Dramatic extensions of experimental possibilities (spacecraft RHESSI,CORONAS-F and others) in solar gamma-ray astronomy call for urgent, detailed theoretical consideration of a set of physical problems of solar activity and solarterrestrial relationships that earlier may have only been outlined. Here we undertake a theoretical analysis of issues related to the production of gamma-radiation in the processes of interactions of energetic (accelerated) heavy and middle nuclei with the nuclei of the solar atmosphere (the so-called i-j interactions). We also make an estimate of the contribution of these interactions to the formation of nuclear and isotopic abundances of the solar atmosphere in the range of light and rare elements. The analysis is carried out for solar flares in the wide range of their intensities. We compare our theoretical estimates with RHESSI observations for the flare of 2002 July 23. It was shown that the 24Mg gamma-ray emission in this event was produced by the newly generated Mg nuclei. With a high probability,the gamma-ray line emission of 28Si nuclei from this flare was generated by the same processes.