CLONAL AND SEASONAL DIFFERENCES IN LEAF OSMOTIC POTENTIAL AND ORGANICSOLUTES OF 5 HYBRID POPLAR CLONES GROWN UNDER FIELD CONDITIONS

Leaf osmotic potential at full turgor (Psi(pi 0)) and the major solute s that contribute to osmotic potential were characterized in five hybr id poplar clones of Populus trichocarpa Torr. & Gray x P. deltoides Ba rtr. (TD) and P. deltoides x P. nigra L. (DN), growing under field con ditions at two sites in eastern Washington and Oregon, USA. Trees were drip irrigated with 46, 76 or 137 cm of supplemental irrigation durin g each grow ing season. Trees at Wallula, WA, which were in their thir d growing season in 1994, were sampled twice a year for two years (199 4 and 1995), and trees at Boardman, OR, which were in their second gro wing season in 1994, were sampled once a year for three years (1994-19 96). At Wallula, the TD and DN clones exhibited lower predawn leaf wat er potentials in the 46-cm treatment than in the 137-cm treatment (-1. 2 versus -0.7 MPa) during a hot, dry period in July 1994. Clone TD had a lower Psi(pi 0) than Clone DN (-1.67 versus -1.56 MPa) during the s ame period and the difference was also evident in 1995 (-1.81 versus - 1.72 MPa) when trees were in their fourth growing season. There was al so a significant treatment effect on Psi(pi 0) in Clone TD, with trees in the 46-cm treatment having lower Psi(pi 0) than trees in the 137-c m treatment in July 1994. At Boardman, Psi(pi 0), was generally high w ith no treatment differences during the 1994-96 samplings. The TD clon es had significantly lower Psi(pi 0) than the DN clones in 1994 (-1.44 versus -1.36 MPa) and 1996 (-1.72 versus -1.54 MPa), but there was no difference between clones in 1995 (-1.40 versus -1.43 MPa). In 1995; at Wallula, osmotic adjustment in Clone TD was largely accounted for b y an increase in sucrose, which constituted 70% of total organic solut es. Although the total concentration of free primary amino acids in th is clone was 28% higher in trees in the 46-cm treatment than in trees in the 137-cm treatment, amino acids constituted only a small fraction of the total solute pool. Sixty-two percent of total solutes were ino rganic ions in Clone TD compared to 52% in Clone DN, and potassium was the main ion constituting about 30% of total solutes and 50% of total ions. However, the clonal difference in Psi(pi 0) was not fully accou nted for by the difference in solute concentration. Osmotic potential at full turgor declined over the growing season and with age. We concl ude that, because the extent of osmotic adjustment exhibited by these clones was small, other drought resistance mechanisms contributed to t he clonal differences in field performance.