Investigating the structure/function relationships of leaves in wheat and its wild relatives

In order to carry out photosynthesis, plants rely upon the exchange of CO2 and water vapour. This is limited by pores on the epidermis (stomata), the internal arrangement of cells within the leaf, and the intercellular airspace network between the cells, through which gas must pass to the chloroplasts. How these leaf structures are co-ordinated, and the extent to which they individually influence photosynthesis, is open to debate and is the subject of this thesis. Using modern bread wheat (Triticum aestivum), a hexaploid plant derived from tetraploid and diploid relatives, as the experimental organism, I first show using gas exchange analysis that diploid lines have the highest stomatal conductance (gs) and lowest intrinsic water use efficiency (iWUE), whereas hexaploid lines have the lowest gs and highest iWUE. These differences are linked to changes in stomatal size and density driven by changes in cell size linked to ploidy.
When the relationship between wheat gs and leaf airspace was investigated using X-ray micro-computed tomography, a positive correlation was found. Analysis of differentiation patterns in the epidermis and subtending mesophyll provided evidence supporting a mechanistic link between gs and airspace, indicating that functional stomata are required for the formation of intercellular airspaces within the leaf. Finally, I report on the development of a 3D imaging protocol using confocal microscopy to extract wheat mesophyll cell geometry. This allowed the elucidation of a link between mesophyll cell size/shape and gas exchange, leading to the hypothesis that the evolution of higher ploidy levels in wheat may have been linked to gas exchange.
The work reported here provides a new insight into the relationship of stomatal function and leaf differentiation, and supports the idea that leaf water use rather than CO2 assimilation may have been a driver in the selection of one of our major food crops.