© 2001 Heron Publishing—Victoria, Canada
Increasing the sink:source balance enhances photosynthetic rate of 1-year-old balsam fir foliage by increasing allocation
of mineral nutrients
M. B. Lavigne (1), C. H. A. Little (1) and J. E. Major (1)
1. Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, P.O. Box 4000, Fredericton, NB E3C 5P7, Canada / Received March 9, 2000
Summary
To determine the impact of altering the sink:source balance on gas exchange rates in 1-year-old foliage of balsam fir (Abies balsamea (L.) Mill.), seedlings were either debudded before the growing season began or left intact. To assess alternative explanations
for the observed photosynthetic response, additional seedlings were variously root pruned, shaded, drought stressed or deprived
of fertilizer in combination with debudding. Foliar gas exchange rates and carbohydrate concentrations, xylem water potential
and current-year growth were measured on three occasions spanning the shoot extension period. In addition, growth, foliar
concentrations of chlorophyll, nitrogen, phosphorus and potassium, chlorophyll fluorescence and photosynthetic rate at different
CO2 concentrations were determined 1 month after shoot extension ceased. Debudded seedlings had higher photosynthetic rates than
budded seedlings, and the difference increased as the growing season progressed. We observed no evidence that debudding increased
photosynthetic rate by improving water relations or by reducing foliar carbohydrate concentrations. Debudding increased foliar
concentrations of nitrogen, phosphorus, potassium and chlorophyll, as well as quantum yield and photochemical quenching as
determined by chlorophyll fluorescence measurements. Therefore, we conclude that debudding increased the photosynthetic rate
by increasing the allocation of nutrients to 1-year-old foliage, thereby enhancing the amount or activity, or both, of photosynthetic
enzymes, as well as increasing chlorophyll concentration.
Keywords:
Abies balsamea, carbohydrates, chlorophyll fluorescence, debudding, fructose, glucose, nitrogen, phosphorus, photosynthesis, potassium,
starch, sucrose, xylem water potential.
