Relative growth rate

RGR is a concept relevant in cases where the increase in a state variable over time is proportional to the value of that state variable at the beginning of a time period. In terms of differential equations, if ${\displaystyle S}$ is the current size, and ${\displaystyle {\frac {dS}{dt}}}$ its growth rate, then relative growth rate is

${\displaystyle {\frac {1}{S}}{\frac {dS}{dt}}}$.

If the relative growth rate is constant, i.e.,

${\displaystyle {\frac {1}{S}}{\frac {dS}{dt}}=k}$,

a solution to this equation is

${\displaystyle S_{t}=\exp ^{(kt)}}$.

A closely related concept is doubling time.

In the simplest case of observations at two time points, RGR is calculated using the following equation:[1]

${\displaystyle RGR\ =\ {\frac {(\ln S_{2}-\ln S_{1})}{(t_{2}-t_{1})}}}$,

where:

${\displaystyle \ln }$ = natural logarithm

${\displaystyle t_{1}}$ = time one (e.g. in days)

${\displaystyle t_{2}}$ = time two (e.g. in days)

${\displaystyle S_{1}}$ = size at time one

${\displaystyle S_{2}}$ = size at time two

When calculating or discussing relative growth rate, it is important to pay attention to the units of time being considered.[2]

For example, if an initial population of ${\displaystyle S_{0}}$bacteria doubles every twenty minutes, then at time interval ${\displaystyle T}$ it is given by the equation

${\displaystyle S_{T}\ =\ S_{0}2^{T}=S_{0}\exp ^{(\ln(2)\cdot T)}}$,

where ${\displaystyle T}$ is the number of twenty-minute intervals that have passed. However, we usually prefer to measure time in hours or minutes, and it is not difficult to change the units of time. For example, since 1 hour is 3 twenty-minute intervals, the population in one hour is ${\displaystyle S(3)=S_{0}2^{3}}$. The hourly growth factor is 8, which means that for every 1 at the beginning of the hour, there are 8 by the end. Indeed,

${\displaystyle S_{t}=S_{0}8^{t}=S_{0}\exp ^{(\ln(8)\cdot T)}}$

where ${\displaystyle t}$ is measured in hours, and the relative growth rate may be expressed as ${\displaystyle \ln(2)}$ or approximately 69% per twenty minutes, and as ${\displaystyle \ln(8)}$ or approximately 208% per hour.[2]

In plant physiology, RGR is widely used to quantify the speed of plant growth. It is part of a set of equations and conceptual models that are commonly referred to as Plant growth analysis, and is further discussed in that section.

• Doubling time
• Plant growth analysis