Summary• Close correlations between specific leaf area (SLA) and relative growth rate (RGR) have been reported in many studies. However, theoretically, SLA by itself has small net positive effect on RGR because any increase in SLA inevitably causes a decrease in area-based leaf nitrogen concentration (LNC a ), another RGR component. It was hypothesized that, for a correlation between SLA and RGR, SLA needs to be associated with specific nitrogen absorption rate of roots (SAR), which counteracts the negative effect of SLA on LNC a .• Five trees and six herbs were grown under optimal conditions and relationships between SAR and RGR components were analyzed using a model based on balanced growth hypothesis.• SLA varied 1.9-fold between species. Simulations predicted that, if SAR is not associated with SLA, this variation in SLA would cause a 47% decrease in LNC a along the SLA gradient, leading to a marginal net positive effect on RGR. In reality, SAR was positively related to SLA, showing a 3.9-fold variation, which largely compensated for the negative effect of SLA on LNC a . Consequently, LNC a values were almost constant across species and a positive SLA-RGR relationship was achieved.• These results highlight the importance of leaf-root interactions in understanding interspecific differences in RGR.

Summary 1.Optimal biomass allocation models predicted that an increase in specific absorption rate (SAR: nitrogen absorption rate per unit root dry weight) increases the optimal leaf N concentration (LNC) which maximizes whole-plant growth rates. From this prediction, we hypothesized that inherent differences in the N absorption ability of roots, which is represented by differences in SAR, causes interspecific differences in LNC and photosynthetic capacity ( P max ). 2. Four deciduous tree species and three herb species were grown under controlled conditions to test this hypothesis. Despite growing under the same soil N conditions, the SAR of these species differed more than sixfold, with the deciduous trees having a smaller SAR than the herbs. Consistent with the hypothesis, there were strong positive correlations between SAR, LNC and P max . Leaf dry mass per area and the LNC-P max relationship, factors correlated with leaf life span, differed little among the species, suggesting a small effect of differences in these properties on the variation in LNC. 3. Simulations were performed using an optimal biomass allocation model and parameter values determined from measurements of each species. These demonstrated that the observed variation in LNC could be explained largely by differences in the N absorption ability of the species. 4. Additionally, the causal relationship between N absorption ability and LNC, suggested by optimal biomass allocation models, was verified by manipulating the biomass allocation between roots and leaves of Morus bombycis using uniconazole, an inhibitor of gibberellin synthesis. 5. These results indicate a close functional link between N absorption ability and LNC, which would account for large variations in LNC and P max across species.

Summary 1.A model was developed to examine effects of the stem biomass fraction on the optimal responses of plants to soil nitrogen availability. 2. Our model predicts that the optimal leaf : root ratio and optimal photosynthetic capacity ( P max ) increase with soil N availability. For a given N availability, the optimal leaf : root ratio decreases and the optimal P max increases with increasing stem fraction. As a result, the increase in optimal leaf : root ratio is smaller, and that in optimal P max is greater, in response to increasing N availability when stem fraction is large. 3. To test these predictions we grew two herbs with different stem fractions: Polygonum cuspidatum Sieb. et Zucc. and Chenopodium album L . Showing excellent agreement with the simulation results, the leaf N concentration and leaf : root ratio of the two herbs increased with increasing N availability, and leaf N concentration was larger for C. album with higher stem fraction than P. cuspidatum . 4. The general tendency for plants with larger stem fractions also to have greater leaf N concentrations and P max was demonstrated for a wide range of temperate herbs. This suggests that stem fraction may be a source of variation in P max among plants in the same functional group.

The applicability of optimal biomass allocation models is fairly high, although constraints in the plasticity of biomass allocation could prevent optimal regulation of the root : leaf ratio in some species. The assumption that regulation of the root : leaf ratio enables maximization of RGR was supported.