Daily light integral for strawberry
Photosynthetic lighting is a supplemental lighting to promote plant photosynthesis and productivity when sunlight does not provide enough light. In greenhouse, we use a cumulative measurement of light over a day (photoperiod), daily light integral (DLI). This is a practical way to evaluate light environment in greenhouse as light intensities vary over time, days and seasons. Under natural light, DLI can exceed 60 mol/m2/d during summer long day conditions and can be less than 5 mol/m2/d during overcast winter short day conditions. Greenhouse structure and glazing would cut 30-50% of DLI by reflection and shading. We consider minimum and optimum DLI inside the greenhouse for strawberry as 10-12 mol/m2/d and 20-25 mol/m2/d, respectively. DLI exceeding 30 mol/m2/d seems to impart stress to strawberry plants.
Linear yield increase over cumulative light
Crop productivity is typically in a linear relation with total light that plants received. Cumulative yields of fruiting crops often exhibit a linear correlation with cumulative light (sum of all DLIs). The slope of the linear response (grams per mole) can be a useful parameter to predict the potential yield under varied DLI over seasons. However, as strawberry often shows cyclic production (flushes) with a gap between, the slope for a shorter cycle of production can be different from one obtained from the entire production cycle. For example, initial productivity of strawberry (1-2 months after transplanting) is largely dependent on the flower initiation status of the planting materials.
Photosynthetic lighting use
Until recently, supplemental photosynthetic lighting was not commonly applied over strawberry, due to its costs. However, recent technological advancement of horticultural LED lamps made the use of supplemental lighting more realistic. For example, the conventional lighting such as high pressure sodium lamps has an energy-to-photon conversion around 1.0 – 1.7 micro-mole/J, while high efficient LED lights today exceeds 3.0 micro-mol/J, a 2-3 time higher efficiency. To further increase the economic impact of using LEDs, we recommend either targeted lighting over the plant canopy or a full-canopy production approach (e.g., using movable gutter system) so that wasteful use of lighting can be minimized (Kubota, 2019?). This consideration of improving lighting efficiency (photon delivery to plants) can further increase the light-based productivity (grams per mole) by more than 50%.
Among limited information, a 100-200 micro-mol/m2/s PPFD seems to be a typical light intensity applied for strawberry. For example, additional DLIs that can be achieved at these light intensities are 4.68-9.36 mol/m2/d for 13 hours and 5.76-11.52 mol/m2/d for 16 hours of lighting when operated continuously. The goal of supplemental lighting is to achieve a minimum of 12 mol/m2/d daily light integral as a total of solar and supplemental lighting. Photoperiod should be selected based on cultivars, specifically their photoperiodic type as well as sensitivity to a dormancy-inducing short-day condition.
Figure. Wasteful lighting application and light energy use in a commercial strawberry production operation. High pressure sodium is not energy-use efficient and light distribution is over a wide horizontal plane. In such a setting, much of the light is wasted by illuminating the walkway.