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Developing alternative methods to synthetic plant growth retardants for controlling stem elongation and plant shape in pot and bedding plants and seedlings have got increasing attention during the last 15-20 years. Two important growth factors have been intensively studied: temperature and light quality. First, day/night temperature (DT/NT) alternations (DIF) and/or a short temperature drop treatment, and, secondly, a change in the ratio of red (R)/far-red (FR) light. Application of a higher NT than DT (negative DIF) or a temperature drop in the morning has successfully reduced plant height in the winter months in locations with cool climate. Screening daylight with selective plastic films for manipulation of the R/FR ratio in the spring and summer months during periods with high solar irradiance seems also to be promising. The physiological mechanisms for thereto- and photomorphogenetic stem elongation are not known. Several studies in pea (Pisum sativm) and Campanula isophylla have shown that the content of gibberellin (GA₁) plays a key role in thermoperiodic stem elongation. The GA₁ level in the elongating tissue is lower under negative (DT$lt;NT) than positive DIF (DT$gt;NT). Our results indicate that 2(3-hydroxylation of precursors to GA₁ or to GA₁ itself might be of importance for controlling the levels of GA₁ under alternating DT and NT. The thermoperiodic response can be modified by light quality. An end-of-day (EOD) treatment or day extension with FR light can partly nullify the inhibition of stem elongation by negative DIF in C. isophylla, pea, and cucumber plants. The response to an EOD FR treatment in the phytochrome B deficient long-hypocotyl mutant (lh) of cucumber developed equally long hypocotyls under positive and negative DIF conditions. This indicates that functional phytochrome B, at least in cucumber, needs to be present to make plants express a thermoperiodic response. Modification of the light quality in daylight by selective plastic films to an high R/FR ratio (1.6) resulted up to 50% reduction in stem length in some plant species, and very little response in others. In general, no or minor interactions between changes in daylight quality and DIF on stem elongation were found.
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