Do you know any specific/unique seed morphology (like color, size etc.) for these 4 varieties of Gleditsia triancanthos, so you can identify them? Or do you know any specific/unique DNA markers for them, so you can identify them through molecular technology?

Different levels of water stress may be your treatments. You can use CRD or RBD design for doing the experiment and accordingly you can go for ANOVA. Moreover, if you want to add, different varieties also for comparison, you can go for factorial experimenter with either CRD or RBD depending upon the situation.