Photosynthesis and Transpiration in Leaves and Ears of Wheat and Barley Varieties

Carbon exchange rate (CER) and transpiration were measured in flag leaves, whole ears, glumes (referring to the total area of glumes and lemmas) and awns, in six hexaploid spring wheats (Triticum aestivum L.), three cultivated tetraploid spring wheats (T. turgidum L.), four wild tetraploid wheats (T. dicoccoides), eight six-rowed barleys (Hordeum vulgare L.) and five two-rowed barleys (H. vulgare L.). Differences between varieties and between species in total ear CER and transpiration were associated largely with differences in ear surface area rather than with rates per unit area. Rates of CER and transpiration per unit area of ears were 40–80% of those of flag leaves, depending on the species. However, since ear surface area was greater than flag leaf area by a factor of 1.1, 3.9, 5.5 and 4.4, in hexaploid wheat, tetraploid wheat, six-rowed barley, and two-rowed barley, respectively, total ear CER reached up to 90% of that of the flag leaf. The contribution of awns to total ear CER depended largely on total awn surface area per ear, rather than on CER per unit awn area. Awns contributed about 40–80% of total spike CER, depending on the species, but only 10–20% of spike transpiration. The disproportionately small contribution of awns to ear transpiration was caused by the very low rate of transpiration per unit area of awns. Thus, while transpiration ratio (CER/transpiration) was about the same in flag leaves and glumes, it was higher by several orders of magnitude in the awns. A large amount of awns in the ear is therefore a drought adaptive attribute in these cereals, for which tetraploid wheat exceeded hexaploid wheat and six-rowed barley exceeded two-rowed barley.