Photoperiodic Time Measurement | Plant Physiology

The following points highlight the two main approaches of photoperiodic time measurement.

Approach # 1. The Conception of Physiological Clock of Erwin Bunning:

This is also known as biological clock or oscillating clock. Bunning performed several experiments in Kalanchoeblossfeldiana. In this species the petals open in the morning and close down in the evening. It is also possible to maintain Kalanchoe plants in complete dark conditions for some time.

Plants exposed to initial light-dark alteration, exhibited succession of movement of their petals even when transferred to complete darkness. It was believed that some sort of chronometric mechanism(s) was set which went through oscillations.

In other words a kind of physiological clock was set which experienced a specific rhythm. In our example given above the initial setting of light-dark alterations for the plant determined the timing of the petal opening and closing, several species of cactii exhibit endogenous diurnal rhythms in regard to the opening of the flowers and their closure.

The time factor or the period varied from species to species. In any case it has become more and more evident that the endogenous circadian rhythm of Bunning plays an important role in photoperiodic timekeeping. Hammer and his associates have clearly demonstrated that photoperiodic timing is neither a matter of measurement of the dark period alone nor of the light period alone.

Approach # 2. The Hour-glass Model:

It is proposed that in a living system a specific process is set in motion as a response to a specific stimulus and this process/reaction continues to the finish. It is suggested that time is measured by the finishing of this specific reaction/process.

The recent available information from higher plants indicates involvement of plant pigment phytochrome whose disappearance acts as a ‘dark period hourglass’ system in photoperiodic timing by measuring the length of the uninterrupted dark period.

One may assume that a process already exists in a plant and is set in action by a specific light-dark alteration. The plant measures time when the process is completed. It is assumed that the process is reversible. Further, during darkness P₇₃₀ is changed to P₆₆₀ whereas in the presence of light it is converted back to P₇₃₀. It is as if, the clock is wound again.

In SD plants like Kalanchoeblossfeldiana interruption of dark period by light does not let it flower. When a continued dark period was interrupted each time with a two hour light break, phase of different sensitivity to light recurred periodically.

On the contrary in-the LD plant like Hyoscyamus the flowering is stimulated by light break. It was subjected to circadian light-dark cycles and transferred to long period of darkness. Subsequently the long period is also interrupted by two hours light break for phases of different light sensitivity. Again, phases of different sensitivity to light recurred periodically. It is assumed that physiological clock is the causative factor.

In a recent communication Heide (1977) has presented a theoretical model for photoperiodic time measurement in higher plants (Fig. 22-15). It is based on the concept that the diurnal change in responsiveness to the phytochrome is dependent upon a circadian rhythm in membrane functioning and configuration, which is reflected in a parallel rhythm in membrane binding capacity for phytochrome (P_ƒr).

In fact phytochrome is an integrated component of the membrane clock which mediates the external light stimuli in phase controlling of the membrane oscillation. According to Heide, the phytochrome and the circadian clock are integrated into one structural and functional unit. Heide’s model can account for most of the conflicting experimental results in both SD and LD plants.

Heide’s model is the unequivocal demonstration that phytochrome plays an important role in photoperiodic timing and that photoperiodism depends upon an interaction of phytochrome with the circadian rhythm. Further, in several experiments, a diurnal changing sensitivity towards R and FR i.e., towards P_ƒr and P_r has been demonstrated in both SD and LD plants. Heide’s model tries to explain how this interaction is faciltated.

Njus, Sulzman and Hastings (1974) have proposed a membrane model for circadian clock. These authors describe the clock as a biochemical Network of self-sustained and feedback controlled oscillations arising due to interactions of membrane and solutes.

It is further believed that a photoreceptor resides in the membrane which modifies membrane properties in response to light quality. Wagner and his co-workers (1974, 76) visualize energy change a basis for circadian rhythm.

Now, it is almost accepted that the phytochrome is the clock photoreceptor. Heide (1977) postulated phytochrome as an integrated part of the circadian clock which comprises membrane transport structure and compartmentalized solutes which mutually interact to form a closed feedback loop.

His hypothesis also takes into account the following assumptions:

(i) Circadian rhythmic changes exist in the phytochrome receptor sites in the membrane. That such changes are affected by energy changes, pH, etc. due to solute oscillator.

(ii) The rhythm is rephased or adjusted due to the P_ƒr acting as an allosteric affecter.

(iii) The light-dark cycles are timed through the “orders” for P_ƒr rhythms (Figure 22-15).

Now an attempt may be made to resolve some of the paradoxical experimental results in photoperiodism in the light of the above hypothesis.

(i) Short day plants:

It has been demonstrated time and again that SD plants exhibit changing responsiveness to R and FR. That the effect is dependent on the particular time when the light exposure is given and also in relation to plant’s transition from light to darkness or vice-versa. This implies a diurnal change in the optimum proportion of P_ƒr which is determined by the phase of the membrane.

In nutshell, the length of both the light and dark periods in 24 hr cycle would be of importance for the photoperiodic timing especially if the light period is short. Alternatively, both the light-on and the light-off signal presumably influences the membrane rhythm and the phytochrome state.

(ii) Long day plants:

These category of plants show maximum flowering in continuous light. Several plant species have been shown to exhibit changing sensitivity to R and FR which points towards a rhythmicity in the optimum proportion of P_ƒr.

The chief point is that LD plants flower only when the photoperiod is longer than a certain critical length. Even SD and LD plants differ in their light requirements for flowering since they have the same basic rhythm. The crucial difference between SD and LD paints lies in the time span of the photoperiod.

It is postulated that the basic membrane rhythm is the same in both categories i.e., LD and SD plants. However, in SD plant rephrasing is avoided to induce flowering, in LD plants rephrasing in scotophase is a must for its flower formation. In other words, the critical photo-period in LD plants becomes the shortest photoperiod which is able to cause rephrasing of the rhythm.