He worked on breeding improved varieties of forage grasses and grains, including oats and barley.
His work moved into research on photosynthesis, effects of temperature on grasses and also developing an understanding of the differences in biochemistry and lipid metabolism as leaves died.
[7] Initially using classical genetics methods but later molecular genetics, Thomas and his collaborators identified a gene, Sid ((senescence-induced degradation), the protein product of which stabilised the pigment-protein-lipid complexes of chloroplasts so that dying leaves remained green.
Later, collaborating with researchers in Switzerland and the USA using molecular genetics, functional analysis and cell biology in pea, Arabidopsis, rice and Festuca pratensis, the researchers showed that this gene was one that Gregor Mendel recorded in 1866 that resulted in green or yellow cotyledons.
He participated in the Hay Literary Festival in 2013 in a panel discussion about Shakespeare and sustainability with English scholars.