Sayan Sdoodee and Porntip Kaewkong

pp. 1161 - 1167

Abstract

In general, water stress causes stomatal closure in citrus, and this leads to higher leaf temperature. Recently, it has been reported that infrared thermometry technique can be used for detecting stomatal closure indirectly to assess plant water stress. Therefore, it was proposed to apply to neck orange. An experiment was arranged as a completely randomized design. There were 3 treatments of watering levels: 1) wellwatering (W1), 2) 3-day interval watering (W2), and 3) 6-day interval watering (W3) with 6 replicates. Eighteen 2-year-old trees of neck orange were used, and each tree was grown in a container (0.4 m³) filled
with mixed media of soil, compost and sand (1:1:1). During 18 days of the experimental period, it was found that leaf water potential and stomatal conductance of the plants in W2 and W3 treatments decreased with the
progress of water stress. There was high correlation (r² = 0.71**) between leaf water potential and stomatal conductance as a linear regression (Y = 0.0044X-1.8635). Canopy temperature (Tc) and air temperature (Ta) of each tree were measured by an infrared thermometer, and Tc-Ta was assessed. At the end of the experimental period, it was found that Tc-Ta was significantly highest in the W3 treatment (3.5ºC) followed by the of W2 treatment (2ºC), while it was lowest in the W1 treatment (1ºC). The relationship between Tc-Ta and stomatal conductance was high as polynomial (Y = 0.0002X² -0.0572X+3.9878, r² = 0.70**). This indicated that stomatal closure or decreasing stomatal conductance caused increasing of Tc-Ta in the leaves. Hence, it suggests that infrared thermometer is a convenient device for the assessment of plant water stress in neck orange.