To determine more precisely the threshold temperature that triggers the damage we observed at 45◦C air temperature, the phenotypes of the null and T-Oa-19 line were determined for plants grown to maturity after exposure to daily maxima of 28, 40, and 45◦C prior to anthesis. For each of the two genotypes, above ground fresh mass(FM)at final harvest was similar whether they were grown at 28 or 40◦C(Figure5),even though there was a small but significant effect of Rca-Oa expression on biomass in the 28◦C control. However, at 45◦C FM declined to below 200 g in both genotypes, reflecting a much-restricted rate of carbon gain. This was more pronounced for the null segregant, with T-Oa-19 accumulating significantly more biomass than the null segregant after 45◦C maxima during the vegetative phase (t = 4.6, p < 0.001).