Estimation of relative growth rate of ten field-grown herbaceous species: the effects of LAR and NAR depend on time scale and type of analysis

Ten herbaceous species were grown under field conditions to assess their relative growth rate (RGR) and if the differences in RGR amongst species were more associated with morphological or physiological traits. Nine harvests (every 14 days) were carried out in both years. Factors explaining RGR differences between species varied, depending on whether short (14 days) or longer periods (56 and 112 days) were considered and whether analysis was performed by single relationships or by a multivariate linear equation system using the three-stage least squares simultaneous estimation method. When the data were analyzed by single relationships, RGR for short periods (14 days) always showed a positive correlation with net assimilation rate (NAR). In contrast, when growth was investigated over two months, RGR was positively correlated also with morphological traits (LAR, SLA and LMR) in the first two months, only with NAR in the second two months, but considering all the 4 months period, RGR was not correlated with all the traits under observation. When the data were analyzed by multivariate relationships, both NAR and LAR were correlated with RGR, but the key role is attributable to LAR, which together with temperature was found to condition NAR. It is concluded that under field conditions, for short time periods, the differences in RGR values among species seem to be explained by NAR. For longer periods, the plasticity of the plant morphological traits reduces the importance of NAR, and LAR becomes more relevant to plant growth