CLIMATE AND WATER RELATIONS OF PLANTS 

 IN THE SUB-ALPINE REGION 



Walter Tranquillini 

 Forschungsstelle fiir Lawinenvorbeugung, Innsbruck, Austria 



INTRODUCTION 



The water balance of a plant is favourable as long as its transpiration and 

 water absorption are equal. If the water loss exceeds the absorption the 

 balance becomes negative. Plants have several means of recovering a good 

 balance or avoiding an increase of water deficit. They can 



1. intensify the water absorption 



(a) through extension of the root system by growth thus reaching 

 parts of the soil still wet 



(b) through increase of their suction forces by which they are able to 

 take up soil water that is held more furmly ; 



2. reduce the water loss 



(a) through reduction of the transpiring surface (shedding of leaves) 



(b) through increase in diffusion resistance of leaves (closing of stomata, 

 incipient drying of cell membranes) . 



Especially the last possibihty is frequently used by plants because it is 

 effective very quickly. An increase in transpiration resistance can be easily 

 measured by checking the relation between transpiration and evaporation, 

 the so-called 'relative transpiration' (Livingston, 1906). As long as the 

 water balance is favourable transpiration is closely related to evaporation, 

 and the relative transpiration remains constant. If the balance becomes 

 negative the stomata begin to close. In such a case transpiration lags 

 increasingly behind evaporation. The stomata will close completely, if a 

 certain water deficit is reached. Now the transpiration resistance of the 

 cuticle becomes effective. If cuticular transpiration still exceeds water 

 absorption, the balance gets worse and worse. This will be recognised by a 

 further decrease of the water content of a plant or by an increase of the 

 osmotic values of the cell sap. Plants tolerate this dehydration only to a 

 certain Hmit. If this hmit is exceeded they suffer drought damage. The 

 saturation deficit beginning to cause injuries is the 'sublethal deficit' 

 (Oppenheimer, 1932; Pisek and Berger, 1938). Walter (195 1) coined the 

 term 'maximum osmotic value' for the osmotic value corresponding to 

 this water deficit. 



