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，白萝卜(Raphanus Sativus L.)从泰国北部引进的植物在低温下具有发芽能力。我们研究了在低温条件下种子萌发的几个主要障碍因素。在5℃条件下培养4天后，两个品种的种皮穿透压力下降幅度没有差异，说明这一发现不能解释种子萌发对冷敏性的差异。种皮中水分的吸收不受温度的影响。种子吸水滞后期后，在20℃时水分含量进一步增加。在吸胀5℃时，甜瓜种子在6天内不再发生进一步的吸水，而吸胀4天后种子的吸水率略有增加，同时伴随着胚根的伸长。为了研究甜瓜种子在低温下萌发的机制，我们克隆了9-顺式环氧芳烃双加氧酶基因。该基因全长包含一个1791bp的开放阅读框，其两侧分别有一个148个核苷酸的3‘非翻译区和一个399个碱基对的5’非翻译区。在起始密码子的上游发现了一个推测的TATA盒，有177个核苷酸。我们在那里搜索了推测的顺式作用基序，发现了四个MYB识别位点。RsNCED基因在萌发过程中不积累。萌发种子中ABA含量在小白菜根伸长前迅速下降，但在Shogoin中保持不变，表明ABA含量的维持与低温下不能萌发有关。

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.