# 6.4.1 Concepts

By analogy with single plants growing exponentially where RGR = NAR × LAR (Equation 6.12), instantaneous rate of dry matter production by a community of plants or crop growth rate (CGR, *sensu* Watson 1958) can be summarised as

where LAI, or leaf area index (*sensu* Watson 1947), is a dimensionless ratio of total (projected) leaf area per unit ground area.

Some crops do sustain gas exchange on both leaf surfaces (amphistomatous) but LAI relates more fundamentally to light absorption than to CO_{2} assimilation and is always based on total projected leaf area (i.e. single-sided leaf area).

RGR of single plants (d^{–1}, or more explicitly g g^{–1} d^{–1}) and absolute growth rate of a plant community, or CGR (g m^{–2} d^{–1}), are interrelated. For a given crop biomass (g m^{–2}) the collective RGRs of individuals in a crop translate to CGR where

Put more explicitly with *A* as canopy area, *W* as plant mass, *N* as the number of plants per unit ground area and d*W*/d*t* as rate of total biomass accumulation per unit time (*t*), then*:
*

so that