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 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.