Australian wild oryza species: growth, canopy structure and atmospheric CO2 effects

The canopy structure of O. sativa and the Australian wild Oryza relatives, O. australiensis and O. meridionalis, were compared developmentally and with digital models. Species differences were found in average leaf angle, self-shading, leaf dispersion and leaf area index (LAI). There was no advantage of the near vertical leaf angles that are associated with high leaf area index in O. sativa and therefore, O. sativa did not have superior light interception efficiency (LIE). There was also no clear difference between the species in light-saturated photosynthetic rates; all species had higher rates in the vegetative than the reproductive phase of growth. Elevated CO2 enhanced photosynthesis, accelerated development of total leaf area, shoot and root weights, tiller number and plant height during vegetative development but at the reproductive stage of development, there were no longer any significant species differences. Oryza meridionalis produced the greatest biomass, with substantial variation between O. meridionalis accessions collected from an arc across tropical Australia. Oryza sativa produced more grain and had much higher harvest indices than O. meridionalis or O. australiensis. It was concluded that the wild Oryza relatives were as efficient as O. sativa in photosynthetic carbon fixation. However, radical re-design of O. sativa canopies to resemble the wild Oryza archetypes might confer advantages in heat dispersal or efficient water use.