Some physiological aspects of evolution in wheat

Abstract

Twenty-one lines representing wild progenitors and cultivated wheats at the diploid, tetraploid, and hexaploid levels were compared during growth under favourable conditions (2l/l6°C, l6-hr days of high intensity natural light, and nonlimiting nutrient supply). Grain weight, which varied 20-fold between lines, increased with increase in ploidy and with the shift from wild to cultivated forms, particularly at the diploid level. The area of individual leaves and the total leaf area of seedlings were proportional to the weight of the grains sown. In turn there was a close relation between the area of the largest leaf on the main stem and the weight of the ear and of the individual grains it supported. Evolution in wheat has thus involved a parallel increase in leaf and grain size, but this has been coupled with a progressive reduction in the rate of photosynthesis per unit leaf area. At low light intensities there was little difference between species in their rate of photosynthesis, but light saturation was approached at lower intensities in the more advanced forms. Photorespiration was also higher in the primitive wheats. Total photosynthesis per flag leaf blade was greatest in the wild and cultivated tetraploids and the cultivated hexaploids, due to their greater leaf size. All wild species showed a rapid fall in flag leaf photosynthesis during grain development, whereas in many of the cultivated wheats the flag leaf rate rose during most rapid grain development. The rate of ear photosynthesis depended on the presence of awns and the number of spikelets, and was highest in the durum wheats and in Triticum dicoccoides. The duration and rate of grain growth has tended to increase during evolution in wheat. Ear photosynthesis was largely sufficient for grain needs in Aegilops speltoides, but the need to import carbohydrates from the leaves has increased in modern wheats, as shown by the increased movement of 14C-Iabelled assimilates to the ear from the flag leaf and from the leaf below it. The loss in dry weight from the stems is also greater in modern wheats. Movement of assimilates to the roots, stem, and tillers has been correspondingly reduced during evolution, and the proportion of the shoot weight in the grain at harvest has increased. © 1970 CSIRO.