Comparison of light interception and field photosynthesis between vertically and horizontally trained watermelon [Citrullus lanatus (Thunb.) Matsum. et Nakai] plants

The light-interception characteristics and field photosynthetic rates of individual leaves were compared between vertically and horizontally trained watermelon plants to determine why the former produced smaller fruit than the latter. The planting density of the vertically trained plants was 3 times greater than that of the horizontally trained plants, because in the former the shoots were trained upward, whereas the latter were spread on the ground. Although the amount of solar radiation received by the upper leaves was similar in both plots, significantly less light was received by the middle and lower leaves of the vertically trained plants than by those of the horizontally trained ones. In the vertically trained plants, the photosynthetic rate was high at the upper leaves and decreased gradually with lower leaf positions. The photosynthetic rate in the horizontally trained plants was generally high, but the difference in the rate among leaves in different positions varied. We conclude that the main reason for the lighter fruits on the vertically trained watermelon plants is that the middle and lower leaves received less light because of shading compared with horizontally grown vines.