Ecophysiological adaptations of water relations of Teucrium chamaedrys L. to the hot and dry climate of xeric limestone sites in Franconia (Southern Germany)

Teucrium chamaedrys is a typical plant of dry grassland communities at xeric sites on calcareous soils in Franconia (Southern Germany). The morphological-anatomical appearance of limestone plants in the form of nanism or dwarfing has been interpreted by Gregor Kraus approximately 100 years ago as a consequence of the dry and hot environmental conditions. In the present study the mechanisms of adaptation to drought were evaluated by comparison of water relations of plants growing in the natural habitat at a xeric site in the nature protection area “Grainberg-Kalbenstein und Saupurzel” with plants cultivated at a mesic site in the Botanical Garden of the University of Würzburg. Cuticular water permeances of astomatous leaf sides and minimum conductances of detached leaves were not significantly different between the two growing regimes indicating that there are no intraspecific/phenotypic adaptations in terms of the establishment of a more efficient cuticular transpiration barrier and a more strict regulation of the stomatal aperture as response to the xeric conditions of the limestone site. Pressure-volume analysis of water potential revealed that plants growing at the xeric site exhibited a lower water potential at the turgor loss point and a higher bulk modulus of elasticity. Both parameters emphasise the ability to regain more rapidly a favourable water status at drought by water uptake from the soil and to maintain turgor at lower water potentials. Gas exchange measurements obtained from attached leaves of intact plants under controlled experimental conditions in the laboratory revealed that under drought stress, at water potentials far below the turgor loss point, stomata were only completely closed in the dark. In the light the water loss by stomatal transpiration contributed with values between 60% and 75% to the total water loss. This physiological response was found both for plants grown at the xeric site and plants cultivated at the mesic site. The maintenance of a low but still measurable stomatal conductance under drought stress at maximum light intensity can be interpreted as a mechanism to adjust leaf temperature below the heat resistance limit by transpiration cooling.