Growth analysis of congeneric annual and perennial grass species

1. A growth analysis was conducted on seven annual/perennial pairs of grasses (six congeneric and one pair taken at random) grown in a hydroponic culture system under constant, productive conditions, in order to investigate the mechanisms responsible for the higher relative growth rate (RGR) of annuals as compared to perennials. 2. All the production parameters (RGR, relative leaf production rate, unit leaf rates), were higher in annuals than in perennials. There were very few differences between the two life forms in biomass allocation to the different organs (except a higher allocation to sheaths in perennials). The specific leaf areas and the leaf area ratios were higher in annuals, whereas the dry-weight:fresh-weight ratios of all the organs were higher in perennials. These differences between annuals and perennials were almost systematic when the comparisons were made within a given genus, but not when they were made between annuals and perennials in different genera. 3. The wide range of RGR obtained for the 14 species also permitted general relationships between RGR and the various components of growth to be discussed. RGR was significantly correlated with: (i) unit leaf rate, (ii) specific leaf area, (iii) leaf area ratio, (iv) dry-weight:fresh-weight ratio of the whole plant, and (v) dry-weight:fresh-weight ratio of roots. Surprisingly, RGR was not correlated with any of the biomass allocation parameters. The single factor that best explained the differences in RGR was the specific leaf area. 4. If, as has been hypothesized elsewhere, the annual life form is derived from the perennial one, the results presented here suggest that the same morphological changes have occurred repeatedly in different genera. It is argued that these changes, which are probably correlated with differences in anatomical features, mainly affect the specific (i.e. per unit weight or area) uptake capacities of the plant organs, leading to the higher seedling growth rate observed in annuals.