LIGHT SOURCES FOR PROMOTING PHOTOSYNTHESIS
In assessing human environment light is normally measured in terms of lux, a unit based on the spectral sensitivity of the eye. However, the spectral response of plants to light is so different to that of the human eye that many photosynthesis experiments in which light has been measured in visual units have been misleading. In an endeavour to improve the description of the radiant flux from lamps for horticultural purposes the Dutch Committee for Plant Irradiation divided the spectrum into a number of bands defined according to their physiological activity. Three of these bands (400 to 510 nm, 510 to 610 nm and 610 to 700 nm) include photosynthetically active light (see Canham, 1966). McCree (1966) argued that a flat response detector sensitive between 400 and 700 nm should be used to measure photosynthetically active radiation and suggested the term "plantwatt" to denote the integrated power in this region of the spectrum.
The three principal factors which govern the ability of a lamp to promote photosynthesis are (1) the photosynthetically-active luminous flux produced by the lamp at the leaf surface, (2) the spectral distribution of that flux and (3) the photosynthetic action spectrum of the leaf. In this paper the possibility is examined of combining these factors to give a photosynthetic flux rating for lamps (Burdett, 1969). The consideration is then extended to the correlation of the net assimilation rate of a crop with the photosynthetic flux from a range of lamps and to the implications of the photomorphogenetic effects of the flux from the lamps.
Cucumber was used as the test plant. It is experimentally convenient and lighting is frequently used commercially in its propagation. Information on the action spectrum for photosynthesis in higher plants is rather sparse in the literature. It is generally agreed that there is a peak in the red, but whereas Hoover (1937) and Litvin and Ho I-T'an (1967) also found a peak in the blue, Gabrielsen (1948) and Vince and Stoughton(1957) did not. It is probable that the exact spectrum varies with species and possibly also with the environmental history of a plant. It was therefore necessary to determine the action spectrum of cucumber before it was possible to compute the photosynthetic flux of the lamps investigated and interpret their effects.