# 6.3 - Vegetative growth and development

Growth is an irreversible increase in plant size accompanied by a quantitative change in biomass. Development is more subtle and implies an additional qualitative change in plant form or function. Development thus lends ‘direction’ to growth and can apply equally well to a progressive change in gross morphology as to a subtle change in organ function, or to a major phase change from vegetative to reproductive development.

Increases in leaf area over time can be a useful basis for measuring plant growth rates than biomass increases, particularly as non-destructive and automated techniques for measuring leaf area are now available. Plant growth rate can be assessed as the relative increase in leaf area over time, by substituting total plant leaf area for total biomass in the conventional RGR equation.

$\text{RGR}_\text{A} =\frac{\text{ln } LA_2 - \text{ln } LA_1}{t_2-t_1} \tag{6.14}$

where RGRA is relative leaf expansion rate, LA is total leaf area and t is time at two time intervals, t1 and t2, preferably 2-3 days apart. This can be done by image analysis. This information can be extrapolated to whole plant growth rates as leaves, stems and roots generally maintain a balance in biomass that can be described by an allometric relationship.

In the first part of this section, growth of individual leaves is described at the cellular level of organisation, how this is influenced by light, and how much the photosynthetic activity of leaves changes with development.

The second part shows how root:shoot ratios change with availability of resources and the third part how these change with ontogeny (allometry).