COOL TEMPERATURE-INDUCED STERILITY IN SPRING WHEAT (TRITICUM-AESTIVUML.) AT HIGH-ALTITUDES IN NEPAL - VARIATION AMONG CULTIVARS IN RESPONSE TO SOWING DATE

Cold stresses during the reproductive development of spring wheat (Tri ticum aestivum L.) cause grain-set failure in the high altitudes (> 15 00 m) of Nepal and other similar parts of the world. Field experiments were conducted during the winter seasons (October to April) of 1992/1 993 and 1993/1994 at the Lumle Agricultural Research Center (1675 m) i n Nepal to study the relations between temperature, phenology and grai n set of spring wheat. Six cultivars of differing response to cool tem perature were planted at 7-d intervals on six dates from 11 October to 15 November in both winters. The effect of temperature was evident on the times taken to heading, from heading to anthesis, total crop dura tion, grain set and grain yield. There was a seasonal difference in th e extent of cold temperature: mean sterility was higher in 1992/1993 ( 19%) than in 1993/1994 (16%). Cool (< 10 degrees C) temperature around heading prolonged the time to anthesis and led to greater degree of s terility in cold-susceptible cultivars. The cooler it was between head ing and anthesis, the longer was the period between them and more grai ns failed to set. This period was longer when the crop was sown earlie r, or when the crop faced a cooler period at and after heading and if the cultivars were cold susceptible. All the early heading cultivars s tudied (i.e., RR-21, BL-1022, BL-1135 and BL-1066) were cold susceptib le and had significantly higher sterility than the late cultivars (NL- 582 and Annapurna-3). The late-heading cultivars not only escaped the cool period, but also had inherent cold tolerance: the rate of develop ment from heading to anthesis was rapid even under comparably cool tem peratures. The reduction in grain set induced by cold stress also vari ed widely between cultivars and within a cultivar over sowing dates. S terility was significantly greater (26%) in 11 October-planted crops f ollowed by 18 October (22%), and reduced to 13-15% in the other later dates of sowing, which were not significant. If the period from headin g to anthesis occurred in warm temperatures, the crop did not suffer m uch even if the pre-ear emergence stage of reproductive development co incided with extreme cold suggesting that the period from around headi ng until anthesis is critically sensitive to cold temperature. Mean te mperatures during the period between heading to anthesis were more cri tical than minimum temperature as a cause of grain-set failure in cold -susceptible cultivars. (C) 1998 Elsevier Science B.V.