Abstract
The weak growth occurring in hybrids derived from crosses between two normal strains is referred to as hybrid weakness. F1 hybrid weakness (F1HW) in a cross between a Peruvian rice cultivar ‘Jamaica’ and Japanese rice cultivars has been reported and was considered to be controlled by a set of complementary genes, Hwc1 and Hwc2. We observed the development of F1 plants between Nipponbare and Jamaica (NJF1s). NJF1s were characterized by short roots, rolled leaves and a short stature. Although the NJF1 embryos were normal, the growth of the primary roots was arrested and in most cases, they could not emerge from seeds of the hybrids. Histological observation of the root apical meristem (RAM) at 10 days after sowing revealed that the zone of cell division in the NJF1 plants was lost, and that the cells were not short enough to explain the reduced root length of the NJF1 plants. Thus, it was suggested that the shorter roots the NJF1 plants might be due to reduced cell division. Although no distinct morphological abnormalities in the shoot apical meristem were observed, leaf primordium production was delayed in the NJF1 plants. Inhibition of root elongation in this type of hybrid weakness was alleviated under high temperature (34°C) conditions in diverse genetic backgrounds. The threshold temperature required for the recovery of root growth from hybrid weakness seemed to range from 29 to 30°C. Among 85 NJF1 plants that germinated and grew at 34°C for 10 days, and then outdoors, 18 plants eventually flowered. These plants were dwarf and their flowering time was delayed compared with that of their parents. In spite of their abnormal development, they were fertile. Thus, it is suggested that high temperature treatment was also effective to overcome hybrid weakness and to obtain next generation in this cross combination.
