Influence of Water and Nitrogen Stress on Stem Sap Flow of Tomato Grown in a Solar Greenhouse

Accurate measurement of crop water use under different water and nitrogen (N) conditions is of great importance for irrigation scheduling and N management. This research investigated the effect of water and N status on stem sap flow of tomato ( Solanum lycopersicum ) grown in an unheated solar greenhouse in northwest China. A water experiment included sufficient water supply (T 1 ) based on in situ water content measurement, two-thirds T 1 (T 2 ) and half T 1 (T 3 ) under a typical N application rate (N 1 ); i.e., 57.4 g·m −2 N. The N experiment included N 1 , two-thirds N 1 (N 2 ), and half N 1 (N 3 ) under T 2 irrigation. Results showed that deficit water supply reduced the stem sap flow by 22.1% and 42.8% in T 2 and T 3 , respectively, compared with T 1 . The average daily stem sap flow between N 1 and N 2 was similar, and both were higher than that of N 3 . Significant differences between N 1 or N 2 and N 3 were only observed on four dates (totally 34 days). Nighttime stem sap flow accounted for 6.0% to 6.9% of the daily value for the water treatments and 5.7% to 8.5% of the daily value for the N treatments. No significant differences for nighttime stem sap flow were found among water and N treatments. The daily stem sap flow was significantly and positively correlated with solar radiation, air temperature, vapor pressure deficit, and reference evapotranspiration under the water and N experiments. The slopes of the regression equations between the daily stem sap flow and these parameters were lower when soil water availability was limited, whereas the slopes of the regressions had no significant differences among N treatments. A parabolic relationship between the ratio of the daily stem sap flow of water deficit treatments to that of T 1 and soil relative extractable water content was observed.