Effects of High Temperatures during Panicle Differentiation with Interaction of Lime Application on Naturally Induced Straighthead of Rice Under Field Conditions.

Straighthead is a physiological disorder that causes floret sterility and poor grain development in rice which results in yield loss. It represents a potential threat to US rice production because of its unknown casual factors and susceptibility of widely grown cultivars in southern major rice producing region. As increasing temperature can cause floret sterility in rice, it was essential to investigate the possible effects of higher panicle differentiation temperature on straighthead.  Areas of Pine Bluff fall in the region of Arkansas with very hot weather. There were about 29 days with daily highest temperature over 33⁰ C between July and August in 2015 which coincides with the panicle differentiation period of most varieties of rice. Straighthead of rice will be increased if rice meets a high temperature during the panicle differentiation period. Forty-four inbred and breeding lines were experimented to investigate the effect of temperature during panicle differentiation with the interaction of calcium application on rice straighthead. Fields treated as control 0-Ca (0 tones lime/acre), Ca_I (2 tones lime/acre), and Ca_II (4 tones lime/acre) differed in the lime application rates.  Individual panicles of varieties for each treatment were randomly tagged for varieties in each treatment plot to record the date of heading so they can be inferred to the panicle differentiation time. Panicle differentiation temperatures were recorded 21 days prior to heading date because in rice panicle differentiation occurs 20 – 25 days before panicle heading. Daily temperature was collected from NRCS Scan Site sensors which is about 50 meters away from the field. The data showed that high temperature during the panicle differentiation period can be an attributing factor to straighthead and effect of increasing panicle differentiation temperature can be reduced with lime application at some level.  Varietal response to lime application was different as susceptible varieties showed higher response than moderately susceptible and resistant varieties. Straighthead severity was also analyzed and conferred with soil temperatures at various depths including 2 inches, 4 inches, 8 inches and 20 inches levels. This analysis was also done with soil temperatures during panicle differentiation stage.