QUINBV: MUTATIONS IN SORGHUMS 189 



Texas, in 1941. Several varieties are sensitive to photoperiod, sev- 

 eral are less sensitive, and one, Dwarf Broomcorn, is quite insensi- 

 tive. Although this point has not been established experimentally, 

 it can be inferred from work with other species that the thermal 

 requirements have not been met if a sorghum variety is not sensi- 

 tive to photoperiod. Likewise, it can be inferred that varying degrees 

 of sensitivity indicate that the thermal requirements have been 

 partially met. Insensitivity to photoperiod is undoubtedly the 

 mechanism that allows late maturity of varieties in the tropics. 

 In some tropical areas, sorghum is planted before a rainy season 

 of 3 or 4 months duration and the crop heads and matures after 

 the rainy season. Such a long duration of growth under 12-hour 

 days would not be possible with varieties sensitive to photoperiod. 

 Three or four maturity genes, allelic series at those loci, and vary- 

 ing degrees of insensitivity to photoperiod might well account for 

 all the maturities found in sorghum. 



Early floral initiation, low leaf number, and small plant size 

 are associated, as shown by data presented by Quinby and Karper 

 (25) and reproduced in Table 3. At the time these data were collected, 



Table 3. — Data Showing the Size of Four Milo Maturity Genotypes Grown 

 Under Normal Photoperiods from a Planting at Chillicothe, Texas, 



Made on June 20, 1944. 



Genotype 



Criteria 



No. of days to anthesis 

 No. of leaves 

 Height of plant, cm 

 Length of leaf, cm 

 Diameter of stalk, cm 

 Weight of head, lbs. 

 Weight of plant, lbs. 



Ryer milo maturity was not known which accounts for the fact 

 that a Ryer population was not measured. The data in Table 3 

 show that the duration of the vegetative period is positively cor- 

 related with plant size, the best measure of which is total dry weight 

 per plant. The figures show that a two-fold difference in size result- 



