Articles
PHOTOMORPHOGENESIS IN ROSES
Plants have at least three photoreceptor pigments by which they can measure the spectral distribution or quality of the light in which they are growing.
By means of the UV-B receptor and the blue-light receptor, information is gained about the amounts of UV-B and blue in the light, whereas the pigment phytochrome enables the plant to measure the red to far-red ratio of the light.
Depending on the plant species, both the amount of blue light and the ratio of red to far-red in the light perceived by the plant can significantly affect its morphogenesis.
A lack of blue light as well as a ratio of red to far-red less than about 1.2 may result in an increase in stem elongation in many species.
In roses (Rosa hybrida ‘Mercedes’) grown in artificial light with relatively low amounts of blue wavelengths, stem elongation is increased when the amount of blue is decreased.
Shoots, developed for a period of six weeks in light from which all the blue wavelengths were filtered out, were about 43% longer and had a 20% higher dry weight than those grown in white light.
The total dry-weight increase of the plants grown at equal photosynthetic photon fluxes (130 μmol m-2 s-1) of white light or light without any blue wavelengths was similar.
This agreed well with the equal rates of photosynthesis of leaves measured in the white and blue-deficient light.
Taken together, these observations indicate that the rates of assimilate production in both groups of plants were about the same, but that the amount of assimilates partioned to the shoots was increased in response to a decrease in the relative amount of blue light in the photosynthetic active radiation.
Although the development of flowers in roses is positively correlated with the assimilate availability in the plant, the increased allocation of assimilates to the shoots in plants grown in blue-deficient light did not affect the flower development of the shoots.
Carbohydrate analyses showed that shoots developed in light lacking the blue part of the light spectrum accumulated more starch than those of the white light control plants.
As photosynthesis was not affected by this difference in light quality, the difference in starch content also indicates a different partitioning of carbohydrates in the plants.
So far, the elevated level of starch as a response to blue-deficient light seems to result from a decreased rate of export of sucrose from the leaves as compared to white light grown leaves.
At present the effects of light quality on carbon metabolism and carbohydrate translocation are studied in greater detail.
Possible consequences of the observed differences in the accumulation of starch for the quality of the harvested shoots with flowers, such as flower opening and keeping quality of the flowers, will also have to be examined.