In order to explore the light and heat use characteristics of crops in continuous drop frost period of later autumn and increase resource utilization thresholds of the season

LI-6400 portable photosynthesis system was used to monitor photosynthetic characteristics during rootstock growing stage of beets

kohlrabi and mustard in cold and arid North China. The limiting factors affecting the daily change of Pn(photosynthetic rate) of three crops were analyzed combined with the changes of Tr(transpiration rate) and WUE(water use efficiency). The results showed that the daily changes of Pn of three crops in continuous drop frost period were bimodal curves. Beets possessed the ability of wide region use of light radiation in quantity and quality

and its daily accumulative dry mass were 1.13 and 1.54 times than that of kohlrabi and mustard

respectively; The daily accumulative transpiration amount of beets were 1.06 and 1.25 times than that of kohlrabi and mustard

respectively

indicating that beets had the highest average daily WUE. The diurnal variation of net photosynthetic rate of three crops exist a midday "Pn inefficient" period

and the reasons may be stomata partial closure caused by leaf water loss and low photosynthetic activity of mesophyll cell. Compared with kohlrabi and mustard

Tr daily variation of beets showed a character of postponed rapidly rising stage

higher at bottom keeping stage and delayed descending stage

which make it to be the dominating crop with higher light

heat and water use efficiency

and had higher productive potential in continuous drop frost period.