2017 ASHS Annual Conference

Light drives photosynthesis and the cumulative amount of light over the course of the day (daily light integral; DLI) influences crop growth. Under low ambient DLIs, supplemental lighting is used to increase growth. However, species-specific responses to DLI affect the efficiency of supplemental lighting. Our objective was to quantify the growth of culinary herbs in response to DLI. Seedlings of sweet basil (Ocimum basilicum L.), parsley (Petroselinum crispum (Mill.) Fuss.), dill (Anethum graveolens L.), mint (Mentha sp. L.), oregano (Origanum vulgare L.), sage (Salvia officinalis L.), cilantro (Coriandrum sativum L.), and thyme (Thymus vulgaris L.) were placed in nutrient film technique hydroponic systems in a glass-glazed a greenhouse. Plants were grown under no shade or shade cloth (33% or 69% shading) and provided with supplemental light from 1000-W high-pressure sodium lamps over the course of numerous seasons from the fall through spring, resulting in DLIs from 2.1 to 15.0 mol∙m^–2∙d^–1 over the course of the study. Data collected or calculated included shoot fresh (SFM) and dry mass (SDM), growth index, and chlorophyll fluorescence throughout the day (F_v/F_{m morning, midday, evening}). As DLI increased from 2.1 to 15.0 mol∙m^–2∙d^–1, SFM (g) and growth index (GI) increased in basil (116.6, 14.4), sage (30.4, 7.4), cilantro (58.0, 13.6), thyme (8.9, 4.4), oregano (7.0, 9.6), dill (33.1, 28.8), mint (67.4, 14.9), and parsley (155.2, 8.8) respectively. Growth of some species, such as cilantro, increased linearly with increasing DLI (r² = 0.92) from 2.1 to 15.0 mol∙m^–2∙d^–1. Other species growth exhibited a quadratic response and saturated at lower DLIs, such as parsley (r² = 0.87). Chlorophyll fluorescence at the end of the day (F_v/F_{m evening}) as affected by increasing DLI was species-specific; F_v/F_{m evening} of light-sensitive species like parsley decreased with increasing DLI, while other species like cilantro exhibited no or minimal reduction in F_v/F_{m evening} with increasing DLIs . Though fresh mass increased with increasing DLI for all species, our models can be used to distinguish which species would benefit most from increasing light. The relationship for growth (SFM, GI) in response to DLI provides a tool to predict growth and yields in response to DLI, as well as determine the economic and physiological efficiency of supplemental lighting decisions on yield and make decisions on environmental management of lighting. Species-specific changes in SFM and F_v/F_m with increasing light illustrate the relationship of the photosynthetic efficiency of PSII and growth.