276 CONTROL OF REPRODUCTION 



Table I. Mean Developmental Responses of Six Soyliean Varieties Planted on 

 Four Different Dates and Subjected to Three Different Photoperiodic Schedules 



Period from Planting Period from Planting 



to Flowering on to Pod Maturity on 



Indicated Photoperiod, Days" Indicated Photoperiod, Days 



Planting 



Date 12 3 12 3 



Mav23 62 58 54 146 141 138 



June 6 54 50 48 135 132 124 



June 20 51 46 44 127 124 118 



July 4 45 41 40 119 116 113 



" Photoperiods were changed at 2-day intervals to simulate the seasonal change in day 

 length at latitudes of 39°N for photoperiod 1, 36°40'N for photoperiod 2, and 34°20'N_for 

 photoperiod 3. The calculated photoperiod for each latitude and date included the period 

 from sunrise to sunset as tabulated in the American Nautical Almanac plus 20.9% of the 

 morning and evening twilight periods. 



progressed. The photoperiodic control mechanism is thus very sensitive 

 and delicately balanced. 



Action Spectra 



A discussion of a plant's mechanism for the control of flowering can 

 best start with a summary of information obtained from action spectra 

 (Fig. 1). Action spectra for control of flowering were measured for 

 two short-day plants, cocklebur (Xanthium pensylvanicum Wallr.) 

 and soybean {Glycine max var. Biloxi) (Parker et al, 1946), and 

 for two long-day ones, barley {Hordeum vulgare L.) (Borthwick et 

 al, 1948) and henbane (Hyoscyamus nigerh.) (Farker et al., 1950). 

 Important features of these action spectra are as follows: ( 1 ) They are 

 closely similar from about 5800 A to about 7000 A (Borthwick et al., 

 1948, 1950) ; (2) the action of radiant energy of this spectral region is 

 reversed by that of the so-called far-red region, which extends roughly 

 from 7000 to 7600 A (Hendricks and Borthwick, 1955); and (3) 

 they are very similar to the action spectra of a number of other plant 

 responses such as germination of light-sensitive seeds (Borthwick et 

 al., 1952, 1954), elongation of internodes (Downs et al., 1957), and 

 pigmentation of parts of fruits (Piringer and Heinze, 1954). 



The region of maximum effectiveness for the so-called red reaction 

 is centered near 6500 A and is broad. Maximum effectiveness for the 

 reverse reaction is centered near 7300 A. The nearness of the peaks of 



