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Photoperiod-Induced Dormancy in Cicer Milkvetch

USDA-ARS Crops Res. Lab., 1701 Center Ave., Fort Collins, CO 80526.

^* Corresponding author.

Cicer milkvetch (Astragalus cicer L.) does not produce as much forage as alfalfa (Medicago saliva L.) under field conditions during the third growth period, which ranges from about 1 August to 15 September in Colorado. This study examined the effect of decreasing photoperiod on the herbage growth of cicer milkvetch. Spaced plants of the component polycross progenies of the cultivar Monarch were grown in the field under irrigation on a Nunn clay loam soil (fine, montmorillonitic, mesic Aridic Argiustoll). These progenies were subjected to one of four clipping treatments, removing the top growth on 18 July, 25 July, 1 August, or 8 August. Substantial variability was detected among plants within each clipping treatment date for extended height, which suggests that photoperiod was the primary factor controlling plant growth. Therefore, growth chamber studies were designed that exposed polycross progenies of selected germ plasm and Monarch to a constant 15-h photoperiod and a gradually decreasing photoperiod equivalent to the natural daylength at Fort Collins, CO, from 1 August through 15 September. Plants under the decreasing photoperiod treatment were 23% shorter and produced 25% less shoot dry matter than those under the constant photoperiod. Under the decreasing photoperiod there were also fewer flowering plants and fewer racemes per plant than under the 15-h photoperioil. Considerable variability for extended height and shoot dry weight was detected among plants under both photoperiods. These studies demonstrated the presence of a photoperiod-induced dormancy trait in cicer milkvetch.

Received for publication April 12, 1993.