of nitrogen and the N/O ratio
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Nitrogen is a key element to understand the evolution of galaxies with few star forming events since it needs relatively long timescales as well as relatively high underlying metallicity to be produced. The reason is that N is believed to be mostly a product of secondary nucleosynthesis, being produced by CNO processing of $^{12}$C or $^{16}$O from earlier generations of stars. However, a primary component can be obtained when the seed nuclei of $^{12}$C or $^{16}$O are produced in earlier helium burning stages of the same star. Generally, N is believed to be secondary in massive stars, and mostly secondary and probably partly primary in low and intermediate mass stars. However, some doubts exist at the moment on the amount of primary nitrogen which can be produced in intermediate mass stars due to the uncertainties related to the occurrence of the third dredge-up in asymptotic giant branch stars (AGB). In fact, if Blöcker and Schoenberner (1991) calculations are correct, the third dredge-up in massive AGB stars should not occur and therefore the amount of primary N produced in AGB stars should be strongly reduced (Renzini, private communication).

The only possible way to produce 