Modeling developmental growth stages in mango on the basis of absolute vs. relative growth rates

Plant development, the change in plant structure, and growth, the increase of organ size over time are often studied as two separate processes. The concept of developmental growth stages has been recently proposed in mango to integrate both morphological traits and growth measurements. These stages rely on a strong match between developmental (i.e., phenological) stages and growth stages deduced from absolute growth rate (AGR) sequences. In this study, we explore growth stages deduced from relative growth rate (RGR) sequences. For a given mango cultivar (Cogshall or Jose) and either growth units (GUs) or inflorescences, a multistage model based on RGR sequences was first built. Growth stages deduced from this model were then compared with developmental stages determined visually. For GUs, the match rates between RGR-based stages and developmental stages were similar to the match rates between AGR-based stages and developmental stages, because of the rich information provided by the four organs modeled (the axis and three selected leaves). The match rates were far lower for the inflorescences where only the main axis was modeled. This is related to the fact that, compared to AGRs, RGRs amplify the variations at the beginning of growth of an organ while damping the variations at the end of growth. This explains the low match rates at the end of inflorescence growth. In plant growth studies, RGRs are often preferred to AGRs with the objective to take account of sink strength and to abstract from the effect of organ size. This latter argument is not relevant in our case since our probabilistic multistage model intrinsically abstracts from the organ size and only relies on the overlap of growth rate distributions for successive stages.