GROWTH AND PHYSIOLOGY OF ASPEN SUPPLIED WITH DIFFERENT FERTILIZER ADDITION RATES

Variable internal plant nutrient content may confound plant response t o environmental stress. Plant nutrient content may be controlled with relative addition rate techniques in solution culture. However, becaus e raising large numbers of plants in flowing solution culture is diffi cult, we investigated the feasibility of raising plants in soil mix us ing relative fertilizer additions. Aspen (Populus tremuloides Michx.) clones (216, 259 and 271) planted in pots containing a peat, sand and vermiculite (2:1:1, v/v/v) soil mix were grown with exponentially incr easing fertilizer concentrations and harvested periodically to assess growth. Addition rate treatments ranged from 0.01 to 0.05 day(-1). The lag phase of growth, in which plants adjusted to the fertilizer regim e, lasted 40 days after which plants entered the experimental period c haracterized by constant relative growth rates equivalent to applied f ertilizer addition rates. Total plant nutrient concentration was (1) u nique for each addition rate, (2) linearly related to addition rate an d growth rate, and (3) tended to increase at the highest, and decrease at the lowest addition rates. Regardless, the plants appeared to have attained steady-stale conditions. Allocation of carbon to roots incre ased with lower addition rate treatments and was not dependent upon on togeny. There were no treatment differences in growth response among a spen clones. Yet there were treatment differences in leaf chlorophyll and photosynthesis within the clones. For the 0.05 day(-1) addition ra te treatment, chlorophyll, leaf N concentration and photosynthetic rat e were strongly correlated with one another, were at a maximum in rece ntly mature leaves, and rapidly declined with leaf age. The rate of de cline in these leaf characteristics was slowest in clone 271, consiste nt with the leaf longevity stress response reported elsewhere. Plant r esponses from these relative fertilizer addition trials in soil mix ag ree closely with those run in hydroponics, indicating that steady-stat e nutrition can be achieved with a technically simple experimental ass emblage.