OAK GROWTH, DEVELOPMENT AND CARBON METABOLISM IN RESPONSE TO WATER-STRESS

The genus Quercus (Fagaceae) contains both deciduous and evergreen spe cies adapted to a wide range of sites differing widely in moisture ava ilability. Different oak species have developed both morphological and physiological adaptations to survive and grow on such sites. Morpholo gical adaptations in leaves, stems and roots aid in both drought avoid ance and drought tolerance. Physiological adaptations involve control of stomatal conductance, leaf water potential, osmotic adjustment and photosynthetic carbon fixation. Carbon fixation can be divided into st omatal and nonstomatal responses. Stomatal response is probably the mo st important factor controlling carbon fixation. The more drought-tole rant species control stomatal function to allow some carbon fixation w ith stress, thus improving water use efficiency, or open stomates rapi dly when water stress is relieved. Nonstomatal responses of carbon fix ation such as photosystem II light energy conversion and the dark reac tions of Rubisco carbon fixation are quite resistant to water stress, although internal resistance to CO2 movement may increase. With water stress, soluble sugar/starch ratios increase, new leaf development dec reases or stops altogether, and carbon allocated to leaf development s hifts to lower stem and root for growth or storage. Many oak species, genotypes and hybrids are available that may be adapted to difficult s ites. Use of such genotypes could greatly improve current forest manag ement systems and horticultural amenity plantings.