Inherent Variation in Growth Rate Between Higher Plants: A Search for Physiological Causes and Ecological Consequences
Section snippets
SUMMARY
When grown under optimum conditions, plant species from fertile, productive habitats tend to have inherently higher relative growth rates (RGR) than species from less favourable environments. Under these conditions, fast-growing species produce relatively more leaf area and less root mass, which greatly contributes to their larger carbon gain per unit plant weight. They have a higher rate of photosynthesis per unit leaf dry weight and per unit leaf nitrogen, but not necessarily per unit leaf
INTRODUCTION
Plants are distributed over a wide range of habitats varying from tundra to rain forests, from wetlands to deserts and from lowland to alpine regions. Coping with such contrasting, sometimes extreme, environments requires a certain degree of inherent specialization. One of the characteristics in which species of different habitats vary is their growth potential. Plants growing on nutrient-poor soils have a lower growth rate than those on fertile soils. But even when grown under optimum
GROWTH ANALYSES
Growth analysis is often used as a tool to obtain insight into the functioning of a plant. Different types of analyses exist, depending on what is considered a key factor for growth (cf. Lambers et al., 1989). In the most common approach, leaf area is assumed to be a key factor. The relative growth rate (RGR) (see Table 1 for a list of abbreviations), the rate of increase in plant weight per unit of plant weight already present, is then factorized into two components, the leaf area ratio and
NET ASSIMILATION RATE AND LEAF AREA RATIO
A wealth of information is available on the comparison of growth of two or three species, but few authors have investigated the relation between RGR and growth parameters for a range of species. Potter and Jones (1977) compared nine crop and weed species, Mooney et al. (1978) investigated five Eucalyptus species and Poorter and Remkes (1990) analysed the growth of 24 wild species common in western Europe. In all of these cases the LAR was the predominant factor explaining the inherent variation
SPECIFIC LEAF AREA
As outlined in Section IV, variation in RGR is strongly correlated with that in LAR. Differences in LAR can be due to variation in LWR or in SLA. The specific leaf area is defined as the amount of leaf area per unit leaf weight. Its reciprocal, specific leaf weight or specific leaf mass, is also frequently used. Various aspects of inherent and environmentally induced variation in SLA have been reviewed by Dijkstra (1989). Large variations in SLA can be found between different types of plants
BIOMASS ALLOCATION
Biomass allocation can be defined in terms of leaf, stem and root weight ratio, the fraction of total plant biomass allocated to leaves, stems and roots, respectively. A more frequently used parameter, the shoot: root ratio or its inverse, does not acknowledge the distinct functions of leaves and stems and is avoided here. Various aspects of inherent differences in biomass partitioning between leaves and roots have been discussed by Konings (1989). A low availability of nitrogen, phosphorus and
GROWTH, MORPHOLOGY AND NUTRIENT ACQUISITION OF ROOTS
The simple growth equation: RGR=NAR×LAR, suggests that any investment in biomass other than leaf area reduces the plant's RGR. Such an approach tends to consider the roots merely as a carbohydrate-consuming organ and does not give credit to their role in the acquisition of nutrients and water or their function in transport, storage and anchorage. In this section we will concentrate on the root's role in the acquisition of ions. Growth can then best be approached from an alternative point of
CHEMICAL COMPOSITION
Plant dry matter is composed of a number of major compounds, which can be grouped into the following seven categories: lipids, lignin, organic N-compounds, (hemi)cellulose, non-structural sugars, organic acids and minerals. Apart from “primary” compounds, there is a wealth of “secondary” compounds, defined by the absence of a clearly defined role in the metabolic processes of the plant (Baas, 1989; Waterman and McKey, 1989). Lignin is often included in the category of “secondary” compounds and
Species-specific Variation in the Rate of Photosynthesis
Fast-growing crop species (Evans, 1983; Makino et al., 1988) and their accompanying weeds (Sage and Pearcy, 1987) tend to have higher maximum rates of photosynthesis (expressed per unit leaf area) than evergreen trees and shrubs (Field et al., 1983; Langenheim et al., 1984). Similarly, sun species have a higher rate of light-saturated photosynthesis per unit area than slower-growing shade species, when the plants are grown at an optimum quantum flux density (e.g. Pons, 1977; Björkman, 1981;
RESPIRATION
Respiration provides the driving force for three major energy-requiring processes: maintenance, growth and ion uptake. Maintenance respiration is mainly associated with turnover of various cellular components and the conservation of solute gradients across membranes. Growth respiration is used to supply ATP and NADH, needed to convert glucose into the different chemical compounds. In roots, respiratory energy is also needed for the absorption of nutrients from the environment.
EXUDATION AND VOLATILE LOSSES
Plants lose photosynthates through exudation and volatilization as well as during respiration. Exudation may occur both above- and below-ground, whereas volatilization predominantly occurs above-ground.
OTHER DIFFERENCES BETWEEN FAST- AND SLOW-GROWING SPECIES
Apart from the above-mentioned traits, which directly affect the growth of a plant, some other aspects of fast-growing and slow-growing species and mutants thereof have been investigated. In recent years fascinating information has become available on the role of a specific class of phytohormones in the control of a plant's growth rate—the gibberellins.
AN INTEGRATION OF VARIOUS PHYSIOLOGICAL AND MORPHOLOGICAL ASPECTS
In the previous sections several aspects of the physiology, morphology, allocation and biochemical composition have been discussed in relation to the potential growth rate of plant species. We now address the question of what proportion each parameter contributes to the observed differences in growth rate, using (Eq. (5)) as a framework.
SPECIES-SPECIFIC PERFORMANCE UNDER SUBOPTIMAL CONDITIONS
Up till now we have paid most attention to plants grown under conditions favourable for plant growth. But how do fast- and slow-growing species perform under suboptimal conditions?
When grown at a low nutrient concentration in the environment, the RGR of potentially fast-growing species is reduced more than that of slow-growing ones (e.g. Christie and Moorby, 1975; Robinson and Rorison, 1987; Boot and Mensink, 1991). However, the inherently fast-growing species are still growing faster than
What Ecological Advantage can be Conferred by a Plant's Growth Potential?
The ecological advantage of a high RGR seems straightforward: fast growth results in the rapid occupation of a large space, which is advantageous in a situation of competition for limiting resources. A high RGR may also facilitate rapid completion of the life cycle of a plant, which is essential for ruderals. But what is the survival value of slow growth? Grime and Hunt (1975) and Chapin, 1980, Chapin, 1988 mention several possibilities:
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Slow-growing species make modest demands and will
CONCLUDING REMARKS AND PERSPECTIVES
Generalizing the above leads to suites of traits of a “typical fast-growing” and a “typical slow-growing” plant species (Table 5). Most of these traits refer to slow-growing species from nutrient-poor sites. Species from other adverse habitats may also have a lower RGR (Section II and XVE), but much less comparative data are available. The difference with species from more favourable conditions is probably less pronounced and information on special traits of such slow-growing species is scanty.
Acknowledgements
We would like to thank all colleagues who generously allowed us to use some of their unpublished data, and the following colleagues for their constructive criticism on (parts of) earlier drafts of this manuscript: Frank Berendse, Arjen Biere, René Boot, Marion Cambridge, Heinjo During, Eric Garnier, Henk Konings, Dick Pegtel, Thijs Pons, Jacques Roy, Adrie van der Werf, Marinus Werger and Chin Wong. We thank Marion Cambridge for her linguistic advice.
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