Mechanical and optical assessment of the ripening of tomato fruit with reduced polygalacturonase activity

The ripening characteristics of modified tomatoes (Lycopersicon esculentum Mill cv Ailsa Craig), which express antisense RNA to polygalacturonase (PG) and thus have very low activity of this enzyme, were compared with control fruit. Previous studies of these fruits showed that although PG activity was reduced to approximately 1% of that in untransformed tomatoes, this reduction had no effect on softening. Further detailed mechanical assessments have now been performed which revealed small, but significant differences in the fruit ripening characteristics between control and antisense fruits. Compression along the polar axis of the PG antisense fruit was significantly reduced relative to the control. Deformation at failure (h), 'modulus' (M), coefficient of compression (Kc), and coefficient of shear (Ks) values from probe tests along the equatorial axis also indicated that the antisense fruit was firmer. Although these significant differences in texture properties were observed between the antisense fruit and the control, they were quite limited in extent, as compared to the normal evolution observed during ripening. Scanning electron microscope studies on the subexocarpic region of the pericarp showed that cell wall separation was reduced in the ripe antisense tomatoes, and this may explain the change in mechanical properties. Optical measurements made with both Hunter Color Difference (Hunter Lab Ltd, Fairfax, VA, USA) and Micromatch 2000 (ICS Texicon Ltd, Altrincham, UK) spectrophotometer systems showed that the 'a' (green/red) component of colour of the antisense tomatoes was increased relative to that of the control samples. The effect of PG on softening and of cell wall breakdown on mechanical and optical properties is also considered.