© 2006 Heron Publishing—Victoria, Canada
Seasonal variations in water relations in current-year leaves of evergreen trees with delayed greening
Hisanori Harayama (1, 2, 3), Takefumi Ikeda (4), Atsushi Ishida (2) and Shin-Ichi Yamamoto (1)
1. Laboratory of Forest Ecology and Physiology, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601,
Japan / 2. Department of Plant Ecology, Forestry and Forest Products Research Institute (FFPRI), Tsukuba, Ibaraki 305-8687, Japan / 3. Corresponding author (harahisa@ffpri.affrc.go.jp) / 4. Department of Forest Science, Kyoto Prefectural University, Sakyo, Kyoto 606-8522, Japan / Received June 28, 2005; accepted November 11, 2005; published online May 1, 2006
Summary
We investigated seasonal patterns of water relations in current-year leaves of three evergreen broad-leaved trees (Ilex pedunculosa Miq., Ligustrum japonicum Thunb., and Eurya japonica Thunb.) with delayed greening in a warm-temperate forest in Japan. We used the pressure-volume method to: (1) assess the
extent to which seasonal variation in leaf water relations is attributable to leaf development processes in delayed greening
leaves versus seasonal variation in environmental variables; and (2) investigate variation in leaf water relations during
the transition from the sapling to the adult tree stage. Leaf mass per unit leaf area was generally lowest just after completion
of leaf expansion in May (late spring), and increased gradually throughout the year. Osmotic potential at full turgor (Ψoft) and leaf water potential at the turgor loss point (Ψwtlp) were highest in May, and lowest in midwinter in all species. In response to decreasing air temperature, Ψoft dropped at the rate of 0.037 MPa °C–1. Dry-mass-based water content of leaves and the symplastic water fraction of total leaf water content gradually decreased
throughout the year in all species. These results indicate that reductions in the symplastic water fraction during leaf development
contributed to the passive concentration of solutes in cells and the resulting drop in winter Ψoft. The ratio of solutes to water volume increased in winter in current-year leaves of L. japonicum and E. japonica, indicating that osmotic adjustment (active accumulation of solutes) also contributed to the drop in winter in Ψoft. Bulk modulus of elasticity in cell walls fluctuated seasonally, but no general trend was found across species. Over the
growing season, Ψoft and Ψwtlp were lower in adult trees than in saplings especially in the case of I. pedunculosa, suggesting that adult-tree leaves are more drought and cold tolerant than sapling leaves. The ontogenetic increase in the
stress resistance of I. pedunculosa may be related to its characteristic life form because I. pedunculosa grows taller than the other species studied.
Keywords:
bulk modulus of elasticity, cold hardening, ontogeny, osmotic potential, pressure-volume (P–V) curve, symplastic water, turgor
maintenance.
