2017 ASHS Annual Conference

Many greenhouses in northern climates use supplemental light to increase growth of crops in low light seasons and environments. The duration and integrated quantity of light, in the form of Photosynthetically Active Radiation, a plant receives over a day impacts growth, quality, and yield for greenhouse crops. The sum of photons in a unit area over a day, known as Daily Light Integral or DLI, is used by growers to plan crop growth schedules. Although DLI is well correlated to growth, this metric does not address duration of light during a day, known as photoperiod.

Work by Jiao et al. (1989) on roses, and Logendra and James (1992) on tomatoes have shown plants grown under longer photoperiods with a similar DLI produce more overall growth and accumulate a higher dry mass when compared to shorter photoperiods. To determine the optimal photoperiod for currently relevant day-neutral horticultural crops we grew plants under the same DLI with different photoperiods to determine an optimal photoperiod for production. Tomatoes and lettuce were grown in a growth chamber under a DLI of 12 mols / day with 12-Hour, 16-Hour, 20-Hour, and 24-Hour photoperiods to test effects on growth, development, and mass. Tomatoes grown for three weeks under a longer photoperiod of 20-Hours averaged a 45%-67% increase in dry mass and grew 31% to 44% taller in height in comparison to plants with a 12-hour photoperiod. Lettuce showed similar results, as the 20-Hour treatment produced plants with 38% more wet mass and 36% more dry mass in comparison to the 12-Hour treatment. Under a 24-Hour photoperiod, tomatoes developed intumescence and lettuce had significantly less wet and dry mass, suggesting the importance of a dark period. These finding will be tested in a full production greenhouse environment, to explore the benefits of achieving both a target DLI and target photoperiod on marketable yield, by adjusting supplemental light intensity in regards to the light provided by the sun to create an optimal lighting environment.