Analysis of interspecific variation in plant growth responses to nitrogen

Plant species differ greatly in their growth responses to nutrients, but little is known about the physiological and morphological factors that are responsible for this variation. To address this question, I measured the responses to added nitrogen of relative growth rate and three of its components (specific leaf area, unit leaf rate, and leaf weight ratio) for 17 C3 grass species. Plants were grown in sand culture in a greenhouse and were fertilized daily with either 5 or 0.05 mM NH4NO3. For most species, growth response to nitrogen was primarily associated with an increased mass allocation to leaves at high versus low nitrogen. Across all species, the average response at high versus low nitrogen was a 37% increase in leaf weight ratio, a 12% increase in unit leaf rate, and a 4% decrease in specific leaf area. Interspecific differences in growth response to nitrogen, however, were associated primarily with species differences in the response of unit leaf area to nitrogen supply. I determined the nitrogen response ratio of each parameter as the value of the parameter at high nitrogen divided by the value at low nitrogen. The rank-order correlation between the unit leaf area response ratio and the relative growth rate response ratio was 0.88. Reanalysis of previous experiments on plant nutrient response showed a similar pattern. In all studies, interspecific variation in the response of relative growth rate to nutrients was associated primarily with interspecific differences in the plastic response of unit leaf area.Key words: leaf weight ratio, net assimilation rate, plant growth analysis, relative growth rate, specific leaf area, unit leaf rate.