Photo-selective films that absorb FR light (FR-absorbing films) have been used to control the growth and development of horticultural crops (Demotes-Mainard et al., 2016; Rajapakse and Shahak, 2007; Rajapakse et al., 1999). Under these films, shoot length, which is an important criterion for the quality of transplants and pot plants, can be shortened without using chemical growth regulators (Cerny et al., 2003; Li et al., 2000; Runkle and Heins, 2002; Wilson and Rajapakse, 2001), thereby improving tolerance to mechanical stress including wind damage (Latimer and Mitchell, 1988). The reduction of shoot elongation is due to the R:FR of the filtered light, which is higher than that of sunlight due to the film’s selective absorption of FR light. This light (hereafter, “high-R:FR light”) increases the proportion of active phytochrome and thereby suppresses shoot elongation (Franklin, 2008; Sasidharan et al., 2008; Smith and Whitelam, 1997). In addition to reducing extension growth, the FR-absorbing films also decrease plant dry matter production (Cerny et al., 2003; Fletcher et al., 2005; Li et al., 2000). Several studies have demonstrated that high-R:FR light decreases plant dry matter production even at equivalent photosynthetic photon fluxes (PPFs) (Hogewoning et al., 2010; Li and Kubota, 2009; Shibuya et al., 2010a), but the exact cause of this decrease has not been firmly established.
The purpose of the present study was to determine the factor or factors that limit plant growth under high-R:FR light by analyzing its effects on the components that contribute to plant growth (Hunt et al., 2002; Radford, 1967). In these methods, the RGR is calculated by multiplying the LAR by the NAR, which are correlated with the light interception area per plant and the net photosynthetic rate per unit leaf area, respectively. The high-R:FR light has been shown to alter these growth parameters. There have been many reports that leaf enlargement (correlated with LAR) was inhibited under high-R:FR light (Fletcher et al., 2005; Heraut-Bron et al., 1999; Hogewoning et al., 2010; Li and Kubota, 2009; Shibuya et al., 2010a). The suppressed leaf enlargement decreases the light-interception area, and this could explain the reduction in dry matter production under high-R:FR light (Fletcher et al., 2005; Li and Kubota, 2009). If so, a smaller LAR can result from decreased leaf enlargement per unit leaf dry matter, decreased allocation of dry matter to the leaves vs. stems resulted from the acceleration of stem elongation, or both (Casal et al., 1987). However, leaf enlargement has not been investigated sufficiently well for plants grown under high R:FR lights to determine which explanation is correct.
Contradictory results have been reported for NAR. Heraut-Bron et al. (1999) demonstrated that the R:FR did not affect photosynthesis per unit leaf area of white clover (Trifolium repens L.). However, other reports demonstrated that low-R:FR light reduces the photosynthetic performance per unit leaf area of common bean (Phaseolus vulgaris L.), cucumber (Cucumis sativus L.), and other several species of different growth habit (Barreiro et al., 1992; Hogewoning et al., 2010; Pons and de Jong-Van Berkel, 2004; Shibuya et al., 2010a, 2012, 2015), although Yang et al. (2013) reported the opposite results for chrysanthemum (Chrysanthemum ×grandiflorum Tzvelv). In these reports, photosynthesis of a single leaf was measured under leaf chamber conditions, and thus the results may be different from results of whole-plant photosynthesis under growing conditions, which are more appropriate to explain RGRs.
In the present study, cucumber seedlings were grown under light with a normal R:FR (similar to that of sunlight) or under high-R:FR light created by FR-absorbing film, and their growth parameters were determined by the growth analysis methods. In addition, net and gross photosynthesis per unit leaf area and of the whole plant were evaluated during the growing period to follow the change in photosynthetic performance and validate the results of the growth analysis.
This research was supported by a Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (B) (KAKENHI 24380140, KAKENHI 15H04575), and by a joint research agreement between Osaka Prefecture University and Yanmar Co., Ltd. The authors thank Ryo Matsuda (University of Tokyo) and Keach Murakami (University of Tokyo) for valuable discussion.
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