6.3.2  Size and ontogeny

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Table 6.9

Vascular plants increase in both size and complexity during vegetative growth and reproductive development, showing changes in growth indices that are characteristic of ontogenetic drift (sensu Evans 1972). Size is a major factor for RGR (Table 6.9). This brief survey of wide-ranging taxa shows how values can range over three orders of magnitude. Single-celled organisms such as bacteria and algae vary between 5 and 20 d–1 (corresponding to a doubling time of 0.14 and 0.04 d respectively). By contrast, RGR for young vascular plants including crop species rarely exceeds 0.4 d–1 even during early vegetative growth and is more commonly around 0.1 d–1. Particular organs on vascular plants can, however, achieve faster growth and most notably young leaves can double in size every day or so during their first week of (exponential) growth.

With size comes complexity, and especially in vascular plants where specialised tissues constantly differentiate as organs and participate in resource exchange as either sources or sinks. Perennial plants represent an extreme case where biomass accumulates as inert structures and where cycles of differentiation and renewal last years rather than days. Whole-plant RGR is typically lower in these species. For example, Jarvis and Jarvis (1964) cite representative values for birch seedlings growing in nutrient solution of c. 0.12 d–1 compared with parallel cultures of sunflower of c. 0.24 d–1.


Figure 6.20 RGR for whole plants (g g-1 d-1) is size dependent and commonly diminishes as growth and reproductive development proceed (ontogeny). Three lines of a semi-dwarf wheat designated here by three different symbols differ in their complement of dwarfing genes (Rht) and thus in final size and absolute growth rate, but when referenced to plant mass there are no intrinsic differences in RGR (Based on Bush and Evans 1988)

Even highly selected crop species show an ontogenetic drift in RGR and a semi-log plot of RGR versus plant mass for different wheat genotypes (Figure 6.20) illustrates this principle. Bush and Evans (1988) grew isogenic lines of tall and dwarf wheat in natural light under Canberra phytotron conditions using four day/night temperature regimes in combination with three daylengths (8, 11–12 and 16 h) and with daily irradiance treatments that ranged between c. 8 and 25 MJ m–2 d–1 (total energy). A strong genotype × environment interaction on whole-plant growth was evident in their experiment. Tall isogenic lines were consistently larger due to faster and more uniform germination (Figure 2 in Bush and Evans 1988) but whole-plant RGR was similar for both tall and dwarf lines, and when plotted as a function of dry mass (log scale in Figure 6.20) genetic differences disappeared.

Other cases of genotype × environment effects on plant growth do embody genetic differences, but once again contrasts in plant size must be accommodated for valid comparisons of RGR to emerge. For example, Dijkstra and Lambers (1989) grew two subspecies of Plantago major (large plantain) in a controlled environment and established a genetic difference between the two subspecies (Figure 6.21). P. major L. is an inbreeding perennial that forms a rosette and is distributed worldwide. P. major ssp. major L. is slow growing and late flowering, but withstands stresses such as soil compaction and mowing, and is thus a common weed in lawns and on road sides. By contrast, P. major ssp. pleiosperma (Pilger) is a fast-growing annual, early flowering and an opportunistic coloniser, producing a great number of small seeds and commonly found on river banks and tilled fields.

Both subspecies decreased in RGR with time (Figure 6.21a) regardless of size class, but any clear genetic differences were obscured in these pooled data. However, when RGR data from the two subspecies were plotted as a function of whole-plant fresh mass (Figure 6.21b) age and/or size effects were accommodated and an intrinsic difference in RGR became apparent.

Applying this same rigour in other comparative studies, Dijkstra and Lambers (1989) report intraspecific differences in nutritional physiology, growth response to irradiance, tolerance to trampling and resistance to soil compaction. By eliminating age and/or size as a factor in growth analysis, and thus removing ontogenetic drift as a confounding variable, genotype versus environmental effects on growth indices have been resolved.


Figure 6.21 Two subspecies of Plantago major known to differ with respect to growth rate under natural condition were raised in a controlled environments (13 mol quanta m-2 d-1 and 20 C day and night). RGR diminished with age in all cases (a) and genetic differences did not become apparent until data were referenced to plant mass (b). The higher RGR in P. major ssp. pleiosperma (solid symbols) compared with P. major ssp. major (open symbols) was associated with higher SLA and lower respiratory losses (Based on Dijkstra and Lambers 1989)