Flower initiation in pasture legumes. IV. Flower initiation in Trifolium pratense L.

An early strain (Cowgrass) and a late-maturing strain (Montgomery) of red clover (Trifolium pratense L.) were grown at Melbourne, lat. 38°S., to study flower initiation and the relationship between initiation and flowering, because of the importance of flowering to the use of the species as a source of hay. The terminal inflorescence in red clover arises from a lateral bulge at the shoot apex, and the main shoot of the plant is strongly vegetative, only becoming reproductive in a proportion of plants under long photoperiod, particularly continuous light. In both strains, whether first-year or older plants, flower initiation and therefore flowering were confined to a short period of the year, the early strain having the longer period. Sowing the two strains at intervals through the year in the field, under natural and long photoperiods, showed the importance of both photoperiod and temperature to initiation. Both strains were hastened by a long photoperiod whatever the temperature, and initiated at a similar low leaf number, but the early strain was able to initiate at a shorter photoperiod than the late one. Seed vernalization had almost no effect; but at photoperiods of 14 to 11 hr, high temperatures strongly retarded Cowgrass and appeared to prevent initiation in Montgomery. The capacity of Cowgrass to initiate at a shorter photoperiod and at higher temperatures than Montgomery explains its longer flowering season in temperate latitudes. These genetic differences suggest that an appropriate environment can be used to differentiate early and late-flowering strains for seed certification, and that late strains would stay vegetative at low latitudes and would therefore be less useful as a source of hay than early ones.