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.