Use of Leaf Water Potential to Determine Water Stress in Field-grown Tomato Plants1

Abstract

Tomato plants ( Lycopersicon esculentum Mill.) were grown under 4 irrigation treatments. In the first three an attempt was made to maintain soil water at a different level for each treatment. The fourth treatment was an approximation of the varied moisture levels that would be encountered with furrow irrigation. Leaf water potential (Ψ L ) was affected more during a day by atmospheric factors than by soil water availability. Hourly changes in Ψ L and air water potential (Ψ air) were highly correlated (range .94*** and .99***). Ψ L decreased as the plant aged, apparently due to decreasing soil water availability, decreasing root activity, and increasing resistance to water flow in stems and leaves. The stress factor, (which is an integration of the area below −6 bars; a critical level for tomatoes) was determined from the second degree polynomial regression of time versus Ψ L and is proposed as a useful integration of Ψ L . By using stepwise regression it was found that plant water status as evaluated by tensiometer reading, stress factor, the ratio of soil water/soil water at field capacity, and daily pan evaporation had the greatest effect on yield and total soluble solids content of tomato fruit.