The effect of light on plant activities

light. Sunlight contains a lot more red light than far-red light. Therefore, during the day, phytochrome exists in the form Pfr. At night it gradually changes back into the Pr form. Phytochrome is involved in a range of plant responses to light, including the germination of seeds, elongation of seedlings, the size, shape and number of leaves, the synthesis of chlorophyll, and the straightening of the epicotyl or hypocotyl hook of dicot seedlings. In particular, many flowering plants use phytochrome to regulate the time of flowering based on the length of day and night (the photoperiod), and depending on which type of plant – short day plant (SDP) or long day plant (LDP) – the time at which it chooses to flower will differ. LDP’s flower with short nights – at the end of a short night, not all the Pfr will be converted back to Pr and it’s this remaining Pfr that promotes flowering. Therefore, in SDP’s Pfr inhibits flowering. In order to decipher which wavelength of light has greatest effect on the plant’s activites, one must first use an action spectrum. An action spectrum simply shows which wavelength of light is most effectively used in a given reaction. In the case of phytochrome, the red light and far-red light parts of the spectrum are used much more efficiently than any other. Therefore, the action spectrum graph would show spikes above the wavelengths representing the colors red and far-red. An absorption spectrum shows which part of the spectrum is most effectively absorbed, in this case, by a plant. The absorption peak of the red-absorbing form of phytochrome is 660nm and the absorption peak for the far-red form is 730nm. It was shown in the 1930s that the light stimulus controlling flowering is perceived by the leaves and somehow transmitted to the flowering apex. This implies that a growth regulator may be involved. Plant physiologists were so convinced by this theory that the regulator was given a name, florigen. Florigen was first described by Russian plant physiologist Mikhail Chailakhyan, who demonstrated that floral induction can be transmitted through a graft from an induced plant to one that has not been induced to flower. It was later shown that several long-day plants could be made to flower by treatment with gibberellin, when grown under a non-inducing photoperiod. This led Chailakhyan to modify his florigen hypothesis to postulate two classes of flowering hormones: gibberellins and anthesin. Chailakhyan postulated that during non-inducing photoperiods, long-day plants produce anthesin, but no gibberellin while short-day plants produce gibberellin, but no anthesin. However, these findings did not account for the fact that SDP’s grown under non-inducing conditions (thus producing gibberellin) will not cause flowering of grafted LDP’s that are also under non-inductive conditions (thus producing anthesin). It is argued that florigen does not exist but instead, a particular ratio of other hormones must be achieved for the plant to flower. However, there are also strong arguments to suggest that it does exist. Recent experiments suggest that florigen does exist. Its existence is substantiated by experiments that indicate that the substance that triggers blooming is produced or activated in the leaves of the plant, and must be given time to pass out of the leaves before the plant can flower. In conclusion, light affects a plant activites in a number of ways. This is reinforced by the fact that that a plants level of activity changes depending on the time of day and therefore the intensity of light.