水淹胁迫诱导喜旱莲子草茎发生类环割效应

在水淹环境中，喜旱莲子草水面上未受淹的茎段常表现出增粗膨大的现象。遭受水淹的植物被淹没的组织和器官会面临氧气缺乏和能量供应不足的问题。植物体内碳水化合物的运输需要消耗能量，当因水淹而使植株被部分淹没（即地下部分全部和地上部分的一部分被淹没）时，由于水淹缺氧导致能量供应不足，碳水化合物在植物被淹组织内的运输可能受限从而在水面上的未淹茎段中积累并对未淹茎段的径向形态产生影响。为探究水淹环境中喜旱莲子草未受淹茎段增粗膨大是否与碳水化合物积累有关，对茎被水淹和茎不受水淹的喜旱莲子草进行对比研究，结果发现：（1）水淹的喜旱莲子草位于水面上的未淹茎段节间平均直径显著大于水面下受淹茎段节间平均直径，未淹茎段与受淹茎段相比发生显著的膨大现象；未水淹的喜旱莲子草其茎的上部茎段节间平均直径与下部茎段节间平均直径相比并无显著差异，上部茎段也无明显膨大现象。（2）水淹的喜旱莲子草位于水面上的未淹茎段碳水化合物含量显著高于未水淹的喜旱莲子草对应茎段的碳水化合物含量。本研究表明，水淹胁迫下喜旱莲子草位于水面上的未受淹茎段中碳水化合物发生积累，导致植株位于水面上的未受淹茎段发生与物理环割后类似的茎膨大现象。

Girdling-like effect elicited in the stem of Alternanthera philoxeroides by partial submergence

Alternanthera philoxeroides(Mart.) Griseb., a perennial terrestrial plant of the Amaranthaceae family, originates in South America, but has spread to many parts of the world and is considered an invasive species in the United States, Australia, New Zealand, Thailand and China. A. philoxeroides is highly tolerant to submergence and able to grow in wetlands, river and lake shores, and other submergence-prone habitats. It is found that under submergence conditions, A. philoxeroides usually develops diameter-widened internodes on its unsubmerged stem parts above water surface. The inundated tissues and organs of submerged plants usually suffer from hypoxia and energy crisis therefore. Carbohydrates transport in plants is an energy-consuming process, when a plant is partially submerged, the impeded carbohydrate transport in submerged plant tissues and organs due to energy deficiency caused by submergence may lead to carbohydrate accumulation in abovewater unsubmerged tissues and organs, thereby affecting the diameter growth of abovewater stem. To explore whether the diameter widening of abovewater stem parts of partially submerged A. philoxeroides correlates with carbohydrates accumulation in the stem parts, we carried out an experiment in which two treatments were applied to two groups of randomly selected A. philoxeroides plants, respectively, plants in the first group were partially submerged and the plants of the second group were set as unsubmerged control. It was shown by the results that: (1) The average diameter of internodes on the abovewater stem parts in partially submerged A. philoxeroides plants was larger than the average diameter of internodes on the belowwater stem parts, and the abovewater stem swelled up markedly compared with the belowwater stem. In contrast, the upper stem parts (counterpart of the abovewater stem parts of partially submerged plants) of unsubmerged control A. philoxeroides plants did not differ with the lower stem parts (counterpart of the belowwater stem parts of partially submerged plants) in internode diameter, and no swelling occurred on the upper stem parts. (2) The content of carbohydrates in the abovewater stem parts of partially submerged A. philoxeroides plants was significantly higher than that of the counterpart stem parts of unsubmerged plants. Our study demonstrated that partial submergence of A. philoxeroides did cause the hampered carbohydrate transport in the belowwater tissues and organs so that the carbohydrates accumulated in the abovewater stem parts and led to the girdling-like swelling of the abovewater stem.