Canopy development of direct-seeded rice and some important grass and sedge weeds in response to water management

Leaf and stem distributions within plant canopies strongly affect plant growth and competition, but have been poorly studied in direct-seeded rice (Oryza sativa) and associated weeds. Importantly, agronomic management practices affect plant development, and water management is arguably the most important cultural practice in direct-seeded rice. Our objectives were to quantify the canopy development of rice and some important weeds in response to water management, and test the effects of those descriptions in DSRICE1, a rice–weed model of competition for light. Here we report results for rice (M-103; Japonica type), early watergrass (Echinochloaoryzoides), and ricefield bulrush (Scirpus mucronatus) in California, and for rice (IR72; Indica type) and Cyperus difformis (smallflower umbrellaplant) in the Philippines. Leaf and stem material were stratified over height. Leaf and stem drymass per meter per plant (g m−1 plant−1; LDH or SDH), plus leaf area per meter per plant (m2 m−1 plant−1; LAH) in the Philippines, were fit over fractional plant height by linear and nonlinear regression and evaluated using likelihood ratio tests. After standardization, no significant differences between rice LDHs and LAHs were detected, indicating that its drymass and area distributions were equivalent. Differences in LDHs and LAHs were detected for C. difformis, however, which was probably the result of having basal leaves. Shoot distributions were mostly consistent within each plant type, grass or sedge. Except for senescent early watergrass at 104 days after seeding (DAS), grass LDHs were best fit by a bottom-skewed function. Changes in skewness of LDHs corresponded with tillering dynamics in both grass species. LDHs for C. difformis and ricefield bulrush were conical, indicating a possibly large disadvantage competing for light. Grass SDHs were always conical, and early season stem height fractions (SF=stem height/plant height) of the grasses were about 0.5. Shoot distributions were very consistent across treatments, but water management modified competitive relationships via effects on plant heights. The model DSRICE1 was used to test the effects of changes in canopy specifications on rice–watergrass and rice–Ammannia spp. (redstem; canopy traits measured previously) competition in the California system. Canopy descriptions in DSRICE1 were changed singly and in combinations, and other tests used observed heights. Results were compared to baseline output. Competition predictions of rice and weed growth were much more dependent on canopy details than monoculture predictions, as expected. Rice–weed competition was most affected by LDH and SDH changes, while SF mediated SDH effects. Using observed heights for rice–watergrass competition further affected outcomes. Model changes were best understood by how they altered the dominance of one species over the other (e.g., watergrass over rice). Overall, results confirmed that shoot distributions were strongly related to plant architecture and leaf demography. Model analyses affirmed the fundamental role of shoot distributions in canopy light dynamics. The quantitative descriptions provided here are robust and should help standardize canopy specifications across rice–weed models.