|
|
|
|
|
One key question to be examined is the issue as to whether plants indeed behave like the 'wise investor' who may follow optimization principles in resource allocation to the various demands in physiology (GRACE 1997). Perhaps, this is the case in short-lived herbaceous rather than woody life forms (HEILMEIER et al. 1997). However, generalization is rather uncertain for woody plants because of their prolonged ontogeny, and their 'defense strategy' in relation to the primary and secondary metabolism appears to be largely determined by the prevailing growth conditions (MAURER & MATYSSEK 1997; GRAMS et al. 1999; POLLE et al. 2000). Nevertheless, modeling concepts show that respiratory costs as part of the carbon balance may be, in proportion, rather similar in forest and pasture systems (THORNLEY & CANNELL 2000). Are trade-offs between alternate pathways of resource investment typically linear as found in relationships between growth rate and lignification or reproduction (SIBLY & VINCENT 1997; LERDAU & GERSHENZON 1997)? The application of economic optimization principles to plants may be problematic, however, for clarifying allocation patterns, as the variable site conditions may bias the resource-based 'interest staking' in complex ways, and as the involved 'interpretation' of signals between external stimuli and effective physiological responses is still poorly understood (BAZZAZ 1997). For clarifying 'allocation strategies', the analysis of 'priority conflicts' - both based on modeling and experimental concepts (as pursued by SFB 607) - gains in importance (GRACE 1997; KIMMINS et al. 1999; BARTELINK 2000)
Starting point for the examination of 'allocation strategies' may be a conceptual model as proposed by HERMS & MATTSON (1992) which claims that increasing (external) resource availability does reduce the proportion of secondary metabolites along with an increasing primary production (Fig. 2). This reduction is believed to occur at the expense of defense but in favor of a stimulated primary metabolism and, as a consequence, fostered plant competitiveness (Fig. 3) - if growth is conceived as biomass investment into sequestration of shoot and root space and, thus, as a pre-requisite of competitive resource exploitation. In these terms, the 'competitive behavior' of a plant can be quantified through a sequence of 'cost/benefit' relationships:
- Efficiency in space sequestration:
Resource investment per unit of occupied shoot or root space (and in relation to the exploitable resource availability);
- Efficiency in space exploitation:
Resource acquisition (gain) per unit of resource investment (or occupied shoot and root space);
- Efficiency of maintenance costs:
Demand for resources (water, respired carbon, nutrients) per unit of resource gain.
|
These efficiencies characterize the 'translation' of resource allocation into allometric relationships and 'cost/benefit' balances that result from space occupation (MÄKELÄ & VANNINEN 1999), and by this, represent the actual mechanism of 'competitive behavior' of plants (TILMAN & GRACE 1990; KÜPPERS 1994; BAZZAZ 1997; HIKOSAKA et al. 1999). Does the competitive advantage arising from a high resource acquisition capacity (as one aspect of fitness) become jeopardized by a simultaneous decline in the ability of resource retention (as one further aspect of fitness), i.e., limited defense capacity (cf. BUNGERER et al. 1999)? Is such a scenario characteristic for economic plants under high resource supply, whereas increasing primary production at low supply might be associated with stimulation in secondary metabolism (cf. Fig. 2; HERMS 1999)? If so, then a trade-off like that depicted in Fig. 3, in particular its linearity, would be questionable. Interrelationships of this kind form the core of the hypothesis that integrates the two major aspects of fitness, competitiveness in resource sequestration and defense, in the following way
Regardless of the kind of stress, plants regulate their resource allocation in a way that increase in stress tolerance and resistance (in particular against pathogens and phytophages) inherently leads to constraints on growth and competitiveness.
The 'translation' of resource allocation into plant allometry reflects an inherent link to defense, as the value of organs in defense may decline along with their increasing proportion in the whole-plant biomass (Fig. 4 ; ZANGERL & BAZZAZ 1992). Hence, defense costs of roots and their value for ensuring an adequate resource sequestration capacity (and by this, for maintaining competitiveness) may be enhanced at a situation of a low root/shoot ratio and limited resource uptake. 'Indirect costs' may complicate such interrelationships, if constitutive defense per se already curtails the assimilate pool for growth (PENNYPACKER 2000). The issue of 'cost/benefit' relationships in resource allocation has become part of a number of theories and modeling concepts (e.g. MÄKELÄ 1990, "Functional Balance Theory"; NIKINMAA & HARI 1990, "Cost-Benefit Principle").
