Bench-top Root-zone Heating Hastens Development of Petunia under a Lower Air Temperature

Heating accounts for up to 30% of total operating costs for greenhouse operations in northern latitudes. Growers often lower air temperatures for production to reduce energy costs; however, this causes delays in development even in cold-tolerant crops, such as petunia (Petunia xhybrida). This delay increases production time and can reduce profitability. Recent studies on low air temperature bedding plant production indicate petunia as a strong potential candidate for using lower air temperatures in combination with bench-top root-zone heating (RZH) to avoid or reduce delays in development. The objectives of this study were to 1) quantify time to flower (TTF) of seven petunia cultivars and two recombinant inbred lines (RILs) when the mean daily air temperature (MDT) was lowered by 5 degrees C and bench-top RZH was used and 2) determine if a high-quality petunia crop can be produced on RZH. Petunia 'Sun Spun Burgundy', 'Sun Spun Lavender Star', 'Sanguna Patio Red', 'Potunia Plus Red', 'Potunia Plus Purple', 'Supertunia Red', 'Supertunia Bordeaux', and two RILs, IA160 and IA349, were grown in a greenhouse with an MDT of 15 degrees C without RZH or with a RZH set point of 21, 24, or 27 degrees C. Additionally, a commercial control (CC) was established by growing plants without RZH at an MDT of 20 degrees C. All plants were grown under a 16-hour photoperiod to provide a daily light integral (DLI) of approximate to 12 mol.m(-2) . dL(-1). Time to flower was shorter at higher RZH set points. For example, TTF of 'Potunia Plus Red' was 56, 52, 49, or 47 days for plants grown at an MDT of 15 degrees C without RZH, or with RZH set points of 21, 24, or 27 degrees C, respectively. When a RZH set point of 27 degrees C was employed, TTF of all cultivars and inbred lines, except 'Potunia Plus Red' and 'Sanguna Patio Red', was similar to plants grown in the CC. Shorter stem length, lower growth index, and smaller shoot dry mass (SDM) at flowering were observed for plants grown under lower air temperatures with RZH, resulting in a more compact and high-quality plant. Producing a compact plant in a shorter time period is beneficial for growers; thus, results suggest that MDT can be lowered to 15 degrees C for petunia production when a RZH set point of 27 degrees C is employed.