6.6  Concluding remarks

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Ironically, growth indices discussed here are not fundamental entities with some intrinsic significance for plants. Instead, they are derived concepts, and products of human intuition. As such they provide terms of reference for quantitative analysis of responses in growth and reproductive development at a single plant or community level, and some-times point to processes responsible for those responses.

In most cases definition of a growth index is intuitive and inspired by features of growth occurring under conditions which allow exponential growth. A colonising population of single-celled organisms or simple floating plants such as Lemna minor can grow exponentially and thus in accordance with the law of compound interest. However, this is not the case with more complex vascular plants. In these, structural differentiation and a division of labour between tissues concerned with resource capture, substrate processing, structural integrity and photoassimilate storage leads to more complex growth patterns. Invariably, whole-plant RGR falls with age (size) even in a constant environment so that, in truth, whole-plant growth is probably never truly exponential, but fortunately these growth indices can still find application.

At any one time, growth indices and relationships between them yield information on plant function. More particularly, the manner in which these indices vary over time or in response to treatments points to changes in plant function during growth and development.

Decades of success in plant growth analysis has hinged upon shrewd insights and skilful construction and application of growth indices. Further refinement will emerge as improved measurements over shorter time intervals target component processes. Elaboration of conceptual or mathematical process-based models for plant growth and reproductive development will focus that effort, and eventually mechanisms responsible for genotype x environment interaction on gene expression in phenotypes will be identified and examined in more detail.