Components of the relative growth rate under multiple environmental conditions

Background/Question/Methods

The relative growth rate (RGR) is affected by a variety of environmental factors, e.g. light, water levels and nutrient concentration. In order to gain a more complete understanding of RGR in plants, it can be separated into several components reflecting physiology (NAR), leaf construction (SLA) and allocation (LMR). Deciding which of these components has a greater impact on the RGR has produced conflicting conclusions, with the most common conclusion being that the NAR (net assimilation rate) has the largest effect. However, the NAR depends on absolute growth rate and so varies considerably with size. As a consequence of this, experimental treatments that increase the variance in plant size will automatically increase the impact that NAR, relative to SLA and LMR, has on RGR (Rees et al., 2010). We propose that this has significant implications for studying the effect of environmental variables on the RGR components, as different environmental conditions produce different variances in plant size and hence NAR and RGR. To investigate this, plants were grown under high and low light and water regimes and the variance in RGR within a treatment group was compared to the NAR for that treatment.

Results/Conclusions

There was a strong positive correlation between the variance in RGR and the proportion of this variance that was attributed to NAR. This indicates that variance in plant size causes an increased relative contribution of NAR to the variation in RGR. The diminishing importance of SLA with increasing light levels has previously been suggested as an explanation for the greater contribution of NAR to the RGR. This suggestion stems from the theory that it is more important for plants in low-light environments to have a specific leaf construction (SLA) i.e. thin leaves with a large surface area; as it is a limiting factor to growth. The size-dependent association of NAR and RGR may reduce the extent and significance of this finding. A different method of studying these associated physiological variables must be examined and our understanding of how these variables cause changes in RGR under different environmental conditions may need to be re-evaluated.

Rees, M., Osbourne, C. P., Woodward, F. I., Hulme, S. P., Turnbull, L. A. & Taylor, S. H. American naturalist, 176, 6, E152-E161 (2010).