Photosynthetic pathway is an important cause of growth rate variation between species, such that the enhanced carbon uptake of C₄ species leads to faster growth than their C₃ counterparts. Leaf traits that promote rapid resource acquisition may further enhance the growth capacity of C₄ species. However, how root economic traits interact with leaf traits, and the different growth strategies adopted by plants with C₃ and C₄ photosynthetic pathways is unclear. Plant economic traits could interact with, or act independently of, photosynthetic pathway in influencing growth rate, or C₃ and C₄ species could segregate out along a common growth rate-trait relationship.

We measured leaf and root traits on 100+ grass species grown from seeds in a controlled, common environment to compare with relative growth rates (RGR) during the initial phase of rapid growth, controlling for phylogeny and allometric effects.

Photosynthetic pathway acts independently to leaf and root functional traits in causing fast growth. Using C₄ photosynthesis, plants can achieve faster growth than their C₃ counterparts (by an average 0.04 g g^-1 day^-1) for a given suite of functional trait values, with lower investments of leaf and root nitrogen. Leaf and root traits had an additive effect on RGR, with plants achieving fast growth by possessing resource-acquisitive leaf traits (high specific leaf area and low leaf dry matter content) or root traits (high specific root length and area, and low root diameter), but having both leads to an even faster growth rate (by up to 0.06 g g-1 day-1). C₄ photosynthesis can provide a greater relative increase in RGR for plants with a ‘slow’ ecological strategy than in those with fast growth. However, aboveground and belowground strategies are not coordinated, so that species can have any combination of ‘slow’ or ‘fast’ leaf and root traits.

Synthesis: C₄ photosynthesis increases growth rate for a given combination of economic traits, and significantly alters plant nitrogen economy in the leaves and roots. However, leaf and root economic traits act independently to further enhance growth. The fast growth of C₄ grasses promotes a competitive advantage under hot, sunny conditions.

Datasets

leaf_root_trait_average_data.csv = Average leaf and root functional trait data

leaf_root_trait_sizestandardised_data.csv = Leaf and root functional traits predicted at a common reference size (the 60th percentile for total dry mass across all species and harvests: 0.044g)

Explanation of abbreviations/code names used in column headings

phylo.name = species name

avg.RGR = average RGR (g g^-1 day^-1)

SLA = ln specific leaf area (cm²g^-1)

LDMC = log leaf dry matter content (%)

SRA = ln specific root area (cm² g^-1)

SRL = ln specific root length (cm g^-1)

N = log Nitrogen concentration (%)

C = log Carbon concentration (%)

root.avg.diam.mm = ln average root diameter (mm)

PC1/PC2 = principal component analysis axis 1 or 2

A = minimum mass; one of the four parameters in the logistic growth model

B = asymptotic or maximum mass

xmid = or t0 -  time when a plant is midway between A and B; one of the four parameters in the logistic growth model

scal = or k - a growth parameter; one of the four parameters in the logistic growth model

.sc.60 (column name suffix) = trait value predicted at a common reference size (the 60th percentile for total dry mass across all species and harvests: 0.044g)