What is the basis of early vigour, being an important trait of weed competitiveness in rice

Cultural control is a promising alternative weed management strategy to help reduce the strong dependency on chemical weed control. One of the components of cultural control is the use of cultivars with improved weed suppressive ability. The possibility to breed for an increased weed suppressive ability has been explored for a variety of crops. Repeatedly, early vigour, or the ability to grow relatively fast during early growth stages, has been identified as an important trait associated with the ability of the crop to better suppress weeds. Often times, screening for early vigour is based on a rather subjective visual score. In this study the aim was to find out whether early vigour in rice is based on specific traits or related to a particular growth strategy. In such case, it might be possible to define more objective and easy quantifiable traits for weed suppressive ability that would favour the screening and selection process. Field experiments with over forty rice cultivars were conducted to identify cultivars with superior weed suppressive ability. Based on these results, four rice cultivars were selected for further study in growth chambers. In these studies, regular plant sampling was used to determine initial growth and leaf area development of the cultivars throughout a four-week-period. These data were used as basis for a classical growth analysis. In addition, a simple plant growth model was used to establish the influence of growth strategy, characterized by assimilate allocation pattern, on early growth. The field experiments revealed that a visual vigour rating at 4 weeks after sowing was highly significantly correlated with weed suppressive ability (Zhao et al., 2006a,b,c). Further experimentation in growth chambers indicated that differences in total accumulated dry weight after 26 days corresponded to the observed weed suppressive ability in the field. The two fastest growing cultivars had opposite growth strategies, with one cultivar investing in a high amount of leaf area and the other investing in high-quality leaves. Model simulations confirmed that differences in growth strategy not necessarily result in differences in overall growth rate. In the growth chamber experiments, differences in relative growth rate (RGR) between cultivars only became apparent after the first fourteen days. From that moment on the RGR of all cultivars dropped steadily, but this reduction in RGR was less strong in the more competitive cultivars. It was hypothesized that the reductions in RGR are a result of self-shading, and that the more competitive cultivars are better able to avoid self-shading, for instance through tillering. Whether the morphology of rice cultivars is indeed a more significant factor in the realization of early growth than initially assumed, is currently being investigated