Relative growth and photosynthetic response of water-seeded rice and Echinochloa oryzoides (Ard.) Fritsch to shade

Echinochloa oryzoides is a serious weed of rice worldwide and one of the mo st important weeds of water-seeded rice in California, USA, where resistanc e to most available herbicides has been detected. Developing new integrated weed management strategies that include rice suppression of E. oryzoides g rowth requires understanding their relative responses to resource limitatio n. The effects of shade on the growth, morphology and CO2 exchange rates (C ER) of E. oryzoides and rice (cv. M-202) were assessed in glasshouse and gr owth chamber experiments. Plants were grown at 100, 50 or 18% of full sunli ght (glasshouse) or at 500 and 250 mu mol PPFD m(-2) s(-1) (growth chamber) within a completely randomized design with two factors and four replicates . Shade was imposed at 21 days after seeding (d.a.s.) (glasshouse) and at 1 4 d.a.s. (growth chamber). Height, leaf area, tillers, and leaf, stem, root and seed dry weights were determined for both species. CER were determined for plants in the growth chamber. Both species increased dry-matter alloca tion to leaves and maintained CER when subjected to low light. However, in contrast to rice, E. oryzoides maintained or increased leaf area, specific leaf area and leaf area ratio (LAR) in response to shade. E. oryzoides till er number and seed output were reduced only at 18% sunlight. Rice lacked su ch adaptive ability and tiller number and seed were reduced at 50% sunlight . Shade alone may not prevent the seed production of E. oryzoides. However, E. oryzoides increased LAR at the expense of allocation to roots. Thus, fo r substantial E. oryzoides control, early nutrient deprivation by rice root s may be as relevant as improving rice ability to intercept light.