Plant growth analysis refers to a set of concepts and equations by which changes in size of plants over time can be summarised and dissected in component variables.
In comparing different treatments, genotypes or species, the simplest type of growth analysis is to evaluate size of plants after a certain period of growth, typically from the time of germination.
When there are enough resources available (light, nutrients, water), the increase of biomass after germination will be more or less proportional to the mass of the plant already present: small right after germination, larger when plants become bigger.
Blackman (1919) was the first to recognize that this was similar to money accumulating in a bank account, with the increase determined by compounding interest.
[2] He applied the same mathematical formula to describe plant size over time.
In the case of more harvests, a linear equation can be fitted through the ln-transformed size data.
The slope of this line gives an estimate of the average RGR for the period under investigation, with units of g.g−1.day−1.
A time-course of RGR can be estimated by fitting a non-linear equation through the ln-transformed size data, and calculating the derivative with respect to time.
For young plants, values are often in the range of 1–20 m2 kg−1, for tree seedlings they are generally less.
This variable indicates the rate of biomass increase per unit leaf area, with typical values ranging from 5-15 g.m−2.day−1 for herbaceous species and 1-5 g.m−2.day−1 for woody seedlings.
Although the ULR is not equal to the rate of photosynthesis per unit leaf area, both values are often well correlated.
LMF characterizes the fraction of total plant biomass that is allocated to leaves.
Thus, by sequentially harvesting leaf, stem, and root biomass as well as determining leaf area, deeper insight can be achieved in the various components of a plant and how they together determine whole plant growth.
As much as RGR can be seen from the perspective of C-economy, by calculating leaf area and photosynthesis, it could equally well be approached from the perspective of organic N concentration, and the rate of biomass increase per unit organic N:
[9] Another way to break down RGR is to consider biomass increase from the perspective of a nutrient (element) and its uptake rate by the roots.
RGR can then be rewritten as a function of the Root Mass Fraction (RMF), the concentration of that element in the plant and the specific uptake rate of roots for the element of interest.
Under the condition that the concentration of the element of interest remains constant (i.e. dE/dM = E/M), RGR can be also written as:
[11] In a period of several days, plant growth rate will vary because of diurnal changes in light intensity, and day-to-day differences in the daily light integral.
Over a longer period (weeks to months), RGR will generally decrease because of several reasons.
The RGR of trees in particular decreases with increasing size due in part to the large allocation to structural material in the trunk required to hold the leaves up in the canopy.
Overall, respiration scales with total biomass, but photosynthesis only scales with photosynthetically active leaf area and as a result growth rate slows down as total biomass increases and LAR decreases.
And thirdly, depending on the growth conditions applied, shoot and/or root space may become confined with plant age, or water and/or nutrient supply do not keep pace with plant size and become more and more limiting.
One way to 'correct' for these differences is by plotting RGR and their growth components directly against plant size.
[12] If RGR specifically is of interest, another approach is to separate size effects from intrinsic growth differences mathematically.
It has been demonstrated that traditional RGR lacks several of the critical traits influencing growth and the allometric dependency of leaf mass and also showed how to incorporate alloemtric dependencies into RGR growth equations.
This has been used to derive a generalized trait-based model of plant growth (see also Metabolic Scaling Theory and Metabolic Theory of Ecology) to show how plant size and the allometric scaling of key functional traits interact to regulate variation in whole-plant relative growth rate.
After canopy closure, plant growth is not proportional to size anymore, but changes to linear, with in the end saturation to a maximum value when crops mature.
Equations used to describe plant size over time are then often expolinear[15] or sigmoidal.
[16][17] Agronomic studies often focus on the above-ground part of plant biomass, and consider crop growth rates rather than individual plant growth rates.
More specifically, the ULR as discussed above shows up in crop growth analysis as well, as: