Interactive effects of elevated [CO2] and drought on cherry (Prunus avium)seedlings I. Growth, whole-plant water use efficiency and water loss

Seeds of cherry (Prunus avium) were germinated and grown for two growing se asons in ambient (similar to 350 mu mol mol(-1)) or elevated (ambient + sim ilar to 350 mu mol mol(-1)) CO2 mole fractions in six open-top chambers. Th e seedlings were fertilized once a week, following Ingestad principles in o rder to supply mineral nutrients at free-access rates. In the first growing season gradual drought was imposed on rapidly growing cherry seedlings by withholding water for a 6-wk drying cycle. In the second growing season, th e rapid onset of drought was imposed at the height of the growing season on the seedlings which had already experienced drought in the first growing s eason. Elevated [CO2] significantly increased total dry-mass production in both water regimes, but did not ameliorate the growth response to drought o f the cherry seedlings subjected to two sequential drying cycles. Water los s did not differ in either well watered or droughted seedlings between elev ated and ambient [CO2]; consequently whole-plant water-use efficiency (the ratio of total dry mass produced to total water consumption) was significan tly increased. Similar patterns of carbon allocation between shoot and root were found in elevated and ambient [CO2] when the seedlings were the same size. Thus, elevated [CO2] did not improve drought tolerance, but it accele rated ontogenetic development irrespective of water status.