Inhibitory effects on flowering of chrysanthemums caused by far-red light at the beginning of certain inductive long nights were investigated. Such effects were found only under photoperiods shorter than those that occur naturally, even in midwinter, in latitudes where chrysanthemums are produced commercially. The results, therefore, do not apply to greenhouse or field practice but contribute to a better understanding of the control of flowering by light. These results are explained in terms of control by phytochrome. The active form of phytochrome (Pfr) is present during, and at the close of, every photoperiod. During the photoperiod it performs a flower-promoting function. If the photoperiod is too short, the promotive function must be completed by the Pfr left in the plant at the beginning of the dark period. In such a case, the promotive action appears to be low during the early hours of darkness, but reaches a climax after about 6 hr. Simultaneously with completion of its promotive action in darkness, Pfr decreases to a very low level, and it must remain at this low level during the remainder of the dark period for flowering to occur. Thus, far-red light at the beginning of darkness removes Pfr before the latter completes an essential promotive function; and red light, at a later hour of darkness, reintroduces Pfr at a time when its presence inhibits flowering.
The germination of Kentucky bluegrass, (Poa pratensis L.), seeds is profoundly influenced by two light reactions. One, the phytochrome reaction (P), is promotive, and the other, the so-called “high-energy reaction (HER)”, is inhibitory to germination. The level of germination displayed in 14 days as the resultant of these two opposing reactions is appreciably influenced by temperature.
In darkness, the seeds germinate well at certain temperature alternations but not at constant temperatures. At 15-25°C, the promotive effects of temperature alternation are accomplished in the first 5 to 6 cycles. The promotive effects of alternations are displayed in darkness when the daily period at 25° is between 4 and 14 hours.
Brief daily high-intensity fluorescent illuminances (approximately 4,000 ft-c) during otherwise continuous darkness at constant 20°C induce high germination in most lots. Continuous medium-to-high intensity illumination (approximately 1,200 ft-c) very weakly promotes germination and in potentially promoted seeds inhibits germination to about the level of the dark controls.
Inhibitory effects of continuous light on potentially promoted seeds are best displayed at 20°C constant although, in ‘Newport’ they are observable at 15-25°. The inhibition of ‘Newport’ at 15-25° is to a level below that of the dark controls but above that caused by prolonged illumination at 20°.
Thus conditions most promotive to germination are 15-25°C alternations and brief daily illuminances of high intensity. Simultaneous application of these 2 conditions causes higher germination than when either is applied alone and in one lot of ‘Newport’ the effects are strikingly additive.