Photosynthesis, stomatal conductance and terpene emission response to water availability in dry and mesic Mediterranean forests

Warmer summer conditions result in increased terpene emissions except under severe drought, in which case they strongly decrease. Water stress results in a reduction of the metabolism of plants and in a reorganization of their use of resources geared to survival. In the Mediterranean region, periods of drought accompanied by high temperatures and high irradiance occur in summer. Plants have developed various mechanisms to survive in these conditions by resisting, tolerating or preventing stress. We used three typical Mediterranean tree species in Israel, Pinus halepensis L., Quercus calliprinos and Quercus ithaburensis Webb, as models for studying some of these adaptive mechanisms. We measured their photosynthetic rates (A), stomatal conductance (g (s)), and terpene emission rates during spring and summer in a geophysical gradient from extremely dry to mesic from Yatir (south, arid) to Birya (north, moist) with intermediate conditions in Solelim. A and g (s) of P. halepensis were threefold higher in Birya than in Yatir where they remained very low both seasons. Quercus species presented 2-3-fold higher A and g (s) but with much more variability between seasons, especially for Q. ithaburensis with A and g (s) that decreased 10-30-fold from spring to summer. Terpene emission rates for pine were not different regionally in spring but they were 5-8-fold higher in Birya than in Yatir in summer (P < 0.05). Higher emissions were also observed in Solelim for the drought resistant Q. ithaburensis (P < 0.001) but not for Q. calliprinos. alpha-Pinene followed by limonene and 3-carene were the dominant terpenes. Warmer summer conditions result in increased Terpene emission rates except under severe drought, in which case they strongly decrease.