Haloxylon ammodendron(C. A. Mey.) is one of the economically and ecologically important desert trees used for sand fixation. The ovary of H. ammodendron is found not to swell after flowering in spring until at the end of August or early September in western China. However, what happens for ovary at anatomic level in that period and which crucial ecological factor regulates the phenomenon of H. ammodendron have not been fully understood. To characterize the phenomenon and explore the crucial environmental regulating factors, we carried out the morphological and anatomic observations at the different development stages of the fruits and three single-factor experiments(low air temperature, sufficient soil moisture, and short day length). Our results showed that under the natural conditions, the ovary of H. ammodendron after flowering developed slowly and the morphological changes of fruits were not significant for the period from May to August and after late August or early September; and then the ovary developed rapidly and matured in October. Cell division in embryo was observed to start approximately 25 days after flowering(DAF) and just developed to globular embryo stage at mid-August. Photoperiod was identified as the pivotal environmental factor regulating the fruit development of H. ammodendron. Moreover, the threshold value of day length for the fruit development was 14.0 h. A long day(>14.0 h) treatment began from 5 DAF could delay fruit development of H. ammodendron while a short day(<14.0 h) treatment could accelerate it. Moreover, a further longer day treatment(>15.0 h) could also delay fruit development even when they had developed for a long time(110 DAF). The present study indicated that H. ammodendron adopted a reproductive strategy of delayed fruit development and this strategy helps it survive and obtain offspring in harsh desert habitats.
