Plant Resource Allocation

Fakhri A. Bazzaz, John Grace, Fakhri A. Bazzaz, John Grace

• Publisher: Academic Press
• Year: 1997
• ISBN:
  9780080539072
• Topic: Ciencias biológicas
• Subtopic: Ecología

Plant Resource Allocation is an exploration of the latest insights into the theory and functioning of plant resource allocation. An international team of physiological ecologists has prepared chapters devoted to the fundamental topics of resource allocation.

• Comprehensive coverage of all aspects of resource allocation in plants
• All contributors are leaders in their respective fields

1

Allocation of Resources in Plants: State of the Science and Critical Questions

Fakhri A. Bazzaz

I The Plant Functions as a Balanced System: Flexibility of Allocation

Plant biologists have long recognized that in order for a plant to complete its life cycle, it must function as a balanced system in terms of resource uptake and use (e.g., Mooney, 1972; Agren and Ingestad, 1987). Communication between carbon gaining and nutrient and water gaining parts of the plant is assumed to be rapid and efficient. Resources obtained from the environment and manufactured in the plant are allocated to various plant parts and functions (growth, reproduction and defense) in accordance with this view. Many models and experiments make the fundamental assumptions that natural selection has molded plant allocation in economics terms (see Field, 1991). Investment in any function should be terminated when the “return” on that investment falls below the investment (Mooney and Gulmon, 1979; Bloom et al., 1985). For example, investment in the enzyme Rubisco is reduced when light is limiting, and investment of nitrogen shifts toward chlorophyll to obtain the more limiting resource, light. Carbohydrate status seems to coordinate the balance between photosynthesis and respiration (Amthor, 1995).

Shifts in allocation patterns under changing environmental conditions have been experimentally proven to maximize plant growth (e.g., Robinson and Rorison, 1988; Mooney et al., 1988; Hirose, 1987). At the physiological level, it is assumed that plants allocate resources so that pool sizes within the plant remain constant (Schulze and Chapin, 1987). It is also assumed that environmental limitations, or excesses, that reduce resource use will also reduce resource uptake (Chapin, 1991a,b). However, there is some evidence that the uptake and transport of some nutrients can exceed demands for growth (Schulze, 1991). Is this merely luxury uptake to deprive neighbors? How quick is the adjustment to achieve this balance? How much of these resources remain in the active pool? What is the cost of their storage, if any? The speed of adjustment to the prevailing environmental conditions varies among plants and is a critical aspect of their strategies. It follows from this expectation that plants with relatively high nonstructural materials (e.g., herbaceous annuals) are more flexible in redeployment than those with relatively high structural materials (e.g., long-lived trees) and that plants which occupy habitats with highly variable environments have a higher flexibility of allocation and redeployment. They must track their environment, i.e., quickly change their resource allocation in response to environmental change. However, there is only limited experimental evidence for this situation at this time. Plantago major, which is more common in repeatedly disturbed habitats, is more responsive, in terms of allocation, to nutrient pulses than its congener P. rugelii, which is common in less disturbed habitats (Miao et al., 1991). In P. major there is a significant increase in allocation to reproduction, and there is a negative correlation between vegetative and reproductive biomass. With a nutrient pulse, P. major increased its leaf relative growth rate (RGR_l) and decreased its root relative growth rate (RGR_r). In contrast, P. rugelii showed only a small increase in reproductive biomass, and the correlation with vegetative biomass was weak.

Early protection against herbivores may result in many benefits, especially if the protected tissue has a high potential to gather additional resources in the future. Resources captured early are worth much more to the plant than similar quantities captured later, if these early-captured resources are allocated to organs, such as leaves, that can collect further resources. The analogy of this situation with compound interest in economics is obvious. However, Lerdau (1992) argues that economic models of investment do not work well for plants because there is no risk-free environment and economic models of compound interest require a risk-free setting. Economic analysis assumes that there is a trade-off among various sinks, and that resources are permanently allocated to these sinks. It is now clear that this may not be the case in all plants or all environments. Also, the allocation of resources such as minerals and proteins, independent of carbon and mass, are possible. Therefore, there is a need for other kinds of models to do a complete evolutionary analysis of allocation.

Whereas much attention has been given to allocation to shoots, including leaves, flowers, and fruits, much less attention has been given to roots and other underground parts. This is understandable because of the greater difficulty in studying roots in general. New techniques that allow a more accurate assessment of root growth and architecture (e.g., fiber optics, video imaging) are aiding biologists in their study of allocation. Whereas leaves of limited numbers of species, especially in wet environments, interact directly with other organisms such as algae and bryophytes (epiphylls), roots in the soil interact with a variety of soil microorganisms (fungi in mycorrhizal associations, nitrogen fixers, other bacteria, etc.). These organisms can greatly aid root function, in terms of both the availability and uptake of ions, and of water. It stands to reason then that roots must supply these organisms with lots of energy-containing compounds to sustain their growth. Measurements of standing roots biomass, therefore, may greatly underestimate the actual allocation to belowground parts. The partitioning of ions and water absorption between fine and coarse roots, respectively, and the discovery that in some plants there is hydraulic lift, i.e., the uptake of water from lower depths and its release in the upper parts of a soil profile (Caldwell and Richards, 1989), add further challenges to the problem. Furthermore, like herbivory on aboveground plant parts, herbivory on roots can be substantial and variable between years and habitats, but we still know very little about the extent and variation of belowground herbivory.

Acclimation, which can mean the restoration of the allometric ratios between plant parts, can occur at different rates in various species and within the same species for different traits. Many plants adjust their allocation in response to a changing environment (trackers) and are said to acclimate. In a heterogeneous environment acclimation is assumed to be functionally adaptive. Environmental shortages and excesses may greatly change the allocation patterns (see Chiariello and Gulmon, 1991). Soil moisture content and light levels can greatly influence the relative allocation to roots and shoots. Generally, it is assumed that this acclimation occurs in species with slow growth rates and therefore slow organ turnover rates (Grime and Campbell, 1991; Thornley, 1991). Long-lived organs in a changing environment must acclimate to that environment to optimize their resource gain. In contrast, short-lived organs can be discarded in favor of newer organs suitable to the environment. These two kinds of responses are seen in understory plants in the forest when a canopy gap is suddenly created above them, drastically changing the light environment.

We need more information to answer the following important questions:

1. How fast can a plant adjust its belowground and aboveground activities when either the shoots or the roots are subjected to severe herbivory and become out of synchrony with each other?

2. Does this adjustment happen by increasing the specific activity of shoots (enhanced photosynthetic rate per unit of leaf) and roots (enhanced specific absorpti...