Wheat leaf structure and function: the role of cell size, shape, and patterning across Triticum evolution

Gas flux into leaves for photosynthesis occurs through small pores on the surface called stomata. Subsequent gas movement to the photosynthetic cells relies on porous mesophyll tissue. However, maximising the density of stomata and porosity of the airspace to improve the rate of gas flow for photosynthesis increases the amount of water lost. Therefore, there is a trade-off of maximising photosynthetic rate while keeping water loss to a minimum. The effect of leaf structure in Triticum (wheat) species on this trade-off is the subject of this thesis, with a focus on how this has changed during the breeding of modern hexaploid wheat from lower ploidy progenitors.

At the level of whole leaf structure, the Triticum mesophyll has traditionally been considered to be homogenous. Here I show that in fact it comprises four distinct cell layers. This patterning is conserved across all Triticum ploidy levels and cultivation statuses, suggesting an evolutionary advantage.

Focussing on tetraploid wheat varieties, I show that that modern cultivated varieties have larger stomatal, pavement and mesophyll cells, fewer stomata and are more water-use efficient than wild, landrace varieties, suggesting that water-use efficiency has been a driver during the selection of modern tetraploid wheat.

Finally, to test the hypothesis that increased cell size is linked to improved water-use efficiency, transgenic hexaploid wheat overexpressing a construct expected to modify cell size were analysed. This resulted in improved water-use efficiency, but the underlying changes in cellular architecture were complicated, with the major influence on gas exchange probably linked to altered pavement traits rather than the mesophyll.

The work in this thesis provides insight into the impact of leaf structure on water-use efficiency during the breeding of modern wheat which has been superimposed on an ancient underlying mesophyll cellular architecture.