Oak growth, development and carbon metabolism in response to water stress

The genus Quercus (Fagaceae) contains both deciduous and evergreen species adapted to a wide range of sites differing widely in moisture availability. Different oak species have developed both morphological and physiological adaptations to survive and grow on such sites. Morphological adaptations in leaves, stems and roots aid in both drought avoidance 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 stomatal and nonstomatal responses. Stomatal response is probably the most important factor controlling carbon fixation. The more drought-tolerant species control stomatal function to allow some carbon fixation with stress, thus improving water use efficiency, or open stomates rapidly when water stress is relieved. Nonstomatal responses of carbon fixation such as photosystem II light energy conversion and the dark reactions 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 decreases or stops altogether, and carbon allocated to leaf development shifts to lower stem and root for growth or storage. Many oak species, genotypes and hybrids are available that may be adapted to difficult sites. Use of such genotypes could greatly improve current forest management systems and horticultural amenity plantings.