Effects of High Humidity and Solar Radiation on Temperature and Color of Tomato Fruits1

Abstract

Tomatoes were grown at Tucson in plastic covered greenhouses with normal or high relative humidity (RH). The fruits were exposed to the sun (Ex) or shaded by foliage (Sh), and some exposed fruits were painted either black (B) or white (W). Temperatures of the surface (T s ), wall (T w ) and center (T c ) of the fruit were 2 to 3° C lower in high than in normal RH, even though the maximum air temperature (T a max ) in high RH exceeded that in normal RH by 1.5°. When T s and air temperature (T a ) were measured simultaneously, T s of Ex and B fruits was always higher than T a , that of Sh fruits lower, and that of W fruits about the same as T a . The exact gradient depended on RH and T a . T w of Ex fruits almost invariably exceeded T s or T c , and thus the wall was a heat sink. Further, T c max exceeded T a max in small (diam 35 to 40 mm) or large (diam 60 mm) fruits. The gradient T c −T a for large Ex fruits grown in normal RH ranged from −5° to 15° C during the day, while that for fruits grown in high RH ranged from 0° to 12°. The respective daily ranges for the gradient T w −T a were −5° to 20° and 0° to 13°. For small fruits all gradients were similar and ranged from 3° to 13°. The incidence of defective coloration of the shoulders or sides of fruits was highest in Ex and seemed to be influenced by infrared and short-wave radiation. The possibility of protecting tomatoes from excess radiation is discussed.