Role of heat girdling in early seedling death of sorghum.

High soil temperatures (>45 °C) can inhibit the field establishment of sorghum [Sorghum bicolor (L.) Moench.) seedlings in the semi-arid tropics even when adequate soil moisture is present. The objective of this study was to demonstrate an apparatus for simulating this situation under controlled conditions. The apparatus applies a high temperature localized near the leaf intercalary meristem while maintaining an adequately moist root environment. With this technique, we tested the hypothesis that seedling death may be due to a heat-girdling, leading to restricted translocation of carbohydrates to the root. Exposure of 10-d-old sorghum seedlings to heatgirdling temperatures of 52 ± 2 °C caused complete cessation of leaf elongation. At the same time, shoot carbohydrate concentrations increased while root carbohydrates declined, indicating that phloem transport from the shoot to the root may have been blocked or restricted. During the first 173 h of stress, turgor of both expanded leaves and leaf growing regions (basal 1 cm of leaf tissue) increased, indicating a favorable water balance in the seedlings. After 198 h of heat-girdling, seedlings died. The apparatus we developed should prove useful for the large scale screening of seedlings for resistance to high soil surface temperatures