Optimal Leaf-to-Root Ratio and Leaf Nitrogen Content Determined by Light and Nitrogen Availabilities

Daisuke Sugiura,Masaki Tateno,Randall Niedz

doi:10.1371/journal.pone.0022236

Abstract

Plants exhibit higher leaf-to-root ratios (L/R) and lower leaf nitrogen content (N area) in low-light than in high-light environments, but an ecological significance of this trait has not been explained from a whole-plant perspective. This study aimed to theoretically and experimentally demonstrate whether these observed L/R and N area are explained as optimal biomass allocation that maximize whole-plant relative growth rate (RGR). We developed a model which predicts optimal L/R and N area in response to nitrogen and light availability. In the model, net assimilation rate (NAR) was determined by light-photosynthesis curve, light availability measured during experiments, and leaf temperature affecting the photosynthesis and leaf dark respiration rate in high and low-light environments. Two pioneer trees, Morus bombycis and Acer buergerianum, were grown in various light and nitrogen availabilities in an experimental garden and used for parameterizing and testing the model predictions. They were grouped into four treatment groups (relative photosynthetic photon flux density, RPPFD 100% or 10%×nitrogen-rich or nitrogen-poor conditions) and grown in an experimental garden for 60 to 100 days. The model predicted that optimal L/R is higher and N area is lower in low-light than high-light environments when compared in the same soil nitrogen availability. Observed L/R and N area of the two pioneer trees were close to the predicted optimums. From the model predictions and pot experiments, we conclude that the pioneer trees, M. bombycis and A. buergerianum, regulated L/R and N area to maximize RGR in response to nitrogen and light availability.

Highlights

Plants have the ability to alter their phenotype to maximize fitness according to the external environment
Model parameters Parameter values obtained from the measurements, determination coefficients, and corresponding equations were listed in Table 1, which showed good correlation
From the model predictions and pot experiments (Fig. 6), we could demonstrate that leaf to root ratio (L/R) and leaf nitrogen content per leaf area (Narea) of the pioneer tree

Summary

Introduction

Plants have the ability to alter their phenotype to maximize fitness according to the external environment. Many researchers have worked with the subject, and proposed the balanced growth hypothesis where plants allocate more biomass to the organ capturing the most limiting resources, such as light and nutrients [4,5,6] According to this hypothesis, for example, producing more leaves at the sacrifice of root growth is favoured in low-light environments to capture more light to enhance growth rate. Since leaf and root functions are closely interrelated, producing excessive leaves may decrease growth rate due to decreased root functions, such as nitrogen uptake capacity This lead to an idea that there will be an equilibrium between leaves and roots for optimal biomass allocation that maximizes whole-plant growth rate [1,7]

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Conclusion