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 (T1) based on in situ water content measurement, two-thirds T1 (T2) and half T1 (T3) under a typical N application rate (N1); i.e., 57.4 gm L2 N. The N experiment included N1, two- thirdsN1(N2),andhalfN1(N3)underT2irrigation. Resultsshowed thatdeficitwater supplyreducedthe stemsapflow by22.1% and 42.8% in T2and T3, respectively,compared withT1.The average daily stemsapflow between N1and N2 was similar, and both were higher than that of N3. Significant differences between N1or N2 and N3 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 withsolarradiation, airtemperature, vaporpressure deficit, and reference evapotranspiration underthe 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 T1 and soil relative extractable water content was observed. The unheated solar greenhouse industry has been expanding rapidly and becoming increasingly important for local econo- mies in the arid region of northwest China in recent years, because the region has abundant light resources but limited precipitation (rain and snow) in winter. However, shortage of water resources and N stress (too little or too much) often affects the sustainable production of greenhouse crops in the region. Water and N stresses not only affect water uptake and nutrient assimilation of plants, but also influence stomatal response and drought resistance ability (Chapin et al., 1988; Claussen, 2002; Davies and Zhang, 1991). Thus, accurate measurement of crop water use under different water and N conditions can provide information for precision irrigation and N management. Water deficit has an adverse effect on transpiration, which has been confirmed in sap flow studies for many woody tree