Effects of soil temperatures on the rooting of cut-sprouts of sweet potato (Ipomoea batatas Lam.). I. Optimum soil temperature for rooting and effects of high soil temperatures on the physiological and anatomical characteristics of roots.

Abstract

The effects of soil temperatures on rooting, and of high soil temperature on rooting and some root characteristics of sweet potato cut-sprouts (Ipomoea batatas Lam.) were investigated in a controlled environment. The results obtained are as follows : 1. In the cultivar Okinawa No. 100 the rooting of cut-sprouts occurred normally at temperatures ranging from 19°C to 37°C. The soil temperature of about 30°C gave the maximum values in the total and average length of adventitious roots and the dry weight of roots at 1 week after transplanting and the number of leaves which recovered from wilting and newly developed at 1 and 2 weeks after transplanting, respectively (Figs. 1 and 2). However, for the number of adventitious roots thc range of optimum temperatures was wider (Fig. 1). 2. Cut-sprouts of cvs. Koganesengan, Kokei No.14 and Okinawa No.100 were exposed to a soil temperature of above 40°C for 0, 1.9, 5, 8, 11 and 24 hours per day during a period of 1 week after transplanting (Table 1). Total length of adventitious roots and all roots decreased due to the high soil temperature (Tables 2 and 3). But, the extent of the decrease in the treatment for 1.9 hours per day was small due to the compensatory increase in the number of adventitious roots, although the average root length was apparently decreased (Table 2). 3. Total sugar content and respiration rate of roots conspicuously increased by the high soil temperature treatments (Table 3). In spite of the decrease in the total length and dry weight of roots, the number of leaves and transpiration rate did not decrease by the exposure to a high soil temperature for 1.9 hours per day, presumably due to the increase of the water uptake per unit root length associated with the activation of root respiration (Tables 2 and 3). 4. In the treatments with high soil temperature one week after transplanting, there was a large number of roots with few cell layers in the primary cambium and the lignification of the stele cells was accelerated (Figs. 3 and 4) and abnormal stele was observed in cv. Koganesengan (Fig. 5). From these results it can be concluded that the optimum soil temperature for the rooting and establishment of cut-sprouts of sweet potato is about 30°C, and that a temperature of 40°C prevents cut-sprouts from rooting. When the exposure to a high soil temperature (above 40°C) lasts less than 2 hours per day the suppression of cut-sprout establishment is not appreciable. From the anatomical point of view, however, it appears difficult for tuberous roots to develop when exposed to a high soil temperature even for only 2 hours per day.