Tetraploid Bell Pepper Shows High In Vitro Pollen Germination at 15 °C

Abstract

Temperature (°C) Plants 10 15 20 25 30 Diploid 0.1 ± 0.1 6.5 ± 2.2 27.4 ± 2.9 36.3 ± 6.4 29.6 ± 5.3 Tetraploid 1.4 ± 0.4 22.2 ± 2.2 31.3 ± 3.1 21.9 ± 2.4 19.5 ± 2.6 Germination rates = (number of germinated pollen grains / number observed) × 100. The experiments were performed two times using separate preparations of tetraploid plants with comparable results. Data show one set of results. Values represent means ± SD (n = 6 to 19). Significant within columns by Tukey’s Studentized range test (P < 0.05). In Capsicum, the normal development of pollen grains was disturbed at low temperature, inducing seedless fruits, which were deformed in size and shape (Polowick and Sawhney, 1985). Mercado et al. (1997) reported that exine developed at low temperature was thinner than that developed at optimum temperature. However, polyploids are generally characterized to have large flowers and pollen grains (Watts, 1980). We hypothesized that the pollen grains of tetraploid pepper, compared with diploid, would be larger with thicker exine, and that the viability of the pollen at low temperature would be higher. Based on this hypothesis, we tested tetraploid plants of pepper and investigated the rates of pollen germination at low temperature. Tetraploid plants of Capsicum annuum L. ‘Chigusa’ (Nihon Horticultural Production Institute) were obtained by colchicine treatment of seeds (Ishikawa et al., 1997). Flow cytometric analysis (Partec PA flow cytometer, Partec, Munster, Germany) showed that ≈20% of the seeds treated with colchicine were tetraploid, comparable with our previous report (Ishikawa et al., 1997). The tetraploid plants had larger flowers than did plants of the diploid counterpart. were 27.4% to 36.3% (Table 1) and agreed with reports by Kato (1989) and Mercado et al. (1994). The germination rate was reduced to 6.5% at 15 °C, reflecting the high sterility level observed in diploid ‘Chigusa’ at low temperatures. However, the pollen germination rate (22.1%) of the tetraploid plants at 15 °C was significantly (P < 0.05) higher than that of the diploid plants. There was no significant difference among the pollen germination rates of diploid plants at 20, 25, and 30 °C, nor among those of tetraploid plants at 15, 20, 25, and 30 °C (by Tukey, P < 0.05).