© 1998 Heron Publishing—Victoria, Canada
Clonal and seasonal differences in leaf osmotic potential and organic solutes of five hybrid poplar clones grown under field
conditions
G. Michael Gebre (1), Timothy J. Tschaplinski (1), Gerald A. Tuskan (1) and Donald E. Todd (1)
1. Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6422, USA / Received January 16, 1997
Summary
Leaf osmotic potential at full turgor (Ψπo) and the major solutes that contribute to osmotic potential were characterized in five hybrid poplar clones of Populus trichocarpa Torr. & Gray × P. deltoides Bartr. (TD) and P. deltoides × P. nigra L. (DN), growing under field conditions at two sites in eastern Washington and Oregon, USA. Trees were drip irrigated with
46, 76 or 137 cm of supplemental irrigation during each growing season. Trees at Wallula, WA, which were in their third growing
season in 1994, were sampled twice a year for two years (1994 and 1995), and trees at Boardman, OR, which were in their second
growing season in 1994, were sampled once a year for three years (1994–1996). At Wallula, the TD and DN clones exhibited lower
predawn leaf water 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 Ψπo than Clone DN (–1.67 versus –1.56 MPa) during the same 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 also a significant treatment effect on Ψπo in Clone TD, with trees in the 46-cm treatment having lower Ψπo than trees in the 137-cm treatment in July 1994. At Boardman, Ψπo was generally high with no treatment differences during the 1994–96 samplings. The TD clones had significantly lower Ψπo 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 by
an increase in sucrose, which constituted 70% of total organic solutes. Although the total concentration of free primary amino
acids in this 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 inorganic 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 Ψπo was not fully accounted for by the difference in solute concentration. Osmotic potential at full turgor declined over the
growing season and with age. We conclude that, because the extent of osmotic adjustment exhibited by these clones was small,
other drought resistance mechanisms contributed to the clonal differences in field performance.
Keywords:
carbohydrate, free primary amino acids, irrigation, osmotic adjustment, osmotic potential, Populus, potassium.
