DEMOGRAPHIC ANALYSIS OF THE GROWTH OF LINUM USITATISSIMUM

SUMMARY Traditional growth analysis techniques have dealt with changes in plant weight and leaf area and have contributed a great deal to the understanding of plant growth. However, they have ignored the fact that the plant may be considered as a population of modules perfectly suited for studies of age structure. Plant growth has many analogies with the growth of a population of organisms. Plant modules (e.g. leaves) have a life cycle–birth, juvenile phase, ‘reproduction’ (i.e. contributing resources to the birth of other leaves and ultimately to flowering and seed set), senescence and death. Plants of Linum usitatissimum were grown in a glasshouse under full and 50% light and at densities of 3.9 and 30/flat. Leaf birth rates were reduced by shading and especially by increased plant density. Light and density also influenced the branching pattern, the intrinsic rate of growth (rm), leaf survivorship, life expectancy (ex), and leaf death rates. Leaf death was strongly influenced by the treatments. Regardless of leaf age, leaf area index, flowering time, density or light intensity, leaf death started when the total biomass of the plants was about 13 g/flat. This biomass apparently exceeded the carrying capacity of the soil medium. Experimentally withholding nutrients, shading the lower parts of the plants, and the removal of flower buds influenced the onset of leaf death but the variable that was most effective, was the withholding of nutrients. Death started in the lower leaves and proceeded upwards regardless of leaf age. The number of seeds per plant was more strongly correlated with the number of leaves than with the total leaf area per plant. This study shows that the application of life-table and other demographic analyses to leaf birth and death is feasible and permits an interpretation of plant response to environmental factors at a more sophisticated level than is possible from classical growth analysis.

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