Abstract
Seeds of cv. T-340 were sown in the field on 10 March and 9 April 1997. The lateral root (LR) production was progressively intensified in early sown plot (ESP) instead of late (LSP). Plants in the ESP produced roots with small mean weights, diameter and length as compared with those in the LSP. Early-sown seedlings were subjected to low soil temperature conditions, below approximately 12°C during the first growth stage. In a second experiment, seeds of cvs. T-340 and T-396 were sown in 2 experimental plots: a control plot and a high soil temperature plot (HTP). The minimum soil temperatures in the control were often remained below 10°C for about 25 days after sowing, but the temperatures in the HTP kept above 15°C during the growth period. When grown under the control, larger number of thickened LRs was observed in roots of cv. T-340, but roots of cv. T-396 did not exhibit this disorder at any soil temperature plot. In both cvs., hinder root elongation was observed in the control as compared with the HTP. In a third experiment, varietal difference in the occurrence of LR in roots of cvs. Oshin, T-340, T-396 and Tenpou was compared. Although no correlation was observed between the different cvs. and root growth, active meristematic activity was observed in the pericycle region outside the protoxylem in roots of susceptible cvs. Oshin and T-340. Conversely, this meristematic activity remained low in roots of resilient cvs. T-396 and Tenpou. These observations led to the hypothesis that the obstruction of root elongation caused by low soil temperatures (below 12°C) during the first growth period often results in activation of the basal region of the LR meristem in susceptible cvs. prompting LR development. However, in resilient cvs., the LR seldom occur, even when root elongation is inhibited by low soil temperatures that normally decrease the meristematic activity of this region.
