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Ji-Jhong Chen, Shuyang Zhen, and Youping Sun

et al., 2020 ; Neilsen et al., 1995 ). Relative chlorophyll content is estimated using the ratio of transmitted red light and NIR light emitted by a red and an NIR LED, respectively, through a leaf ( Monje and Bugbee, 1992 ). Transmitted red light

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Toshio Shibuya, Ryosuke Endo, Yoshiaki Kitaya, and Saki Hayashi

-halide lamps (MHLs) transmitted through a shade net that did not change the light quality, and ( B ) light from MHLs transmitted through the far-red-light-absorbing film. The red to far-red ratio (R:FR) was defined as the ratio of photon flux between 600 and

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Yohei Kurata, Tomoe Tsuchida, and Satoru Tsuchikawa

cross-correlation function, which is used to calculate the similarity between reference and transmitted light ( Kurata and Tsuchikawa, 2009 ). In previous research, Tsuchikawa and Hamada (2004) applied TOF-NIRS for the detection of sugar and acidity in

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Heidi C. Anderson, Mary A. Rogers, and Emily E. Hoover

the spectral range important for photosynthesis, 400–700 nm ( Björn, 2015 ). The standard films most commonly used in horticultural production transmit lower levels of ultraviolet light, allowing little or no transmission of ultraviolet-B (280–315 nm

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Maria E. Cramer, Kathleen Demchak, Richard Marini, and Tracy Leskey

portion of UV-B, UV-A, and visible light transmitted through the five plastics are presented in Table 1 . The NP treatment had 100% transmittance in every range because there was nothing to filter wavelengths. TIV and UVT transmitted the greatest

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Steven P. Arthurs, Robert H. Stamps, and Frank F. Giglia

750 nm. The red net had a minor peak ≈400 nm and major transmittance beyond 600 nm. Pearl nets transmitted more light above 400 nm compared with black nets but did not otherwise alter spectral composition in the visible range compared with black nets

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Heather H. Friend and Dennis R. Decoteau

Alterations in spectral distribution as affected by selective light transmission of row cover materials were evaluated for effects on early watermelon (Citrullus lanatus cv. Sugar Baby) growth and development. Selected commercially available row covers were analyzed for light transmission properties. Results suggest that row cover materials function as selective light filters and influence parameters of light that can affect plant morphogenesis. Clear polyethylene row covers caused little variation in transmitted PAR (photosynthetically active radiation) and photomorphogenic light (FR/R and blue light). White polyethylene row covers decreased the transmitted PAR and blue light but had no effect on the FR/R ratio. Watermelon plants grown under a white polyethylene row cover with a greater FR/R ratio of light were taller (longer stems) and had longer petioles than plants grown under a clear polyethylene row cover with a smaller transmitted FR/R ratio.

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Michel Génard and Frédéric Baret

Gap fractions measured with hemispherical photographs were used to describe spatial and temporal variations of diffuse and direct light fractions transmitted to shoots within peach trees. For both cultivars studied, spatial variability of daily diffuse and direct light transmitted to shoots was very high within the tree. Diffuse and daily direct light fractions transmitted to shoots increased with shoot height within the tree and for more erect shoots. Temporal variations of hourly direct light were also large among shoots. Hourly direct light fractions transmitted to shoots were analyzed using recent developments in multivariate exploratory analysis. A gradient was observed between shoots sunlit almost all day and other shoots almost never sunlit. Well sunlit shoots were mostly located at the top of the tree and were more erect. Shoots located in the outer parts of the tree crown were slightly but significantly more sunlit than others for one cultivar. Principal component analysis additionally discriminated shoots according to the time of the day they were sunlit. This classification was related to shoot compass position for one cultivar. Spatial location of the shoot in the tree explained only a small part of light climate variability. Consequences of modeling light climate within the tree are discussed.

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Brent Loy and Otho Wells

Near infra-red (NIR)-transmitting plastic mulches transmit between 30 to 50% of the total solar energy to the soil, but block most photosynthetically active radiation (PAR). These mulches warm the soil more effectively than black plastic, but less effectively than clear mulch. Weed growth under NIR-transmitting mulches is suppressed through a combination of lowered light intensity and high temperatures under the mulch surface. With bell pepper and melons, IRT-76, a blue green, NIR-transmitting mulch, enhances growth to about the same extent as black mulch plus a floating rowcover. In turn, rowcover performance is enhanced by IRT-76 as compared to black mulch. In melons, early yield is about doubled and total yields increased by 20 to 30% with IRT-76 as compared to black mulch. Pepper yields are generally higher with IRT-76 as compared to black mulch, but the yield response to IRT-76 is less consistent than with melon.

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Mathieu Ngouajio and Jeremy Ernest

Weed control is one of the benefits associated with the use of plastic mulches used for vegetable production. The mulches decrease light transmission and prevent development of most weed species. Plastics chemistry has developed films varying in their ability to reflect, absorb, and transmit light. Laboratory and field experiments were conducted to 1) measure light transmitted through colored mulches, 2) evaluate weed populations under each mulch type, and 3) determine if light transmission could be used as an indicator for weed populations in the field. The polyethylene mulches were black, gray, infrared transmitting brown (IRT-brown), IRT-green, white, and white-on-black (co-extruded white/black). On average, 1%, 2%, 17%, 26%, 42%, and 45% light in the 400 to 1100 nm range was transmitted through the black, white/black, gray, IRT-brown, IRT-green, and white mulches, respectively. In field experiments, density and dry biomass of weeds growing under the mulches were evaluated. The white mulch had the highest weed density with an average of 39.6 and 155.9 plants/m2 in 2001 and 2002, respectively. This was followed by the gray mulch, with 10.4 and 44.1 weed seedlings/m2 in 2001 and 2002, respectively. Weed density was <25 plants/m2 with the other mulches in both years. Weed infestation was correlated with average light transmission for white, black, white/black, and gray mulches. However, both light quantity and quality were necessary to predict weed infestations with the IRT mulches. Weed infestation under the IRT mulches was better estimated when only wave lengths in the photosynthetically active radiation range (PAR; 400 to 700 nm) were considered. Low weed pressure and high light transmission with the IRT mulches would make them appropriate for use in areas where both weed control and soil warming are important factors.