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.