However, only the clarification of the underlying mechanisms can decide, as to whether relationships as proposed by Figs. 2 to 4 possess a general validity - or are realistic at all. Do such mechanisms comply with the 'Growth-Differentiation Balance Theory' which claims the capacity and quality in parasite defense to result from the ratio between productivity and demand for carbon during organ differentiation (LOOMIS 1953; LORIO 1988)? Or, are mechanisms consistent rather with the 'Carbon-Nitrogen Balance Theory' (BRYANT et al. 1983) which predicts the biochemical quality of defense to be determined by the adjustment between the carbon and nitrogen flux through the plant. These two theories, although being those of major relevance in the context of competitiveness and defense, are not uncontradicted in terms of their general validity (RHOADES 1979; COLEY et al. 1985; LINCOLN & COUVET 1989; HAMILTON et al. 2000). This is the case, in particular, in relation to woody plants with a regulation that potentially favors the constitutive rather than induced defense (FEENY 1976). Or, are these 'classical' theories even obsolete as soon as trade-offs like shown in Figs. 2 and 3 are subjected to a 'full-cost analysis' - as has been postulated recently (BAZZAZ 1997; LERDAU & GERSHENZON 1997)? In this latter case, 'cost/benefit' balances do not only require the synthesis costs of metabolites to be taken into consideration, but also additional costs are relevant for maintaining the involved, enzymatic apparatus as well as for storage, transport and turnover without de novo synthesis.
Because of its complexity, a 'full-cost analysis' is a theoretical aim (BAZZAZ 1997) which, to date, has been realized in one rather specific case only: monoterpenes in pine needles (GERSHENZON 1994; LERDAU & GERSHENZON 1997). Even this example needed to be based, in part, on estimations. Nevertheless, this kind of analysis provides a perspective to be pursued when unraveling 'conflicts' in resource allocation, and attempts appear to be promising, if metabolites are considered which - as far as known - do not seem to be associated with specific costs for storage and transport. A pre-requisite of such an approach is the determination of the pool sizes and fluxes of the central compound classes of plants. According to POORTER & VILLAR (1997), these are carbohydrates, proteins, lipids, phenolics, lignins and the nutritional (mineral) elements. As pointed out by these authors, the regulation between the pool sizes of these compound classes is poorly understood. In particular, it has turned out that synthesis costs of biomass production is rather similar often in different plant life forms, although their chemical composition may differ quite substantially: For example, herbaceous plants may display higher proportions in the contents of proteins, minerals and organic acids relative to woody plants, whereas the latter may be characterized by high levels of phenolic compounds and lignins. Do negative correlations between compound classes of contrasting, energetic costs exist that provide a basis for general rules in resource turnover, regardless of the plant life form?
If so, it appears to be promising to examine to what extent such 'correlative pre-settings' may be affected, if biotic impact through parasites or symbionts alter the balance between primary and secondary metabolism (BECKMAN 2000; NEHLS & HAMMP 2000; GORISSEN & KUYPER 2000). Do general rules of this kind reflect the ultimate basis of similar relationships between the size, number and maximum (ground area related) population density of plants, irrespective of the stand conditions of forest or pasture systems (REINEKE 1933; YODA et al. 1963)? The analysis of resource allocation may provide the mechanistic clarification of such common, unifying rules.
The pool size analysis of plant compounds also offers the advantage to assess those substrates which may be part of resource accumulation (as often seen in herbaceous plants) or storage (both herbaceous and woody plants), and which may compete, plant-internally, with the physiological demands for growth and defense (STITT & SCHULZE 1994). A clear separation of these latter demands on the one hand from resource accumulation and storage on the other is questionable, however, as these different pools may be subject to substrate exchange between each other prior to the ultimate resource use. This aspect gains in importance with increasing plant longevity and is significant, in particular, in woody plants. Scaling of such pool interactions is essential across the different stages of plant age, an issue which is relevant not only for the current discussion on growth limitation during tree ontogeny (RYAN et al. 1997; BOND 2000; KOLB & MATYSSEK 2001), but also for the hypothesized relationships between competitiveness and parasite defense (cf. Figs. 2 - 4). It is one of the major aims of SFB 607 to integrate the mechanisms in resource allocation in long-lived, economic plants, i.e. forest trees, across their characteristic stages in ontogeny.
|
|
|
|
