MECHANICAL AND OPTICAL ASSESSMENT OF THE RIPENING OF TOMATO FRUIT WITH REDUCED POLYGALACTURONASE ACTIVITY

The ripening characteristics of modified tomatoes (Lycopersicon escule ntum Mill cv Ailsa Craig), which express antisense RNA to polygalactur onase (PG) and thus have very low activity of this enzyme, were compar ed with control fruit. Previous studies of these fruits showed that al though PG activity was reduced to approximately 1% of that in untransf ormed tomatoes, this reduction had no effect on softening. Further det ailed mechanical assessments have now been performed which revealed sm all, but significant differences in the fruit ripening characteristics between control and antisense fruits. Compression along the polar axi s of the PG antisense fruit was significantly reduced relative to the control. Deformation at failure (h), 'modulus' (M), coefficient of com pression (K-c), and coefficient of shear (K-s) 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 q uite limited in extent, as compared to the normal evolution observed d uring ripening. Scanning electron microscope studies on the sub-exocar pic region of the pericarp showed that cell wall separation was reduce d in the ripe antisense tomatoes, and this may explain the change in m echanical properties. Optical measurements made with both Hunter Color Difference (Hunter Lab Ltd, Fairfax, VA, USA) and Micromatch 2000 (IC S Texicon Ltd, Altrincham, UK) spectrophotometer systems showed that t he 'a' (green/red) component of colour of the antisense tomatoes was i ncreased relative to that of the control samples. The effect of PG on softening and of cell wall breakdown on mechanical and optical propert ies is also considered.