A role for shoot protein in shoot-root dry matter allocation in higher plants.
OBJECTIVE
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 NO3- and 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.
Andrews
M
M
School of Sciences, University of Sunderland, Sunderland SR1 3SD, UK. mitchell.andrews@sunderland.ac.uk
Raven
J A
JA
Lea
P J
PJ
Sprent
J I
JI
D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D018515:Plant Leaves / Q000254:growth & development / Q000378:metabolism; D010940:Plant Proteins / Q000032:analysis / Q000502:physiology*; D018517:Plant Roots / Q000254:growth & development* / Q000378:metabolism; D018520:Plant Shoots / Q000254:growth & development* / Q000378:metabolism; D014026:Nicotiana / Q000737:chemistry / Q000254:growth & development* / Q000378:metabolism
D009566:Nitrates; D010940:Plant Proteins; D009584:Nitrogen