为了探讨紫花苜蓿根颈芽越冬抗寒的细胞学机理，本研究以 WL168为材料，应用透射电镜技术跟踪观察了根颈芽在自然越冬过程中幼叶细胞的超微结构变化。结果显示，1）在越冬期间，苜蓿根颈芽幼叶细胞超微结构发生了一系列积极适应低温的变化，主要表现为：质膜内陷，中央大液泡分割成多个小液泡，染色质凝集，质体变为月牙形或马蹄状，淀粉粒减少甚至消失，质壁分离等，细胞超微结构经过适应性调整，逐渐提高了根颈芽的抗寒能力。2）根颈芽在越冬期间同时存在白芽与褐芽两种发育状态，其中白芽幼叶细胞对越冬前的降温反应迟缓，直到土壤封冻后才完全形成抗寒结构，但是到了翌年早春，细胞超微结构变化快速，很早便解除抗寒状态，恢复生长。在整个越冬过程中白芽的抗寒性表现出4个不同的发育阶段：抗寒性增强期（10月下旬至12月中旬），抗寒性保持期（12月下旬至1月中旬），抗寒性减退期（1月下旬至2月下旬）和抗寒性解除期（3月初至3月中旬）。相反，褐芽幼叶细胞在入冬前对气温下降提前作出了反应，但在翌年早春对气温的回升却反应缓慢，到3月中旬，细胞结构依然保持抗寒特性。越冬期两种根颈芽的存在以及细胞超微结构响应低温的不同步性，是苜蓿越冬抗寒的一种适应策略。%In order to explore the cytological mechanism of cold resistance in alfalfa crown buds during winter, a study of ultrastructural changes in young leaf cells of crown buds in the alfalfa variety ‘WL168’was conduc-ted by means of TEM.The results showed that:1)a series of active changes in cell ultrastructure adapted to cold temperature take place during overwintering.The major changes included plasmalemma invaginations,the large central vacuole segmentation into multiple small vacuoles,chromatin condensation,plastid becoming crescent or horseshoe-shaped,starch grains depletion or even disappearance and plasmolysis.The capacity for cold resistance in buds was improved gradually by these adaptive adjustments of cell ultrastructure.2 )Two kinds of crown buds were found in winter:white bud and brown bud.These two kinds have different cellular growth states.The white bud cells had a slow response to cooling before winter and did not fully form special-ized structures for winter hardiness until soil freezing,but they regrew quickly after cold resistance release in early spring.The cold resistance development of white buds during overwintering went through 4 phases:en-hanced phase (late Oct-mid-Dec),maintenance phase (late Dec-mid-Jan),decline phase (late Jan-late Feb) and termination phase (early Mar-mid-Mar).In comparison,brown bud cells responded earlier to low tem-peratures before winter but more slowly to the warming of temperatures in spring.By mid-March,the cellular structure of brown buds still maintained their cold tolerance.The coexistence of brown and white buds during winter,whose cell ultrastructure changes asynchronously in response to low temperature,could be an adaptive strategy of alfalfa to cold stress.
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