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Root-zone temperature fluctuates both diurnal and during the growing season, Cold or frosty nights are followed by clear sunny days in desert conditions.

The aim of this work was to study the effects of continuous and a sudden cooling event of the root zone on transpiration, sap flow rate, and level of cytokinins as well as gibberellins in the xylem exudates of tomato (Lycopersicon esculuntum Mill.). The plants were grown for 3 weeks in three growth cabinets with separate controls for top and root sections. The top part of all the cabinets were set at the same light, temperature and humidity conditions while three root temperature regimes: 12, 16/8 (D/N) and 20 degrees C were kept in the root compartment.

Sap flow, in the tomato stem, was measured by the heat balance method. Within one hour after the root temperature dropped from 20 degrees C to 12 degrees C the sap flow rate fell from 37 to 17 g h(-1) plant(-1), the same value was found in plants that grew continuously for 3 weeks at 12 degrees C. This quick response suggest that low root temperature is affecting mainly the viscosity of root membranes and as a result water transport is reduced. Hormonal concentration differences cannot explain such huge drop in water transport in less than an hour.

Cold root temperatures are common in early mornings of dry-land and desert conditions and may result in short supply of water to the leaves in the early day hours. Warming the soil by irrigation before dawn may prove valuable to annual glasshouse and plantation crops in desert and dry land conditions.