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The rate of internodal extension of chrysanthemum (Dendranthema grandiflora Tzvelev. cv. Envy) under various temperature and photoperiod conditions was studied to determine whether reproducible diurnal patterns of growth existed and whether any such patterns conformed to an endogenous circadian rhythm. Stem growth was monitored continuously by means of linear displacement voltage transducers. At constant temperature and under 11 h light/13 h dark photoperiod, stem elongation followed a clearly defined pattern consisting of a peak in rate immediately after the dark to light transition and then a gradual decline until the start of the dark period. During darkness, elongation rate increased and reached a maximum approximately 8 hours after the light to dark transition. This pattern differed when light period temperature was either above or below dark period temperature, but these patterns were also highly reproducible. When plants were subjected to continuous light at constant temperature, the rhythm of stem elongation initially showed a periodicity of approximately 27 hours. After 2 or 3 diurnal cycles the rhythm was less distinct and the rate became essentially constant. Furthermore, the interruption of a long period of continuous light with a 13 h dark period did not restore the rhythm. These findings do not support the existence of an endogenous circadian rhythm of stem elongation. Diurnally-cued rhythms do, however, exist and can be modified by temperature.
The rate of internodal extension of chrysanthemum (Dendranthema grandiflora Tzvelev. cv. Envy) under various temperature and photoperiod conditions was studied to determine whether reproducible diurnal patterns of growth existed and whether any such patterns conformed to an endogenous circadian rhythm. Stem growth was monitored continuously by means of linear displacement voltage transducers. At constant temperature and under 11 h light/13 h dark photoperiod, stem elongation followed a clearly defined pattern consisting of a peak in rate immediately after the dark to light transition and then a gradual decline until the start of the dark period. During darkness, elongation rate increased and reached a maximum approximately 8 hours after the light to dark transition. This pattern differed when light period temperature was either above or below dark period temperature, but these patterns were also highly reproducible. When plants were subjected to continuous light at constant temperature, the rhythm of stem elongation initially showed a periodicity of approximately 27 hours. After 2 or 3 diurnal cycles the rhythm was less distinct and the rate became essentially constant. Furthermore, the interruption of a long period of continuous light with a 13 h dark period did not restore the rhythm. These findings do not support the existence of an endogenous circadian rhythm of stem elongation. Diurnally-cued rhythms do, however, exist and can be modified by temperature.
Stem elongation rate (SER) in Dendranthema grandiflorum (Ramat.) Kitamura was determined in light and in darkness under various temperature regimes. Stem growth as measured with linear voltage displacement transducers on plants in growth chambers. Under alternating 11-hour days and 13-hour nights, SER was strongly temperature dependent and showed patterns that were characteristic of the particular photoperiod-temperature regime under which the plants were grown. Total daily elongation was similar at constant 18.3C and at 11.5C days and 24C nights, but was much greater at 25.7C days and 12C nights. SER was rhythmic in continuous light with a period of slightly less than 24 hours. In continuous darkness, however, SER declined rapidly and the rhythm disappeared within 11 hours. Low-temperature pulses (a rapid decline from 18.3C to 8.3C) applied for 2, 4, 6, 8, or 11 hours during the day induced an immediate decline in SER followed by a slow recovery and peak shortly after the end of the pulse. Total diurnal stem growth declined with increasing pulse length, although short (2-hour) duration pulses apparently had little effect on growth. The results are discussed in relation to the influence of day and night temperature differentials (DIF) on stem growth in Dendranthema.