing to higher instantaneous water use efficiency for both species. Isoprene e mission rates were higher in Q. pubescens trees growing in the CO2 spr ing. The maximum difference between control and CO2 spring trees occur red in late afternoon. In contrast, Q. ilex exhibited isoprene emissio n near background level. Foliage and branch carbon and nitrogen status showed increased concentrations of starch and tannins in Q. ilex and of soluble sugars in Q. pubescens in the elevated CO2 environment, whi le nitrogen concentration decreased in both species. Wood gravity incr eased 6 and 3% in Q. ilex and Q. pubescens, respectively, growing in t he CO2 spring. Q. ilex exhibited afternoon recovery of water potential compared to Q. pubescens which had better night-time recovery. Q. ile x and e. pubescens exposed to elevated CO2 for prolonged periods exhib it different mechanisms for dealing with additional reduced carbon and do maintain an altered carbon physiology, even in midst of the region 's characteristic summer drought. (C) 1998 Elsevier Science B.V. All r ights reserved.