Air Temperature Affects the Growth and Development of Basil (Ocimum) Species

Basil (Ocimum L. spp.) is the most popular fresh culinary herb. An increased demand for locally grown food is causing greenhouse producers to expand production of hydroponically grown culinary herbs. However, the effect of air temperature on growth and development of basil is not well understood. Our objective was to quantify the effect of temperature on growth and development of basil species. Seeds of four cultivars [sweet basil (Ocimum basilicum L. ‘Nufar’), holy basil (O. tenuiflorum L. ‘Holy'), lemon basil (O. ×citriodorum Vis. ‘Lime’), and lemon basil (O. basilicum L. ‘Sweet Dani’)] were sown in 288-cell plug trays filled with a soilless peat-based germination substrate, placed in an environmental growth chamber with a target air temperature of 24 °C, and grown for three weeks. Seedlings were then transplanted into 10-cm containers filled with a commercial soilless substrate comprised of sphagnum peat moss and perlite. Ten plants of each species were placed in five different growth chambers with target air temperatures of 11, 17, 23, 29, or 35 °C. Plants were fertilized once a week with 200 ppm N from a complete, balanced, water-soluble fertilizer. Three weeks after placing plants into temperature treatments, chlorophyll fluorescence, plant height, node and branch number, and flowering data were recorded. Plants were harvested and shoot fresh mass was recorded. Shoots were then placed in a forced air drier maintained at 67 °C for 3 d then weighed and dry mass was recorded. The leaf unfolding rate was influenced by temperature and varied among species. For example, the node appearance rate of sweet basil increased from 0.03 to 0.30 nodes per day as the temperature increased from 11 to 29 °C and then decreased to 0.28 nodes per day as the temperature further increased to 35 °C. Similarly, fresh mass was influenced by air temperature and species. As temperature increased to 29 °C fresh mass increased; as temperature increased from 29 °C to 35 °C fresh mass decreased for all species. Using the data from our experiments, we were able to calculate the base and optimal temperatures for all four basil species in this experiment. Furthermore, we were able to use data from plants grown within the linear range of air temperatures to develop models that may be used to predict growth and development of basil grown at different air temperatures.