Solar neutrinos and grand unification

We consider the grand unification (GU) scenario for neutrino masses which is based on the seesaw mechanism with the mass of the heaviest right handed (RH) neutrino at the GU-scale: M-3 similar to Lambda(GU), and on the quark-lepton symmetry for fermions from the third generation. The scenario predicts for the light neutrinos m(3) similar to(2-4)x10(-3) eV and m(2) similar to(0.3-3)x10(-5) eV tin the case of a linear mass hierarchy of the RH neutrinos or/and in the presence of the Planck scale suppressed non-renormalizable operators). It also predicts large nu(e)-nu(mu) mixing: sin(2) 2 theta(e mu)greater than or similar to 0.2. In this scenario the solar neutrinos (nu.) undergo both the nu e-->nu(tau) resonance conversion in the Sun and substantial nu(2)-->nu(mu) vacuum oscillations on the way from the Sun to the Earth. The interplay of both effects enlarges the range of neutrino parameters which solves the nu.-problem. In particular, the nu(e)-nu(tau) mixing angle can be as small as the corresponding quark mixing: sin(2) theta(e tau)greater than or equal to(2-5) x10(-4). The scenario predicts a peculiar (oscillatory) distortion of the boron neutrino energy spectrum and seasonal variations of signals. Manifestations of these effects in the Super-Kamiokande and SNO experiments are studied.