Water Relations of Hedgerow Shrubs in Northern Central EuropeI. Bulk Water Relations

Abstract

Summary Bulk water relations of Corylus avellana , Alnus glutinosa , Sambucus nigra and Sorbus auctuparia growing in a North German hedgerow were measured during the growing season. Special attention was paid to diurnal and seasonal patterns of water content, osmotic and water potential in leaves from the north and south side and from the center of the shrubs. The diurnal amplitude of water potential was on an average 0.9 MPa in S.nigra , 1.1 MPa in C. avellana and A. glutinosa , and 1.9 MPa in S. aucuparia. The lowest osmotic potential was -3.0 MPa in S. aueuparia and -2.2 MPa in the other three species. These values were found at the south side of the shrubs, whereas potentials were high and variation was low in the inner part of the shrubs. Leaf water content of north- and south-facing branches ranged between 300 and 130% d. wl. in C. avellana and S. aueuparia , and 400 and 200% d. wt. in A. glutinosa and S. nigra , in all cases decreasing from high spring values during the season. By contrast, leaf water content in the inner part of the shrubs did not vary much, and in S. nigra there was even an increase from 400 to 570% d. wt. within the period May to October. In the latter case a redistribution of water from the axes to the leaves is assumed. This is supported by the observation that stem water content decreased in S. nigra by 60% d. wt. during the season, but in the other three species only by 20 to 30% d. wt. From the relations between osmotic potentials and water contents, seasonal and local differences in the amount of osmoregulation can be inferred. Solutes must have increased during early summer in C. avellana and S. aueuparia , and — to a higher extent — between July and September in A. glutinosa and S. nigra . The versatility in the use of solutes is in accordance with the species-specific seasonal demands for growth and fruit sel. The findings suggest a high degree of functional co-ordination of the various parts of the crown. Thus, time- and space:limited osmoregulation improves the leaf water relations under mild water stress conditions. In S. nigra and S. aueuparia desiccation tolerance of the leaves is similar, however, their water loss avoidance differs drastically. S. nigra leaves reach critical water deficits already within 1/3 of the time that was necessary to cause initial injury in leaves of S. aueuparia pinnae. C. avellana and particularly S. aucuparia may be characterized as euryhydric species, and S. nigra as a strictly stenohydric one. A. glutinosa is more flexible.