探討外源激勃素在稉稻種子休眠性分類中的可行性

Abstract

穗上發芽是指生理成熟的種子在收穫前於母體上發芽。以臺灣來說，每年5、6月梅雨季及暫時性強降雨造成水稻倒伏，皆易造成一期作穗上發芽情形發生，以致稻穀減產、品質下降。藉由提升種子休眠性是目前常見降低穗上發芽的育種手段，然而具有強休眠性的種子在收穫後短期內不易發芽、發芽不整齊的特性對於臺灣二期作稻苗生產時程上相當不利。此外，同品種穗上發芽抗性的等級判定在不同年份有顯著不同的結果並非不常見。以黃熟末期濕穀種子的發芽百分率作為判定水稻穗上發芽抗性等級的常規方法，可能是造成上述差異現象的原因。國外研究顯示小麥濕穀的「後熟」期間，即新鮮收穫的成熟種子在室溫下乾燥保存期間，種子具有先提升GA感應性、後降低ABA感應性的現象。該研究利用GA與ABA感應性的變化作為種子休眠性與穗上發芽抗性的分級指標。本篇研究參照此小麥研究的作法，先進行預備試驗，觀察外加GA與ABA對稉稻種子發芽的影響，結果發現稉稻種子的後熟過程也有先提升GA感應性、後降低ABA感應性的現象。隨後的大規模田間試驗對219個稉稻品種檢測新鮮收穫種子的發芽百分率，並篩選出52個發芽百分率不到50%的品種。接著藉由新鮮收穫種子、後熟14天種子、及後熟14天施加GA處理種子共三類種子發芽百分率的多重比較統計分析，可將這些品種的種子休眠特性分為四群。同時篩選出3個水稻品種，其收穫時仍維持強種子休眠性、但能快速打破種子休眠特性。還有於溫室施行小規模重複試驗顯示，不同的栽培環境，能極大程度地影響種子休眠的表現。本篇研究結果可望作為未來開發辨識稻種種子休眠性等級新方法的參考、及耐穗上發芽育種材料選用的依據。

Pre-harvest sprouting (PHS) is defined as the germination of physiologically mature seeds on the mother plant before harvest. In the case of Taiwan, during the rainy season in May and June, the lodging of rice plants due to temporary heavy rains are the major causes of PHS at the end of the first crop season, resulting in reduced rice production and rice quality. Improving seed dormancy is a common breeding strategy to reduce PHS. However, strong dormancy seeds are not easy to germinate and show irregular germination in a short period of time after harvest, which is an unfavorable trait for rice seedling production in the second crop season in Taiwan. Moreover, it is not unusual that the observed PHS levels were significantly different for the same rice variety in different years. The ordinary method to evaluate the PHS level based on the germination percentage of freshly harvested rice seeds at the end of yellow ripe stage might be the cause of the discrepancy mentioned above. A study on wheat has shown that grains gained GA sensitivity and then lost ABA sensitivity during after-ripening (AR), the period of dry storage at room temperature for freshly harvested mature seeds. The changes in GA and ABA sensitivity was then used to serve as landmarks to define the level of seed dormancy and PHS. By adopting the strategy of the wheat study, we investigated the influences of exogenous GA and ABA application on the germination percentage of japonica rice in the preliminary experiment. The results showed that rice seeds also gained GA sensitivity and then lost ABA sensitivity during AR. In the following large-scale field experiment, we investigated the seed germination percentage of 219 japonica rice varieties, and selected 52 varieties with germination percentage less than 50%. Multiple comparison analysis testing for seed germination percentges measured from three different treatments: the freshly harvested seeds, AR14 seeds, and GA-treated AR14 seeds of each rice variety classified seed dormancy of the selected rice varieties into four groups. We also picked up three rice varieties that showed strong seed dormancy at the time freshly harvested but broke seed dormancy quickly. Besides, the small-scale repetitive trial in the greenhouse revealed that different cultivated environments greatly influence seed dormancy. These results can serve as a reference for the development of new methods to classify the level of seed dormancy in rice, as well as the information for selection of rice breeding materials for PHS.