Off the top of my head - heterosis. This can be further improved, if one of the parental components is sterile (particular case: self-fertile crops, like tomato or cucumber; however, the rule also applies for cross-fertile crops, with significant differences between the components and/or high inbreeding at the stage of crossing).
I attached 2 papers for your reference as you requested. Inside the papers, some technologies/ techniques use for studying and improvement of plants (not only vegetable plants though) are mentioned. Hope these papers help.