缺镁纽荷尔脐橙叶脉肿裂的发生机制

Leaf vein enlargement or corkiness of citrus is ususally a typical symptom of boron deficiency. However, in recent years, many NewhalL navel orange orchards in southern Jiangxi province and Fujian province suffered leaf vein enlargemengt or corkiness and chlorosis. According to the nutrient element analysis of leaves from 149 orchards, we found that leaf boron content was enough, and leaf magnesium was deficient in most sympton orchards. The symptoms of leaf vein enlargemengt or corkiness and chlorosis disappeared by applying magnesium fertilizer but not by boron fertilizer in the sympton orchards. Furthermore, sand cultivation experiment also confirmed that magnesium deficiency could induce the symptoms. At present, he mechanism of leaf vein enlargement or corkiness of Newhall navel orange induced by magnesium deficiency is not known. However, cell abnormal proliferation and growth were observed in enlarged or corkied veins. Some researches have found thatmagnesium deficiency could lead to sugar accumulation in organs and tissues. Because sucrose can induce the expression of cyclin, and glucose can regulate the synthesis of auxin, the abnormal cell proliferation and growth in the leaf veins may be related to sugar metabolism. In this program, we plan to induce leaf vein enlargement and corkiness by hydroponic experiment of Newhall navel orange budlings under magnesium deficient stress, and then to research the sugar transport and accumulation, hormone metabolism,cytology process, and the expression of cell cycle-related genes and sucrose transporter gene during the leaf vein enlarging and corking process in order to study its mechanism. It may provide theoretical guidance for correcting magnesium deficiency of Newhall navel orange.

柑橘叶脉肿裂通常为缺硼典型症状。然而，近十多年在赣南和福建等地出现大面积纽荷尔脐橙叶脉肿裂以及叶片黄化症状，经我们对149个脐橙园的叶片全营养元素分析发现，绝大多数症状果园的叶片硼含量充足而镁缺乏，而且田间施镁后症状消失，但是施硼效果甚微或无效，进一步的缺镁砂培试验也证实缺镁的确会导致此症状出现。目前，缺镁导致的柑橘叶脉肿裂机制尚不明确，不过，初步的解剖观察发现，叶脉肿裂与细胞异常增生有关。前人的研究发现，缺镁会导致糖在器官组织内的累积。由于蔗糖可诱导细胞周期蛋白表达，而葡萄糖能调节生长素的合成。因此，叶脉细胞异常增生可能与糖代谢有关。本项目拟通过纽荷尔脐橙缺镁水培试验，诱导叶脉肿裂的发生，从叶源糖类运输与累积、激素代谢与细胞增生、细胞结构变化和细胞周期相关基因与蔗糖转运蛋白基因表达等方面，探索缺镁纽荷尔脐橙叶脉肿裂的发生机制，以期丰富柑橘缺镁的理论，并为纽荷尔脐橙缺素矫治提供生产指导。

中国是世界最大的纽荷尔脐橙产地，而赣南是中国最大的纽荷尔脐橙产区。近年来，赣南众多纽荷尔脐橙园相继出现大面积的叶片缺素黄化且叶脉肿裂现象。通常，叶脉肿裂被认定是柑桔缺硼的典型症状，但是生产实践和野外取样研究均证明赣南纽荷尔脐橙叶片黄化和叶脉肿裂主要与缺镁有关，缺镁砂培试验也重现了叶脉肿裂症状。因此，缺镁纽荷尔脐橙叶脉肿裂发生机制的研究，可丰富柑橘缺镁的理论，并为纽荷尔脐橙缺素矫治提供生产指导。.本项目主要开展如下研究：首先，在缺镁胁迫进程中，测定了叶片可溶性糖（葡萄糖、蔗糖和果糖）和淀粉含量，观察了维管组织中淀粉的积累状况，并检测了柑橘蔗糖转运蛋白基因CitSUT1的表达，结果表明随着缺镁胁迫进程的延长，CitSUT1基因表达显著降低，叶片糖类呈积累状况，特别是缺镁后期缺镁纽荷尔脐橙淀粉量累积尤为显著；在缺镁胁迫条件下叶片淀粉粒累积由分布上皮栅栏组织和邻近的海绵组织细胞，逐渐扩散至大部分的海绵组织、韧皮部、木射线薄壁细胞和髓部，且侧脉淀粉粒累积比主脉更为显著。其次，在缺镁胁迫进程中，观察了叶肉和维管组织的显微动态变化，并且测定了激素含量，结果表明在缺镁中期玉米素核苷（ZR）含量显著增加，主脉和侧脉韧皮部细胞显著增加，叶脉肿大；在缺镁后期缺镁叶片生长素（IAA）含量也显著增加，韧皮部细胞显著生长、增大，木质部、髓部和上表皮细胞受到挤压，导致个别主脉和部分侧脉的爆裂。第三，测定了生长素IAA和细胞分裂素关键基因的表达差异，结果表明缺镁胁迫条件下细胞分裂素和生长素合成关键酶基因产生了程序性的差异表达。因此，缺镁胁迫条件下糖类累积诱导ZR、IAA合成关键酶基因差异表达，促进了玉米素核苷（ZR）、生长素（IAA）合成量的依次增加，导致韧皮部细胞分裂和增生，是纽荷尔脐橙叶脉肿裂的根本原因。