Objective The study was conducted to investigate the development laws of in-season and off-season fruits of pitaya and to set up growth models, which would provide theoretical basis and guidance for high-quality and high-efficient fruit production.

Method Dahong pitaya, the main domestic cultivar, was selected as test material. The vertical diameter (VD) and horizontal diameter (HD) of in-season and off-season fruits were measured at regular intervals during fruit development. The temperature and day length were recorded from flowering to maturing every day. Logistic growth equations of fruit development were fitted.

Result The development of in-season fruits completed in 28 days. The VD and HD of in-season fruits grew faster than off-season fruits per day. In-season fruits were oblong and much smaller. The mean values of daily lowest temperature (DLT), daily average temperature (DAT), daily highest temperature (DHT), day length (DL) and daily temperature range (DTR) were higher than those of off-season fruits. The development of off-season fruits completed in 63 days. The fruit was nearly spherical and larger. The accumulations of DTR, DL, DHT and DAT were higher than those of in-season fruits. Growth curves of both fruits were in S shape. Inflexion point of VD curves of both fruits and HD curve of in-season fruits appeared before flower wilting. HD of off-season fruits was still growing faster and faster until the inflexion point appeared in 9-12 days after flowering, And the growth rate decreased more and more after inflexion point. VD and HD of ripened off-season fruits were 1.03 and 1.25 times those of in-season fruits, respectively.

Conclusion Higher growth rate of in-season fruit may be relevant to higher means of temperature and day length in the course of fruit development. Off-season fruit was larger, which may be relevant to higher accumulations of temperature and day length. The full development of HD of off-season fruit was and one of the direct factors causing differences in fruit shape of the two fruits. Single factor and multi-factor models of fruit development were fitted well, which could be used for analysis and prediction of fruit development.