Evaluation of Growth under Non-24 h Period Lighting Conditions in Lactuca sativa L.

Nearly all living organisms have an endogenous circadian clock that generates a circadian rhythm with a periodicity of approximately 24 h. This rhythm is synchronized with the external environment and regulates many of the physiological processes of organisms. In animals, the circadian rhythm is generated by an endogenous circadian clock core located in the brain, and its resynchronization to different time zones is commonly referred to as “jet lag” (Yamaguchi et al., 2013). Conversely, in plants, the indigenous clock functions at the level of individual cells, which interact to produce the circadian rhythm in plant tissues, organs, and the entire organism (Fukuda et al., 2007; 2012). The circadian rhythm in plants regulates the timing of gene expression, which may for example result in peak expression of photosynthesis genes from early morning to noon, sugar transport genes from late afternoon to evening, and genes involved in fragrance production from late evening to early morning (Harmer et al., 2000). The circadian clock is comprised of a basis of three components: the input pathway, which transmits the lightdark cycle of the environment and other external stimuli to the endogenous oscillator; the endogenous oscillator itself, which generates the circadian rhythm with a periodicity of approximately 24 h; and the output pathway, which transmits the rhythm generated by the oscillator to control various physiological activities (Harmer, 2009). Most of the molecular genetics studies on circadian clocks in higher plants have been undertaken in the model organism, Arabidopsis thaliania (Mizoguchi et al., 2002; McClung, 2006; Harmer, 2009; Pruneda-Paz and Kay, 2010). Some of these studies have demonstrated how gene clusters, such as CCA1 (CIRCADIAN CLOCK ASSOCIATED 1), TOC1 (TIMING OF CAB EXPRESSION 1), PRRs (PSEUDO-RESPONSE REGULATORs), and LHY (LATE ELONGATED HYPOCOTYL) are involved in the oscillator, and how these “clock genes” play a central role in the formation of the circadian rhythm (Alabadí et al., 2001; Nakamichi et al., 2004; 2010; 2012). Proteins involved in the input pathway include phytochromes and cryptochromes, which are redand blue-light photoreceptors, respectively (Pruneda-Paz and Kay, 2010). Other factors known to be involved in the input pathway include phototropins, which are capable of both greenand bluelight photoreception (Briggs and Christie, 2002), as well as the F-box protein ZEITLUPE, which are the new blue-light photoreceptor proteins (Somers et al., 2000; 2004; Kim et al., 2007). Numerous other factors are involved in the output pathway, as evidenced by the circadian clock regulation of photosynthesis, respiration, stomata opening/closing, stem elongation, leaf opening, flowering, and a variety of other higher-plant functions (Harmer et al., 2000; Graf et al., 2010; Farré, 2012). A key factor in the regulation of physiological proc-