6.3.1 Biomass distribution
Roots and shoots are functionally interdependent and these two 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). Whole-plant growth rate and root : shoot ratio are thus an outcome of genotype × environment interaction, but source of control is ambiguous.
According to one argument, internal (genetic) control over root : shoot ratio will be expressed throughout growth and development and will thus dictate resource capture both above and below ground, and hence whole-plant growth rate. Change in root : shoot ratio during a plant’s life cycle is then regarded as part of a gene-controlled ontogeny. An alternative view, and well supported by observation, is that growth rates of roots and shoots continually adjust in response to resource capture with photoassimilate (hence biomass) allocated on a ‘needs basis’.
In practice, both models apply because developmental morphology is ultimately gene dependent 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 (discussed earlier in connection with Table 6.2).
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.
In parallel 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, inoculated 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 c. 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 by their very nature ratios are not a deﬁnitive measure because values change as plants grow. 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). Trees in a plantation forest would also 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.
Meaningful comparisons of root : shoot ratio must therefore be referenced to whole-plant biomass. Some examples cited by Bastow Wilson (1988) meet this criterion, and in so doing exclude ontogenetic effects. Broad generalisations coincide with examples cited above, namely 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 can be artefacts of contrasts in whole-plant biomass. Equally, some 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.
During whole-plant growth in a stable environment, roots and shoots maintain a dynamic balance such that
where y is root biomass and x is shoot biomass. More generally, x and y can be any two parts of the same organism that are growing differentially with respect to each other, but root–shoot relations are the most common candidate in such analyses of plant growth.
The allometric equation y = bxk (Equation 6.17) was formalised by Huxley (1924) but can be ln transformed to become
This formulation enables a straight-line plot of ln y as a function of ln x with slope k (i.e. the allometric coefﬁcient) and intercept ln b. This empirical model does not explain the nature of growth controls between roots and shoots but does offer a simple description which is not confounded by plant size. Moreover, any departure from a particular root : shoot relationship is immediately obvious, and sources of variation in root : shoot ratio can be resolved into starting conditions (differences in intercept, ln b) versus biomass partitioning during growth (differences in slope, k).
Leaf, stem and root growth under controlled conditions in Eucalyptus grandis seedlings demonstrate such application (Figure 6.18a, b; Cromer and Jarvis 1990). Nitrogen input in nutrient spray chambers was used as a driving variable for growth where ﬁve relative addition rates (RARN) generated a wide range in whole-plant RGR (from 0.039 d–1 on lowest RARN to 0.111 d–1 on highest RARN).
Data from all treatments and harvests were pooled to reveal a strict allometric relationship between root and leaf growth (Figure 6.18a) with a nitrogen effect on intercept but not slope. Nitrogen nutrition had influenced biomass allocation to the extent that low RARN had initially promoted root growth relative to leaves (hence higher intercept), but subsequent to this early adjustment, and once growth had stabilised, biomass allocation to roots and leaves maintained a constant relationship irrespective of RARN. In this case k = 0.982, indicating a net bias towards leaf growth over root growth — a ‘net bias’ because carbon loss via excretion, root renewal and respiration was not measured so that more photoassimilate would have been allocated to roots than was ﬁxed in biomass.
Stem and leaf biomass also maintained a strict allometric relationship (Figure 6.18b) where k = 1.261. A value for k greater than unity implies a consistent bias towards stem growth relative to canopy growth, as would be expected in a eucalypt with a high rate of stem growth (and favoured in plantation forestry). Signiﬁcantly, nitrogen treatment was without effect on either intercept or slope (Figure 6.18b) and emphasises the highly conserved relationship between leaves and stem in these seedlings.
Developmental events also influence allometry and Italian ryegrass (Lolium multiflorum) provides a nice example (Figure 6.19) where a log–log plot of root mass as a function of shoot mass showed an abrupt change in slope when flowering occurred. In that case, k decreased from 1.121 to 0.553, and although shoot dry mass was about 10 times root biomass, a change in allometry was clearly evident.
Allometry is most commonly applied to roots and shoots, but other functional interrelations within plants are equally amenable, and especially where non-destructive measurements are involved. Length and breadth of leaves, or length and circumference of fruits enable calculation of k values that categorise shape, and can reveal heritabilities in developmental morphology. The two variables can even carry different dimensions as in stem volume and leaf area or canopy area and plant mass. In that case, a ‘ratio’ of area to mass coincides with leaf area ratio (LAR, Section 6.1). Compared with that cumulative but static index, the allometric relationship between canopy area and plant mass (termed ‘a’ by Whitehead and Myerscough 1962) is a more dynamic indicator of ‘... the proportion of dry weight increment surplus to that required to maintain the morphogenetic proportions of the plant as an efﬁcient photosynthetic form alone. When a is unity all the dry-weight increment is used up in maintaining the proportions of the plants as a ‘photosynthetic entity’...’. Soon after germination, seedlings gain leaf area at the expense of dry mass and a will be <1.0. Similarly, during latter phases of maturation when leaf area can be decreasing while whole-plant mass is still increasing, a will again be <1.0, and in both cases a is simply reflecting normal ontogenetic drift. However, in a plant community where individuals are competing for light, if a remains <1.0 during that early phase of a plant’s life cycle when both leaf area and plant mass are increasing exponentially, such individuals will fail to survive. Time trends in a can thus be used to predict future performance with respect to biomass gain, or to analyse adjustments in biomass distribution under contrasting environmental conditions.