Although low levels of light might cause subtle changes in the growth of plants via the phytochrome system, these changes are likely to be so small as to be undetectable. The potential effects of nighttime light on the flowering process, however, might have more far-reaching consequences. Plants are classified as being day neutral (relatively unresponsive to day length), long-day (flower when days exceed a certain minimum) or short-day (flower when days are shorter than a certain minimum). The flowering of short-day or long-day plants can be influenced by light-breaks which occur at nighttime. In short-day plants, light breaks during the long-night will nullify the flower promoting effects of the long night. Conversely, in long-day plants, a light break during a long night will nullify the inhibitory effects of the long night. Ground irradiance at night has the potential to affect the flowering of important crops which flower according to prescribed daylength requirements. Among crop plants which are short-day or long-day plants are soybean (Glycine max), rye grass (Lolium temulentum), and barley (Hordeum vulgare) (Table 4). It is clear from the data in Table 4 that although both short- and long-day plants are sensitive to low levels of irradiation this irradiation must be supplied continuously throughout the night. It is clear that the total energy from these low irradiances greatly exceeds the energy levels from the SPS in any configuration. Another means of comparing the potential effects of irradiation from SPS comes from an evaluation of the effects of moonlight on the flowering -3 2 process. Moonlight has been estimated at the order of 10 W/m but even in the most sensitive of all flowering plants (Xanthium strumarium), the effects of continuous nighttime irradiation with moonlight are marginal (9). It is agreed that very few plants are as sensitive to lightbreaks as Xanthium. For example, Table 4 shows that Cannabis, Poinsettia
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