Low Temperature‐induced Floret Sterility in Rice¹

Sterility of 296 rice breeding lines grown in the field at Davis, Calif., ranged from 3.6 to 96.8%. Mean sterility was 30.1% from an April 28 seeding and 35.2 for a May 7 seeding. A large part of the variability was caused by differential tolerance to cool night temperature at the young microspore stage of pollen development (10 to 15 days before heading). Higher sterility from the later seeding was probably associated with cooler temperature during this critical period. Growth chamber trials indicated that percent sterility increased in almost direct relation to cool‐temperature exposure. No difference was found between cool‐temperature treatments only during the dark and cool temperature during both dark and light so long as the total length of exposure was the same. Floret and pollen sterility were highly correlated, whereas cool‐temperature‐treated plants fertilized with pollen that was not cool treated were equal in sterility to self‐pollinated, cool‐temperature‐treated plants. Comparisons of plants exposed to various light and darkperiod temperatures showed least sterility at 28 to 24 C and most at 36 to 16 C. Proposed procedures for screening rice germplasm for tolerance to temperature extremes are discussed.