As in many other regions, Ontario’s ornamental greenhouses often produce spring crops in more than one layer by placing one or more rows of HBs above the main crop growing on the greenhouse floor or bench (lower level). Although HB production maximizes the use of production area, shading at the lower level by the HB crop may be a significant challenge to the production of high-quality plants. Shading by HBs logically reduces the quantity of light reaching the lower crop and may also alter its spectral quality through the preferential absorption/transmission of specific wavelengths by the upper canopy. The combined effects may have both photosynthetic and photomorphological implications for the lower crops.
The designs of HB systems vary considerably, with positioning and density of HBs depending partially on greenhouse design (i.e., gutter heights, load-bearing capacity of the structure, and orientation) and largely on grower preferences. One-, two-, and three-tiered HB architectures are all commonly used with no clear distinctions as to the merits and drawbacks of each. In general, increasing the number of tiers also raises the production intensity by increasing HB density (number of HBs per square meter) and percent area coverage [square meters of HBs ÷ square meters of floor area (expressed as a percentage)]. What is not known is how HB production intensity affects the production of the lower level crops. Some Ontario floriculture growers have noted slower growth rates and decreases in crop quality (e.g., stretched internodes) when their main crop is produced under HBs, but the specific causes and the magnitudes of their effects are unknown. It is common for plants grown below another plant canopy to have slower growth, longer times to maturity, stretched internodes, thin stems, reduced branching, and fewer flowers (Faust et al., 2005; Lopez and Runkle, 2008; Thomas, 2006). These effects are due, in part, to a reduction in the ratio of red to far red light caused by preferential absorption of red light by the upper plant canopy.
It has been postulated, by various growers, that reductions in plant quality when grown below HB crops are due to a combination of insufficient PAR and reductions in the R:FR at the lower crop level. However, there are limited data available in peer-reviewed literature, with none specific to Ontario, related to the shading effects caused by HBs in commercial greenhouses.
Photosynthetically active radiation is a quantification of photons within the photosynthetically active spectrum (i.e., 400 to 700 nm) that are incident on a defined surface in a given period, usually measured in micromoles of photons per square meter per second (Barnes et al., 1993). In greenhouse production, continuously recorded PAR measurements are integrated over each daylight period to form DLIs (moles of photons per square meter per day). DLI accounts for the temporal fluctuations in PAR measurements, taken at fixed locations, due to variable weather patterns and changes in the sun’s path across the sky (i.e., time of day and time of year). There is abundant literature describing the effects of DLIs on various aspects of production (e.g., propagation, vegetative growth, and flowering.) for numerous floriculture commodities (Wook et al., 2009). Korczynski et al. (2002) converted 30 years of absolute irradiance measurements from over 200 weather stations into DLI maps for the continental United States. Growers can use the maps to estimate the seasonal outdoor DLIs for their specific location and thus make informed cropping management decisions. There is no similar dataset available for Canada.
One of the ways that plants evaluate their light environment is by sensing the R:FR, where the wavelengths are absorbed by the pigment phytochrome. Since plants readily absorb red light (used in photosynthesis) but transmit or reflect far red light, the R:FR decreases within dense canopies or below an upper layer of foliage (e.g., a forest understory). The R:FR of ambient sunlight is generally between 1.0 and 1.2, although this varies considerably in the literature depending on location and how the ratio is calculated (Blom and Kerec, 2003; Clifford et al., 2004; Erwin et al., 2006; Feldhake and Glenn, 1997; Mata and Botto, 2009; Smith, 1982; Turnbull and Yates, 1993; Yamada et al., 2009; Yang et al., 2012). When exposed to subambient R:FR, many common floriculture plants will shift their morphological development toward growing taller in an effort to access direct light. This is the so-called shade avoidance response and, in floriculture production, results in lower quality plants characterized by thinner stems, longer internodes, less branching, and fewer flowers (Blom et al., 1995; Fletcher et al., 2005; Runkle and Heins, 2001). It is not clear from the literature the magnitude of the decrease in R:FR needed to induce shade avoidance. This is likely due to the interaction of many varying factors including different species and cultivars, environmental parameters during production, specific lighting environments (e.g., natural vs. supplemental), and how various light characteristics are measured and reported.
A lighting survey was undertaken, during the Spring 2012 production season, to help characterize the influence of HBs on the light environment at the lower crop level in Ontario greenhouses. This survey comprised an extensive sampling regimen deployed in several production greenhouses representing the range of HB architectures typically used in this province.
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