Differences in ecophysiological responses to summer drought between seedlings of three deciduous oak species

Seasonal variations in water status and gas exchange were assessed in 2-year-old seedlings of Quercus pubescens Willd., Quercus ithaburensis Decaisne subsp. macrolepis (Kotsch) Hedge & Yalt, and Quercus frainetto Ten. during the 2004 growing season characterized by the typical summer drought conditions observed in Greece. The seedlings were grown under field conditions. After establishment half of the seedlings were well watered, while the rest received no irrigation other than natural rainfall. Measurements were conducted from April to October.Progressive drought reduced water potentials and gas exchange in all species. However, Q. ithaburensis displayed significantly higher water and osmotic potentials and assimilation rate than the other species. In contrast, non-watered plants of Q. frainetto exhibited the lowest leaf conductance and finally leaf loss after predawn water potentials dropped to as low as −5.8 MPa. Predawn water potentials were affected by the temperature of the day before measurements and afternoon water potentials of stressed plants were controlled by humidity. Stomatal conductance correlated positively with water potentials of non-watered seedlings of Q. pubescens and Q. frainetto. The results suggested that water potentials in Q. ithaburensis appeared to be independent of stomatal regulation. Based on interspecific variations in water relations and gas exchange patterns under drought, Q. ithaburensis exhibits drought-avoidance characteristics and seems the most suitable candidate among studied Quercus species for growth in xerothermic environments. Despite its substantially lower water potentials Q. pubescens also shows drought resistance potential, hence suitability for reforestation purposes by recovering to pre-drought patterns, which could indicate desiccation tolerance response promoted by osmotic adjustment. Finally, Q. frainetto appears to be most prone to summer drought.