EFFECTS OF ELEVATED CO2 AND TEMPERATURE ON DEVELOPMENT IN SOYBEAN AND5 WEEDS

Developmental rates of soybean [Glycine max (L.) Merr. 'Braxton'],john songrass [Sorghum halepense (L.) Pers.], quackgrass [Elytrigia repens (L.) Nevski], redroot pigweed (Amaranthus retroflexus L.), sicklepod ( Cassia obtusifolia L.), and velvetleaf (Abutilon theophrasti Medic.) w ere compared among plants grown in all combinations of two temperature levels (avg. day/night of 26/19 degrees C and 30/23 degrees C) and tw o CO2 levels (350 and 700 ppm). Neither temperature nor CO2 affected j ohnsongrass tillering rate, but plants began tillering earlier at high er temperatures. Adverse effects of higher temperatures on quackgrass development were alleviated by elevated CO2 conditions. Plastochron ra te was higher at higher temperatures in all dicot species (soybean, re droot pigweed, sicklepod, and velvetleaf), and was higher at elevated CO2 in all dicots except velvetleaf. Calculating plastochron rates on a degree day basis removed differences between temperature treatments, but did not affect responses to CO2. Responses of dicot branch and br anch leaf production to treatments varied among species. Branch produc tion per day increased with higher temperatures in redroot pigweed, de creased with higher temperatures in sicklepod, and was unaffected by t emperature in soybean. The relationship between main axis and branch d evelopmental rates was altered by temperature in soybean, and by both temperature and CO2 in sicklepod, but was unaffected by either treatme nt in redroot pigweed. These results indicate that developmental respo nses to temperature and CO2 depend on both the species and the aspect of development being considered.