Plant Growth Analysis and Modelling

The process of infestation and its effects on plant yield involves a complex of interactions between the insect and the plant, a process that cannot be fully understood until some attention is paid to the effects of the insect or plant growth (Bardner and Fletcher, 1974). Insect infestation is just one of the factors that can affect a plant's growth during its normal life cycle, the main influencing factor being the environment. One of the methods used by plant physiologists to study the effect of the environment on plant growth is called plant growth analysis. This is a powerful method of estimating net photosynthetic production and analysing the physiological adaptations of different plant species (Jones, 1983); it can be readily applied to estimating the effect of insect infestation on plant growth.

Plant growth analysis is based on readily available measurements of plant dry weights and leaf dimensions, obtained from regular harvests at different times during a plant's life cycle. The analyses can be directed at changes in the growth of individual plants, plant parts or plant stands. Plant growth can be followed through a series of relatively infrequent large harvests (the classical approach to plant growth analysis) or by smaller more frequent harvests (the functional approach; Hunt, 1982). Both these approaches produce a series of sequential measurements from which up to four principal types of derived quantity can be constructed.

1. Simple rate of change.

2. Simple ratios between two quantities.

3. Compounded rates of change (rates involving more than one variable).

4. Integral durations (the area beneath a time series of primary or derived quantities; Hunt, 1982).

Also a number of important relationships exist between these derived quantities.

Most photosynthesis occurs in plant leaves, hence the leaf area will influence the amount of photosynthesis that can take place. The leaf area ratio (LAR) is used as an index of plant leafiness. The LAR is the ratio of the total leaf area to whole plant dry weight and in a broad sense represents the ratio of photosynthesizing to respiring material within the plant (Hunt, 1978).

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