Tribulus terrester is an annual plant commonly found in the suburb of Ürümqi, Xinjiang, China. One fruit of this species is composed of five synaptospermies. This study aims to explore the morphological traits and intermittent germination characteristics of synaptospermy and further investigate the adaptability of its seeds under changing enviroment.

Shape, color and appendage of synaptospermy, and the mass, length of burs, percentage of appendage mass, number of seeds per synaptospermy, were determined; and the vigor, water absorption and germination characteristics of synaptospermies matured at different month were compared in the laboratory.

The synaptospermies became hardened after ripening, with scale-form appendages, and they are significantly different in mass, the percentage of mass of appendage, amount of seeds, vigor, water absorption rate, germination percentage and germination rate. The averages of vigor and germination percentage were increasing and the water absorption rate was decreasing following the order of maturing month in July, August and September. The average of vigor was positively correlated with the ripening time of fruit, as well as germination percentage, while the water absorption rate was negatively correlated with the ripening time of fruit. This suggests that the earlier the fruit ripening, the lower the vigor and germination rate and the higher the water absorption rate. Under the best germination conditions, all synaptospermies matured in September could germinate, whereas only some synaptospermies could germinate if matured in the other two months. Moreover, only those seeds located in the position of the long thorn of one synaptospermy could germinate. Part of the seeds could germinate in the season of ripening; the others retain in the dispersal unit and germinate after several weeks, months or years. This germination behavior was defined as intermittent germination. These results indicate that there is a change of gradient in morphology, vigor and water absorption among fruit ripening in different months and among different synaptospermy of the same fruit. We conclude the variation is a main factor responsible for intermittent germination, which can reduce competition among siblings; based on these strategies, Tribulus terrester can spread the risk of survival and ensure its breeding success.