Abstract
The genes Never ripe (Nr) and ripening inhibitor (rin) were introduced by backcross programmes into the tomato variety Ailsa Craig to give nearly isogenic lines. The effect of each of these genes on the major constituents of fullyโdeveloped tomato fruit has been studied as part of a programme to select breeding lines bearing fruit having prolonged shelfโlife. The Nr gene produced high acid, low sugar, slowโripening tomatoes that were very firm. Because of the adverse effect of this gene on tomato composition, even in heterozygous form, it has little or no commercial potential. The rin gene also led to the production of poor quality fruit. The heterozygote (Rin rin) gave fruit that were similar in compositon to that of the recurrent line, but despite possessing lower polygalacturonase activity were not significantly firmer. The distribution of isoenzymes in the normal and mutant tomato tissues was examined by gel electrophoresis. The majority of the enzymes investigated showed a diminished activity in Nr and rin fruit compared with normal. The very low activities of polygalacturonase, phosphofructokinase and NADP^+^โmalic enzyme in the fullyโdeveloped mutant lines offers an explanation for the weak climacteric respiration rise and slow ripening of Nr fruit and the nonโclimacteric behaviour of rin fruit.