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 difficult to study empirically. A rice:weed model was used to analyse the effects of weed leaf area densities (LAD; m² m^–3), leaf angles (as leaf light extinction coefficients, k _leaf) and maximum heights ( H _m, m) on growth and competition with rice. Weed morphologies were hypothetical but empirically based, as follows: LADs were skewed to the bottom or conical, k _leaf values varied from 0.2 (erectophile) to 0.8 (planophile), and H _m values were 0.5 H _R, 1 H _R and 1.25 H _R, where H _R was rice maximum height. Other parameters were equal to those of rice. Growth and competitiveness were evaluated using mature seed dry weights (g m^–2). Short weeds and weeds with conical 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 competitiveness. All else being equal, weeds with highly skewed LADs produced the most seed dry weight. Planophile leaves were particularly beneficial for short weeds, giving over five times more seed dry weight than erectophile leaves.     

Shading and the growth and photosynthetic responses of Ammannia coccinnea

The effects of shade on the growth and CO₂ exchange rate (CER) of Ammannia coccinnea Wild., a noxious weed in water-seeded rice, were determined under glasshouse conditions. Shade substantially reduced A. coccinnea growth when imposed early and maintained throughout the growing season. However, plants transferred from full light to 18% or 50% sunlight 30 or 45 days after seeding had only slightly less dry weight than unshaded plants. Constant shade reduced growth severely (94%). In contrast, plants that were shaded for 45 days and then placed in full light largely recovered from the effects of shading by final harvest. The ability of A. coccinnea to reduce the effects of shade appears related to increased partitioning to leaves, increased specific leaf area and decreased dark respiration rates. The ability of the weed to recover from shade appears related to its ability to greatly increase CER in new leaves in response to increased light. Plasticity in A. coccinnea morphology and physiology appears to explain its persistence in California rice fields and suggests that management strategies that rely on shade alone may not improve control of this weed.     

The relative importance of root and shoot competition between water-seeded rice and Echinochloa phyllopogon

The relationship between root and shoot growth were determined for water-seeded rice and Echinochloa phyllopogon in greenhouse experiments. When grown in monoculture at four nitrogen rates (0, 60, 120, 180 kg N ha⁻¹), root dry weight was highly correlated with canopy structure for both species. Echinochloa phyllopogon showed a significantly stronger response to the nitrogen rate than rice. When rice and E. phyllopogon were grown in competition, with roots either separated or allowed to mingle, root competition contributed more than shoot competition to reductions in the growth of the target species. The results suggest that root competition may be the primary mechanism determining competitive outcomes between water-seeded rice and E. phyllopogon . The importance of root competition and the relationship between root and shoot growth demonstrated in this study suggest that researchers should not rely solely on correlations between shoot traits and competitive ability as evidence that competition is primarily for light. Our results also suggest the importance of considering the whole plant when assessing rice cultivars for competitive ability.