Photoperiodic lighting is used to create a photoperiod longer than natural daylength (or to create dark periods shorter than natural dark period). Photoperiodic lighting is a unique lighting technology used widely for flowering crops. Lighting is applied to plants as stimulus to induce a target action (e.g., flowering) and so the intensity required for this type of lighting applications is minimum (only a few micro-mol/m2/s photosynthetic photon flux density, or 1/100 the light intensity of typical photosynthetic lighting). When use of incandescent lamps were common, intensity of photoperiodic lighting was expressed using the input power per plant growing area (watt/m2). This was a confusing practice as the same unit can be used for irradiance measured at the target surface. We recommend using either photosynthetic photon flux density (PPFD, 400-700 nm) or biologically active photon flux density (300-800 nm) in a unit of micro-mol/m2/s. Photoperiodic lighting is widely studied in greenhouse flowering crops for which recommended intensities are a minimum of 1-2 micro-mol/m2/s measured over the plant canopy. LED-based flowering lamps can be used and can significantly reduce the lighting power consumption compared with incandescent lamps.
Controlling plant morphology and vigor by lighting
Photoperiodic lighting is also applied for dormancy control in strawberry. This is a unique application practiced over many years specifically for off-season production of strawberry. In general, perennial plants including strawberry enter dormancy (or semi-dormancy) state when they are exposed to low temperature (<5C) and short daylength. In most cases, maintaining optimum growing temperature in greenhouse can prevent plants from entering dormancy. However, some cultivars (e.g., ‘Tochiotome’) are very sensitive to short day and temperature alone cannot prevent them from entering dormancy-like state. Typical signs of plants in dormancy state that we observe in the greenhouse are rosette-like stunted morphology (short petiole) with slow overall growth and low productivity. Photoperiodic lighting can create a long day condition provided either as extension lighting (4-6 hours of lighting before or after the natural daylength) or night interruption lighting (3-4 hours of lighting around midnight to break up the long dark period). In Europe, photoperiodic lighting is applied to break the dormancy (or semi-dormancy) after the winter-resting period of their double cropping system. For that application, a cyclic lighting is common, rather than extension/night-interruption lighting. Cyclic lighting consists of repeated cycles of 15 min light and 45 min dark during the night and seems to have a stronger influence than extension/night-interruption lighting in our observation. When photoperiodic lighting is applied to short-day type cultivars, we recommend starting with relatively low range of optimum temperature (for example, average daily temperature 16C instead of 18C) to assure that lighting does not negatively affect the repeated flowering of the plants. But the sensitivity to long-day conditions for short-day plants at reproductive stage seems to be cultivar-specific. Therefore day-extension lighting needs to be carefully applied so that it would not inhibit flowering induction of short-day plants. Some grower practices include applying only a short period of day-extension lighting. As soon as growers confirm improved plant architecture (longer petiole), they would stop lighting so that flower initiation is not affected.
Lighting technologies for photoperiodic response
Typical light quality for photoperiodic lighting is traditionally incandescent lamp which emits good amount of far-red light (700-800 nm) relative to red light (600-700 nm). Although the light intensity to induce long-day response is as low as 1-3 micro-mol/m2/s photosynthetic photon flux density (PPFD), use of more energy-efficient lighting is desired. Traditionally, irradiance was measured using lux and footcandle, units considering the response to the human eye, which should be avoided for assuring expected plant response using different light sources including light emitting diodes (LEDs). A quantum sensor should be used which measures photon flux (micro-mol/m2/s PPFD). Research has been done by many greenhouse scientists to find a better lighting option.
Application of lighting seems to vary, from night interruption lighting (a few hours around mid night) to intermittent lighting. Night interruption lighting is typically for 4-hour photoperiod extension at 2-3 micro-mol/m2/s light intensity (or ~20 footcandle) around mid night. Intermittent cyclic lighting (15 min lighting for every hour of night) seems to be widely used in the Netherlands to bring the plants quickly from mid-winter dormant status to active production stage in their ‘double cropping’ production system. This intermittent lighting works well for long-day ever-bearing cultivars. Prolonged use of intermittent lighting may suppress flowering of short-day cultivars. Long-day conditions can be also provided as high intensity photosynthetic lighting (in order to supplement DLI as well as extending day length). When we do extension lighting, we typically add hours before sunrise instead of after sunset, so that natural light quality after sunset is still far-red rich which promotes extension growth.