The time of day when the root zone is heated may affect production of
greenhouse tomato (Lycopersicon esculentum Mill) grown under a diurnal
variation in air temperature (DIF), To test this effect, the root zon
e was unheated, or heated to 21 degrees C constantly, or during 6 h in
the day or in the night. Plants were grown in greenhouses with 14 deg
rees C day/14 degrees C night or 22 degrees C day/6 degrees C night mi
nimum air temperatures, which resulted in 5 degrees C or 14 degrees C
DIF, respectively. Seedlings were transplanted in early or late March
in 1994 and 1995. Plants grown under 14 degrees C DIF yielded more tha
n those grown under 5 degrees C DIF after both early-March and late-Ma
rch planting, This increase in yield resulted from earlier fruit set a
nd ripening and an increase in fruit size. For plants transplanted in
early March, average yields with root-zone heating were greater than t
hose with no heating because of increased fruit number. Heating the ro
ot zone had no effect on the yield from late-March planting, perhaps b
ecause unheated roots were never exposed to mean temperatures cooler t
han 16 degrees 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 degrees C air DIF. The time when the root
zone was heated was not critical for increasing yield, and forcing th
e diurnal variation in temperature of the root zone to more closely ma
tch that of the air was not necessary, even with a large diurnal varia
tion in air temperature.