Water stress responses of seedlings of four Mediterranean oak species

Effects of water stress on phenology, growth, stomatal activity and water s tatus were assessed from April to November 1996 in 2-year-old seedlings of Quercus frainetto Ten. (Quercus conferta Kit.), Quercus pubescens Willd., Q uercus macrolepis Kotschy (Quercus aegilops auct.) and Quercus ilex L. grow ing in containers in northern Greece. All four species developed more than 50% of their total leaf area before the beginning of June-an adaptation to arid climates. Well-irrigated plants tended to develop greater individual l eaf area, number of leaves per plant, total plant leaf area, height and roo t:shoot ratios than water-stressed plants, but the difference between treat ments was not significant for any parameter in any species. Quercus macrole pis appeared to be the most drought-tolerant of the four species. It mainta ined the highest number of leaves of the smallest size and increased the pr oportion of fine roots during drought. In all species, drought caused signi ficant decreases in stomatal conductance and predawn and midday water poten tials from mid-July until the end of August, when the lowest soil water con tent and highest mean daily air temperatures and midday leaf temperatures o ccurred; however, the responses were species-specific. Among the four speci es, Quercus macrolepis sustained the highest stomatal conductance despite v ery low water potentials, thus overcoming drought by means of desiccation t olerance. Quercus ilex decreased stomatal conductance even before severe wa ter stress occurred, thereby avoiding dessication during drought. Quercus p ubescens had the highest water potential despite a high stomatal conductanc e, indicating that its leaf water status was independent of stomatal activi ty. Quercus frainetto was the least drought-resistant of the four species. During drought it developed very low water potentials despite markedly redu ced stomatal aperture.