Growth Maintenance

Three primary activities

Resources for reproduction are diverted from growth. Maintenance costs are constant. Polycarpic species

Resources for reproduction are augmented from other sources and diverted from growth. Maintenance costs are constant. Polycarpic species

Resources for growth and reproduction are augmented from other sources. Maintenance costs are constant. Polycarpic species

Fig. 7.7. Theoretical resource budgets of plants over one growing season. Resources are allocated to maintenance (M), growth (G) and reproduction (R) (redrawn from Willson, 1983).

Possible allocation pattern for a monocarpic species

Fig. 7.7. Theoretical resource budgets of plants over one growing season. Resources are allocated to maintenance (M), growth (G) and reproduction (R) (redrawn from Willson, 1983).

reproduction (Fig. 7.7a). The 'principle of allocation' states that plants have a limited supply of resources and that this is allocated to various structures in a way that maximizes lifetime fitness (Bazzaz, 1996; Barbour et al., 1999). Obviously, plants do not make conscious decisions on where to allocate resources. This is determined by the interaction of their genotype and their environment. How a plant allocates resources is important because if too much is spent on one function, then other functions may suffer. For example, if a perennial species allocates too many resources to reproduction and not enough to storage, then it may not be able to survive a harsh winter.

The amount of resources allocated to various functions will vary with the life history strategy and will change over the course of a plant's life cycle. Early on, plants accumulate biomass/nutrients in roots, shoots and leaves. In annual species, reproduction events require expenditures of resources towards the production of reproductive structures (gametes, protective tissue, attraction structures) and towards the care of maturing embryos (Willson, 1983); therefore, as the season progresses, more resources will be devoted to reproduction

(a) Corn marigold 100

S 60

oi m

(a) Corn marigold 100

S 60

June

July

BSeeds E3 Receptacles

H Vegetative buds

□ Underground structures

June

July

(b) Jerusalem artichoke 100

underground stem::

toots underground stem::

toots

10 15 20

Weeks after planting

H Seeds

□ Leaves H Branches

□ Underground structures

Fig. 7.8. Allocation of dry weight to vegetative and reproductive structures of: (a) an annual species, corn marigold and (b) a perennial species, Jerusalem artichoke (adapted and redrawn from Harper, 1977, and Swanton and Cavers, 1989).

and less to vegetative structures (Fig. 7.8a). If resources are limiting, then the individual may not be able to reproduce, or may reproduce, but at the cost of future fitness or sur vival. Allocation patterns differ among species. Common groundsel (Senecio vulgaris), for example, allocates proportionally more resources to stems and less to flowers than corn marigold (Chrysanthemum sege-tum), and yet both are annuals (Harper, 1977). Within species, allocation patterns will vary with the environment. In common groundsel, for example, more resources are allocated to roots in stressful environments.

In perennials, allocation patterns differ primarily because fewer resources are allocated to reproduction. For example, the Jerusalem artichoke (Helianthus tuberosus) is a herbaceous perennial well adapted to invading open areas, particularly cultivated fields (Swanton and Cavers, 1989). Here, a relatively large proportion of biomass is allocated to structural organs such as stems, leaves and branches (Fig. 7.8b). Over the season the allocation to storage organs such as roots, rhizomes and tubers increases and is much larger than biomass allocated to flowers and seeds. This pattern of allocation ensures long-term survival through clonal structures, as well as seed production.

Each reproductive event comes at a cost that must either be compensated for through the accumulation of new resources, or through a trade-off within the plant. In poly-carpic species, for example, the plant diverts only some of its resources towards each reproductive event (Fig. 7.7b-d). In some cases the cost occurs at the expense of growth (Fig. 7.7b), while at other times the cost may be covered by the uptake of additional resources which will partially or totally compensate (Fig. 7.7c and d). This increase in resource supply occurs when reproductive structures take up more resources (e.g. flower and fruit may be pho-tosynthetic) or when they enhance the uptake of resources through vegetative structures. During reproduction, for example, leaf photosynthesis of quackgrass may increase or decrease, depending on the plant's genotype and nutrient status (Reekie and Bazzaz, 1987). Where the change in leaf photosynthesis is positive, the cost of reproduction is offset. In monocarpic species, the plant diverts most of its stored resources towards reproduction at the end of its life cycle (Fig. 7.7e).

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