Seasonal and diurnal gas exchange characteristics and water relations of the drought semi-deciduous shrub Phlomis fruticosa L. under Mediterranean field conditions

Abstract

Summary Diurnal and seasonal fluctuations in gas exchange and water relations were examined in the field during a 21 month's period in the drought semi-deciduous shrub Phlomis fruticosa L., in an attempt to reveal environmental factors that may limit its growth and distribution. P.fruticosa is an eastern Mediterranean malacophyllous plant dominating the so called “phryganic” ecosystem in Greece (synonym to coastal sage in California and garrigue in France). Maximum photosynthetic rates (c.a. 30 μmol CO 2 m −2 s −1 ) were observed during the spring, with a secondary maximum at late autumn, a deep minimum during summer and early autumn and a secondary minimum during mid-winter. The deep minimum (c.a. 2 − 3 μmol CO 2 m −2 S −1 ) coincided with minimal shoot water potentials and a well-developed stomatal regulation of excessive water loss and CO 2 assimilation. Photosynthesis, however, was additionally limited during the dry period by a substantial drop in photosynthetic capacity, as judged by CO 2 exchange measurements at saturated (5 %) CO 2 . The secondary minimum in photosynthesis during mid-winter, occurring at a period of relatively high pre-dawn shoot water potentials and low atmospheric evaporative demand, was partly related to low temperatures in a rather indirect way. Indeed, diurnal changes in shoot water potential revealed a characteristic delay in the replacement of evaporated water at low temperatures, leading to leaf water shortage and stomatal closure, even during the wet period. We may conclude, therefore, that growth in P.fruticosa is restricted by low water potentials during the warm dry period and, possibly, by increased hydraulic resistance when the winter temperatures fall to relatively low values.