Comparative responses of soybean (Glycine max), sicklepod (Senna obtusifolia), and Palmer amaranth (Amaranthus palmeri) to root zone and aerial temperatures

Experiments were conducted to compare germination efficiencies and vegetati ve growth of soybean and the competing weed species, sicklepod and Palmer a maranth, over a range of temperatures in the root zone and aerial environme nts. From genetic origins we hypothesized that the weeds would have a highe r temperature optimum, which would help explain competitive interactions se en in the southeastern U.S. Germination experiments indicated that germinat ion efficiency of the weeds was much more sensitive to low temperature than soybean, being markedly inhibited below 18 C. Similarly, experiments in an automated, temperature-controlled hydroponic system revealed that the weed species were less tolerant of low root zone temperature but more tolerant of high root zone temperature than soybean. At 16 C, dry weight of soybean was 74% of the control dry weight at 24 C, whereas dry weights of sicklepod and Palmer amaranth were 5 and 20% of the control, respectively. At 32 C, soybean root dry weight was only 80% of the 24 C treatment, whereas root dr y weight of the weed species was not significantly different. When plants w ere grown at a low aerial temperature, growth of all plants was strongly in hibited, but the negative effects were somewhat more severe in the weed spe cies than with soybean. An increase in aerial temperature from 26/22 C to 3 4/30 C (day/night) had a positive influence on dry matter accumulation of t he weed species, stimulating sicklepod 150 to 200% and Palmer amaranth 150 to 1,600% compared to their respective controls, whereas soybean remained a t about 80 to 90% of the control. All species grew taller with increasing t emperature. Leaf area of the weeds increased but leaf area of soybean did n ot increase. Consistent with our original hypothesis, the results clearly s how that the weeds, which originate from warm geographical regions, respond more negatively than soybean to low temperatures in the growth environment but more positively to high temperatures. The temperature characteristics help to explain why the intensity of weed pressure increases as the soybean growing season progresses, even after canopy closure.