Comparative drought resistance among six species of birch (Betula): influence of mild water stress on water relations and leaf gas exchange

Abstract

Responses of plant water relations and leaf gas exchange to mild water stress were monitored and compared among six species of birch; monarch birch (Bet& manimowicziana Reg.), river birch (B. nigra L.), paper birch (I?. pupyrifera Marsh.), European white birch (B, pendula Roth.), ‘Whitespire’ Japanese birch (B. platyphylla var. japonica Hara. ‘Whitespire’), and gray birch (B. populifolia Marsh.). Before imposition of water stress, ‘Whitespire’ Japanese birch and river birch maintained the highest stomatal conductances and net rates of photosynthesis of the species examined. After irrigation was withheld, stomatal conductance and rate of net photosynthesis gradually declined for most species. After 28 days without irrigation, ‘Whitespire’ Japanese birch maintained significantly higher stomatal conductance and rate of net photosynthesis than did the other species despite having one of the lowest midday water potentials. There was no evidence of osmotic adjustment by any of the species in response to the imposed drought. However, there was substantial variation in the water potential at the turgor loss point among the species, from a high of -1.34 MPa for river birch to a low of -1.78 MPa for ‘Whitespire’ Japanese birch. Stomata1 conductance and net photosynthesis under mild water stress (average predawn leaf water potential of -0.6 1 MPa) were negatively correlated with leaf osmotic potential at full turgor and leaf water potential at the turgor loss point. Thus, the greater net photosynthesis of ‘Whitespire’ Japanese birch under water stress compared with the other species appears to have resulted from a superior capacity to maintain turgor at low leaf water potentials, which in turn provided for greater stomatal conductance and CO2 uptake. These results indicate that ‘Whitespire’ Japanese birch is better adapted to dry sites than the other species.