Seasonal germination responses of seeds in fruits of the cold desert shrub Zygophyllum xanthoxylon buried in the field

We hypothesized that seeds of the central Asian cold desert shrub Zygophyllum xanthoxylon exhibit dormancy cycling in nature, which would help ensure that germination occurs at a favorable time for seedling survival. Fresh mature fruits of Z. xanthoxylon were placed on the soil surface and buried at a depth of 3–5 cm in soil in a natural habitat in August 2015. At regular intervals until July 2017, fruits/seeds were taken to the laboratory and evaluated for viability and germination. Fresh seeds in fruits germinated to 22–62% over a range of temperatures. After 6 weeks in the field, seeds on the surface germinated to 72–99% and those in soil to 70–100%. From 12 to 98 weeks, seeds on and in soil germinated to 72–100% at low (15/5 and 20/10 °C) temperatures, but germination at high (25/25 and 30/15 °C) temperatures was higher in summer than in spring. Seeds in fruits on the soil surface had higher germination percentages than those in the soil, especially at high temperatures. Although about half the seeds were dormant at maturity, they became nondormant in the field during summer. There was a decrease in ability to germinate at high temperatures in spring, especially for seeds in buried fruits, which indicates conditional dormancy/nondormancy cycling. Lack of germination at high temperatures in spring would help delay germination until the onset of the summer rainy season in the desert. Burial of fruits in the soil enhances the formation of a short-lived persistent seed bank.