Weed shoot morphology effects on competitiveness for light in direct-seeded rice

The effects of weed shoot morphology on competitiveness for light in rice ( Oryza sativa L.) have not been well described quantitatively and are diffic ult to study empirically. A rice:weed model was used to analyse the effects of weed leaf area densities (LAD; m(2) m(-3)), leaf angles (as leaf light extinction coefficients, k(leaf)) and maximum heights (H-m, m) on growth an d competition with rice. Weed morphologies were hypothetical but empiricall y based, as follows: LADs were skewed to the bottom or conical, k(leaf) val ues varied from 0.2 (erectophile) to 0.8 (planophile), and H-m values were 0.5H(R), 1H(R) and 1.25H(R), where H-R was rice maximum height. Other param eters were equal to those of rice. Growth and competitiveness were evaluate d using mature seed dry weights (g m(-2)). Short weeds and weeds with conic al LADs were weakly competitive, regardless of other traits. For other weed types, interference with rice was positively related to H-m, LAD skewness and more planophile leaves. All three traits were critical determinants of weed interference but no single morphological trait guaranteed competitiven ess. All else being equal, weeds with highly skewed LADs produced the most seed dry weight. Planophile leaves were particularly beneficial for short w eeds, giving over five times more seed dry weight than erectophile leaves.