A Role for Shoot Protein in Shoot–Root Dry Matter Allocation in Higher Plants

Background and Aims It is stated in many recent publications that nitrate (NO3-) acts as a signal to regulate dry matter partitioning between the shoot and root of higher plants. Here we challenge this hypothesis and present evidence for the viewpoint that NO3- and other environmental effects on the shoot : root dry weight ratio (S:R) of higher plants are often related mechanistically to changes in shoot protein concentration. Methods The literature on environmental effects on S:R is reviewed, focusing on relationships between S:R, growth and leaf NO3and protein concentrations. A series of experiments carried out to test the proposal that S:R is dependent on shoot protein concentration is highlighted and new data are presented for tobacco (Nicotiana tabacum). Key Results/Evidence Results from the literature and new data for tobacco show that S:R and leaf NO3- concentration are not significantly correlated over a range of environmental conditions. A mechanism involving the relative availability of C and N substrates for growth in shoots can explain how shoot protein concentration can influence shoot growth and hence root growth and S:R. Generally, results in the literature are compatible with the hypothesis that macronutrients, water, irradiance and CO2 affect S:R through changes in shoot protein concentration. In detailed studies on several species, including tobacco, a linear regression model incorporating leaf soluble protein concentration and plant dry weight could explain the greater proportion of the variation in S:R within and between treatments over a wide range of conditions. Conclusions It is concluded that if NO3- can influence the S:R of higher plants, it does so only over a narrow range of conditions. Evidence is strong that environmental effects on S:R are often related mechanistically to their effects on shoot protein concentration.