The effects of environmental heterogeneity on root growth and root/shoot partitioning

Abstract

Aims: The purpose of this Botanical Briefing is to stimulate reappraisal of root growth, root/shoot partitioning, and analysis of other aspects of plant growth under heterogeneous conditions.

Scope: Until recently, most knowledge of plant growth was based upon experimental studies carried out under homogeneous conditions. Natural environments are heterogeneous at scales relevant to plants and in forms to which they can respond. Responses to environmental heterogeneity are often localized rather than plant-wide, and not always predictable from traditional optimization arguments or from knowledge of the ontogenetic trends of plants growing under homogeneous conditions. These responses can have substantial impacts, both locally and plant-wide, on patterns of resource allocation, and significant effects on whole-plant growth. Results from recent studies are presented to illustrate responses of plants, plant populations and plant communities to nutritionally heterogeneous conditions.

Conclusions: Environmental heterogeneity is a constant presence in the natural world that significantly influences plant behaviour at a variety of levels of complexity. Failure to understand its effects on plants prevents us from fully exploiting aspects of plant behaviour that are only revealed under patchy conditions. More effort should be invested into analysis of the behaviour of plants under heterogeneous conditions.

Fig. 1. The relationship between percentage of root systems of plants of Arrhenatherum elatius (closed circles), Briza media (closed squares) and Poa annua (closed triangles) located in a nutrient‐enriched quadrant of a pot in which the plant was grown, and total root weight. Harvests were taken 22, 36, 45 and 57 days after the start of growth. The solid line has the form y = 30·49 + 6·29x + 6·61x² (r² = 0·44, P < 0·001). See text for further details of the experimental design.

Fig. 2. Predicted patterns of adjustment of biomass allocation between roots and shoots for (A) non‐clonal plants growing in conditions of high light availability and low nutrient availability (left), and high nutrient availability and low light availability (right); (B) connected ramets of a clonal plant, of which one ramet (left) is growing in conditions of high nutrient availability and low light availability while the other (right) is growing in conditions of high light availability and low nutrient availability; and (C) separated ramets of a clonal plant, of which one ramet (left) is growing in conditions of high light availability and low nutrient availability while the other (right) is growing in conditions of high nutrient availability and low light availability. The diagrams show relative adjustments, rather than absolute growth of above‐ and below‐ground parts. The grey boxes indicate scarcity of either light or soil‐based resources. Diagram modified from Alpert and Stuefer (1997).

Fig. 3. (A) Relationship between root/shoot ratio and total clone weight for Glechoma hederacea grown in heterogeneous conditions with large scale patches (closed circles and solid line) and small scale patches (open circles and dashed line). Total nutrient supply is the same in each case. For clones in heterogeneous environments with large patches, y = 0·17 + 0·04x, r = 0·69, P < 0·001. For clones in heterogeneous environments with small patches, y = 0·41 + 0·02x, r = 0·54, P < 0·01. (B) Mean (± s.e.) root/shoot ratio for whole clones of Glechoma hederacea grown in large‐ and small‐patch environments with different degrees of contrast between patches. Patch scale had a significant effect on root/shoot ratio (F_1,48 = 14·75, P = 0·0004), whereas neither contrast (F_5,48 = 1·23, P = 0·31) nor the interaction between scale and contrast (F_5,48 = 0·56, P = 0·73) significantly affected root/shoot ratio. Redrawn from Hutchings et al. (2003). (C) Mean (± s.e.) root/shoot ratio of clone parts in nutrient‐rich and nutrient‐poor patches in large‐ and small‐patch treatments at different patch contrasts. Closed circles = rich patches in large‐scale environments, closed inverted triangles = rich patches in small‐scale environments, open circles = poor patches in large‐scale environments, open inverted triangles = poor patches in small‐scale environments. Re‐drawn from Wijesinghe and Hutchings (1999).