Three hundred and thirty lines of guar (Cyamopsis tetragonoloba (L.) taub.) were planted in five locations throughout central and southwestern United States to find diverse photoperiod response types for closer physiological and genetic study. Dates of planting studies were done in 1982 and 1983 in hopes that the photoperiod responses would be obvious in field conditions but they were not. The 1982 dates of planting studies in Arizona, Kansas, and Texas indicated that the date of planting was more important than the selection of cultivar in expectations of high yield even though cultivar selection was very important. The 1983 dates of planting experiment in Tucson, Arizona showed suggestions that photoperiod existed in guar but it took controlled, greenhouse conditions to characterize photoperiodism in guar and to be able to conduct genetic analysis. In greenhouse studies, guar was found to be a quantitative short-day plant, the initiation of buds and floral development were accelerated under short-day conditions. Six guar lines were characterized for the critical photoperiod in days from first true leaf to the first floral bud and from first floral bud to the first flower. No effect of photoperiod on the growth and development from emergence to the first true leaf was observed. The critical photoperiod for days from first true leaf to first bud for the lines are as follows: PI217925-1-1, Mesa, and Mills are between 14 and 15 hours, Kinman and SEAH-90 are between 13 and 14 hours, and PI217925-2 is between 12 and 13 hours. The critical photoperiod for days from first floral bud to first flower for the lines are: PI217925-1-1, Mesa, Kinman, and PI217925-2 are between 12 and 13 hours, SEAH-90 is between 13 and 14 hours, and Mills is day-neutral. Different photoperiodic responses occur for days from first true leaf to first floral bud and days from first floral bud to first flower. This follows a proposed genetic system of photoperiodic actions that has genes for photoperiod sensitivity, short-day versus long-day reaction, critical photoperiod, and genes for the amount of time delay for each developmental stage. The segregations of the guar crosses were explained by the model.

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