Diurnal Temperature Variation of the Root and Shoot Affects Yield of Greenhouse Tomato

The time of day when the root zone is heated may affect production of greenhouse tomato (Lycopersicon eseulentum Mill) grown under a diurnal variation in air temperature (DIF). To test this effect, the root zone was unheated, or heated to 21 °C constantly, or during 6 h in the day or in the night. Plants were grown in greenhouses with 14 °C day/14 °C night or 22 °C day/6 °C night minimum air temperatures, which resulted in 5 °C or 14 °C DIF, respectively. Seedlings were transplanted in early or late March in 1994 and 1995. Plants grown under 14 °C DIF yielded more than those grown under 5 °C DIF after both early-March and late-March planting. This increase in yield resulted from earlier fruit set and ripening and an increase in fruit size. For plants transplanted in early March, average yields with root-zone heating were greater than those with no heating because of increased fruit number. Heating the root zone had no effect on the yield from late-March planting, perhaps because unheated roots were never exposed to mean temperatures cooler than 16 °C. For the early-March planting, heating the root zone in the day did not increase yield significantly compared with heating in the night, even with a 14 °C air DIF. The time when the root zone was heated was not critical for increasing yield, and forcing the diurnal variation in temperature of the root zone to more closely match that of the air was not necessary, even with a large diurnal variation in air temperature.