Factors of germinability at low temperature in seeds of rat's tail radish (Raphanus sativus L.) introduced from northern Thailand

一种鼠尾萝卜，cv。泰国北部引种的Pakki-hood （Raphanus sativus L.）具有低温萌发性。我们研究了低温条件下种子发芽的几个主要障碍因素。在5℃条件下培养4 d后，两个品种的渗透种皮压力的下降没有差异，说明这些发现不能解释萌发时冷敏感性的差异。通过种皮吸收水分不受温度的影响。经过一段吸水滞后期后，种子含水量在20℃下进一步增加。在5℃条件下，Shogoin种子在6 d内没有进一步吸水，而Pakki-hood种子在吸胀4 d后吸水量略有增加，同时胚根伸长。为了确定Pakki-hood低温萌发的机制，我们克隆了9-顺式环氧类胡萝卜素双加氧酶（NCED）基因。全长cDNA包含一个1791 bp的开放阅读框，两侧是一个148 bp的3‘未翻译区和一个399 bp的5’未翻译区。在起始密码子上游177个核苷酸处发现了一个假定的TATA盒。我们搜索了假定的顺式作用基序，发现了四个myb识别位点。RsNCED mRNA在萌发过程中不积累。发芽种子中ABA含量在根系伸长前迅速下降，但在Shogoin中保持不变，表明ABA含量的维持与低温下无法发芽有关。

One of rat's tail radishes, cv.Pakki-hood (Raphanus sativus L.) introduced from northern Thailand has germinability at low temperatures. We examined several factors that are the main barrier to germination of seeds under low temperature conditions. There was no difference in the decrease of pressure to penetrate seed coat of the both cultivars after 4 days incubation at 5℃, indicating that these finding cannot explain the difference in cold sensitivity in germination. Water uptake through the seed coat in was not affected by temperature. After a lag phase of water uptake, the water content of seeds showed a further increase at 20℃. At 5℃, no further water uptake took place in seeds of Shogoin within 6 days, while the uptake of water in the seed of Pakki-hood increased slightly after 4 days of imbibition, concurrent with radicle elongation.To identify the mechanisms involved in germination at low temperature in Pakki-hood, we cloned the gene for 9-cis-epoxycarotenoid dioxygenase (NCED). The full-length cDNA contains an open reading frame of 1791 bp, flanked by a 148-bp 3'-untranslated region and a 399-bp 5'-untranslated region. A putative TATA box was found 177 nucleotides upstream of the initiation codon. We searched for putative cis-acting motifs there and found four MYB-recognition sites. RsNCED mRNA was not accumulated during germination. The ABA content in germinating seeds decreased rapidly before elongation of roots of Pakki-hood, but it was maintained in Shogoin, suggesting that maintenance of ABA content is correlated with the inability to germinate at low temperatures.