Highly precise temperatures (sigma=10-15K) have been determined from line depth ratios for a set of 248 F-K field dwarfs of about solar metallicity (-0.5<[Fe/H]<+0.4), based on high resolution (R=42000), high S/N echelle spectra. A new gap has been discovered in the distribution of stars on the Main Sequence in the temperature range 5560 to 5610K. This gap coincides with a jump in the microturbulent velocity Vt and the well-known Li depression near 5600K in field dwarfs and open clusters. As the principal cause of the observed discontinuities in stellar properties we propose the penetration of the convective zone into the inner layers of stars slightly less massive than the Sun and related to it, a change in the temperature gradient.