Physiological Activity of Awns in Isolines of Atlas Barley1

Exchange rates of CO2 for four isogenic lines of ‘Atlas’ barley (Hordeum vulgare L.), Full‐awned, Half‐awned, Quarter‐awned, and Awnless, were measured on intact and de‐awned spikes in light and darkness from anthcsis through 30 days after anthesis. Full‐awned spikes exhibited the greatest photosynthetic rate followed by Halfawned, Quarter‐awned, and Awnless, respectively. The awns of Full‐, Half‐, and Quarter‐awned spikes contributed an average of 90, 80, and 50%, respectively, of the intact spikes’ net photosynthetic rate throughout the measurement period. Reasons for the significantly greater contribution of increased awn length to net photosynthesis of the spike were investigated. Longer awns increased the area of chlorophyll‐containing tissues on a spike and possessed a greater number of stomata which thereby increased gaseous exchange. In addition to the effect on spike photosynthesis, longer awns increased the transpiration rate of the spike, thereby indicating that awns function in dissipating excess heat energy from spikes of barley plants. The rate of dry weight accumulation in intact spikes was much greater from anthesis through 15 days after anthesis than from 15 days after anthesis through spike maturity. This essentially corresponded to the period from anthesis through 12 days after anthesis when maximum spike photosynthetic rates were obtained for each of the four isolines.