Roots, stems and leaves are functionally interdependent and these three systems maintain a dynamic balance in biomass which reflects relative abundance of above-ground resources (light and CO₂) compared with root-zone resources (water and nutrients) (Poorter et al. 2012). Whole-plant growth rate and summary measures such as root:shoot ratio are thus an outcome of developmental stage and of environmental influences.

Change in root:shoot ratio during a plant’s life cycle is part of an intrinsic ontogeny, but growth rates of roots and shoots continually adjust to resource availability with photoassimilate (hence biomass). In herbaceous plants, root:shoot ratios typically decrease with age (size) due to sustained investment of carbon in above-ground structures (root crops would be a notable exception). Developmental morphology is inherent, but expression of a given genotype will vary in response to growing conditions (hence phenotypic plasticity).

Irradiance is a case in point where shoot growth takes priority in low light, whereas root growth can be favoured under strong light. For example, Evans and Hughes (1961) grew Impatiens parviflora at five light levels and demonstrated a steady increase in root mass relative to whole-plant mass (root mass ratio) from 7% to 100% full sun. Stem mass ratio showed the opposite sequence. Leaf mass ratio increased somewhat at low light, but increased SLA was far more important for maintenance of whole-plant RGR in this shade-adapted species.

If light effects on root:shoot ratio are translated via photosynthesis, then CO₂ should interact with irradiance on root:shoot ratio because carbon assimilation would be maintained by a more modest investment in shoots exposed to elevated CO₂. Chrysanthemum morifolium behaved this way for Hughes and Cockshull (1971), returning a higher NAR due to CO₂ enrichment under growth cabinet conditions despite lower LAR which was in turn due to smaller leaf weight ratio. Adjustment in SLA exceeded that of leaf weight ratio, and so carried more significance for growth responses to irradiance × CO₂.

Consistent with shoot response to above-ground conditions, root biomass is influenced by below-ground conditions where low availability of either water or nutrients commonly leads to greater root:shoot ratio. For example, white clover (Trifolium repens) growing on a phosphorus-rich medium increased root:shoot ratio from 0.39 to 0.47 in response to moisture stress; and from 0.31 to 0.52 when moisture stress was imposed in combination with lower phosphorus (see Table 1 in Davidson 1969b). A positive interaction between low phosphorus and low water on root:shoot ratio was also evident in perennial ryegrass (Lolium perenne) grown on high nitrogen. In that case, root:shoot ratio increased from 0.82 to 3.44 in response to moisture stress when plants were grown on low phosphorus in combination with high nitrogen.

Adding to this nutrient × drought interaction, a genotype × phosphorus effect on root:shoot ratio has been demonstrated by Chapin et al. (1989) for wild and cultivated species of Hordeum. Weedy barleygrass (H. leporinum and H. glaucum) was especially responsive, root : shoot ratio increasing from about 0.75 to 1.5 over 21 d on low phosphorus. By contrast, cultivated barley (H. vulgare) remained between 0.5 and 0.75 over this same period. Held on high phosphorus, all species expressed comparable root:shoot ratios which declined from around 0.55 to about 0.35 over 21 d. High root:shoot ratios on low phosphorus in weedy accessions would have conferred a selective advantage for whole-plant growth under those conditions, thus contributing to their success as weeds.

Even stronger responses to phosphorus nutrition have been reported for soybean (Fredeen et al. 1989) where plants on low phosphorus (10 µM KH₂PO₄) invested biomass almost equally between roots and shoots, whereas plants on high phosphorus (200 µM KH₂PO₄) invested almost five times more biomass in shoots than in roots (daily irradiance was about 30 mol quanta m^–2 d^–1 and would have been conducive to rapid growth).

Root:shoot ratios are thus indicative of plant response to growing conditions, but ratios are not a definitive measure because values change as plants grow. Trees in a plantation forest would show a progressive reduction in root:shoot ratio, and especially after canopy closure where a steady increase in stem biomass contrasts with biomass turnover of canopy and roots and thus predominates in determining root:shoot ratio.

Broad generalisations are that root:shoot ratio increases with nutrient deficiency and moisture stress or under elevated CO₂, but decreases in strong light. Too often, however, reports of treatment effects on root:shoot ratio have can overlooked differences in developmental ontogeny or size, and real responses may be obscured. Allometry then becomes a preferred alternative where repeated measurements of size or mass provide an unambiguous picture of carbon allocation.