Studies on Xanthium and many other plants have shown that leaves are the organs of photoreception. Even if a single leaf (or even leaf) is given the required SD and long-night period in Xanthium (with all other parts in LD-conditions), it leads to flower initiation in the whole plant. Defoliated plants do not show any flowering response. Generally, young, fully expanded leaves are most sensitive and leaves still in bud are not effective. This indicates that whatever the flowering stimulus is, it should move from leaves to floral primordia, before flowering occurs. This stimulus seems to be hormonal in nature.
In xanthium grafting of photo induced leaf from one plant to another plant, which is in non-inductive conditions, promotes flowering on the receptor plant. The effect is found to be more pronounced if the receptor plant is defoliated before grafting the photo induced leaf.
Importance of Dark Period
A more important basic discovery about photoperiodism was the significance of the dark period in photoperiodic response, as demonstrated by Hamner and Bonner in 1938. They showed that Xanthium (SDP) could be kept from flowering even though one correct photo inductive cycle was given, by interrupting the dark period by a brief light period (light break). Breaking up the light period with a brief period of darkness had no effect. Thus a continuous, long, dark period is essential for flowering in short-day plants such as Xanthium. Plant physiologists now feel that the so-called SD-plants should be called long-night plants (LNP). They also found that in Hyoscyamus (LDP), the shortness of the dark period was equally important, thus necessitating its being called a short-night plant (SNP). However, the old terminology still continues.
Phototropism and Geotropism
Quality of LightScientists in the early 1950 showed that Xanthium failed to flower if the long night of its photoinductive cycle was broken by a brief flash of light (light break). They noticed that a red-light break caused maximum inhibition in flowering of these plants. Far-red radiation when used alone had no inhibiting effect. It was soon realised that far-red radiation could reverse the light break effect of red light. However, if alternate red and far-red light breaks were given in the dark phase of a photo inductive cycle of a SDP, it was the last treatment which determined flowering response. We have already seen the effects of such red-far-red treatment on seed germination. From these results it is clear that the phytochromes are involved in the initiation of flowering response also.
There is ample evidence to indicate that the photoperiodic behaviour of plants is based on the interaction of night length with a specific internal timer or biological clock. There is some sort of an internal mechanism within the cells of eukaryotes which enables them to measure the time (light and dark phase of the day). Their activities show a 24 hour rhythm or circadian rhythm. In plant systems, two phases of such endogenous rhythms have been suggested: a photopil phase in which light promotes plant processes including flowering, and a skotophile phase, during which light inhibits flowering and other processes. According to scientists these two phases alternate and the degree to which each of these phases overlap any particular set of light and dark periods to which the plants is exposed, greatly influences photoperiodism.
It has been suggested that the photoperiodic response such as flowering in plants is mediated by a specific hormone called florigen. However, this hormone has not been identified or isolated so far, although many attempts have been made in that direction.
In addition to the hypothetical florigen, several plant hormones have been noticed to have some influence on flowering. Synthetic auxins such as naphthalene acetic acid (NAA) are known to induce flowering in pineapple. Abscisic acid (ABA) also induces flowering in SDPs such as currants, Japanese morning glory and pigweed under long days, but ABA inhibits the flowering of spinach (LDP) under long days. Gibberellins induce flowering in many LDPs under SD-conditions but they have no effect on flowering of SDPs. Thus, it may not be possible to consider any one of these hormones as florigen, and therefore the hormonal factor in the induction of the photoperiodic response remains still elusive.