Effects of climatic and edaphic factors on soybean flowers and on the subsequent attractiveness of the plants to honey bees

Soybean ( Glycine max) plants were grown at various day and night air temperatures, soil temperatures and soil concentrations of N, P and K, to investigate effects of environmental conditions on flower characteristics, including flower production, color intensity, openness, size, nectar secretion and aroma emanation and on attractiveness of the plants to honey bees. Most flower characteristics increased as day air temperatures at which plants were grown increased from 20 to 24°C and reached maximum values at 28°C before plateauing or declining at 32°C, although flower size and nectar secretion continued to increase as growing temperature increased to 32°C. Of two flower aroma components, emanation of one component increased while the other decreased with increases in growing temperatures. The hypothesis suggested is that the two aroma chemicals may communicate flower-readiness information to pollinators. Flower production and flower openness responded linearly to night air temperature at which plants were grown, attaining highest values at higher (22, 26°C) vs. lower (14, 18°C) temperatures. Flower production also responded linearly to soil temperature, attaining highest values at higher (28–32°C) vs. lower (16–20°C) temperatures. Of two levels each of N (75 and 175 ppm) and P (15 and 30 ppm) tested, the higher level of N stimulated greater flower production, flower size and nectar secretion while the higher level of P decreased the same three flower characteristics. Conversely, lower N and higher P promoted flower openness. Honey bee attractiveness of plants varied positively with flower characteristics such that plants grown at a day air temperature of 28°C, night air temperatures of 22 and 26°C, the higher level of N of the lower level of P were the most attractive to honey bees.