Growth and Development of Petunia (Petunia ×hybrida) Cultivars Grown Under Reduced Air Temperature Production in Combination with Bench-top Root-zone Heating

Heating accounts for up to 30% of the total operating costs for greenhouse operations in northern latitudes. Growers often lower air temperatures to reduce energy costs; however, development is delayed even in cold-tolerant crops such as petunia (Petunia ×hybrida). This delay increases production time and reduces profitability. Recent studies indicate that petunia is a strong candidate for production using reduced MDT in combination with bench-top root-zone heating (RZH) to prevent or reduce delays in development. The objectives of this study were to: 1) quantify time to flower (TTF) of nine petunia cultivars when the MDT was reduced by 5 °C and bench-top RZH was utilized 2) determine if high-quality petunias can be produced on RZH. Petunia ‘SunSpun Burgundy’, ‘SunSpun Lavendar Star’, ‘Sanguna Patio Red’, ‘Potunia Plus Red’, ‘Potunia Plus Purple’, ‘Supertunia Red’, ‘Supertunia Bordeaux’, and two recombinant inbred lines were grown in a greenhouse with a constant air temperature of 15 °C without RZH or with a RZH set point of 21, 24, or 27 °C. Additionally, plants were grown in a separate greenhouse without RZH and a constant air temperature of 20 °C. All plants were grown under a 16-h photoperiod with day-extension lighting delivered from high-pressure sodium (HPS) lamps to provide a DLI of 10 to 12 mol∙m⁻²∙d⁻¹. As RZH temperature increased, TTF significantly decreased for all cultivars. For example, TTF of ‘Potunia Plus Red’ was 56, 52, 49, or 47 d for plants grown at a MDT of 15 °C without RZH, or with RZH set points of 21, 24 or 27 °C, respectively. When a RZH set point of 27 °C was employed, TTF of all cultivars, except ‘Potunia Plus Red’ and ‘Sanguna Patio Red’, was similar to plants grown at a MDT of 20 °C and no RZH. Reduced stem elongation, growth index, and shoot dry mass were observed when plants were grown with RZH. For example, stem elongation was reduced by 1.6, 1.7, and 3.8 cm for ‘Potunia Plus Red’, ‘Sun Spun Lavender Star’, and ‘Supertunia Bordeaux’ grown at a MDT of 15 °C with a RZH set point of 27 °C compared to without RZH. Producing a compact plant in a shorter time period is beneficial for greenhouse growers; thus, results suggest that MDT can be reduced to 15 °C for petunia production when a RZH set point of 27 °C is employed.