Three-stage transformation of chlorophyll transient fluorescence pattern under sustained dehydration and the discovery of critical water content in seaweeds

The chlorophyll fluorescence kinetics of marine red alga Grateloupia turutunt Yamada, green alga Ulva pertusa Kjellm and brown alga Laminaria japonica Aresch during natural sustained dehydration were monitored and investigated. The pulse amplified modulation (PAM) system was used to analyze the distinct fluorescence parameters during thallus dehydration. Results proved that the fluorescence kinetics of different seaweed all showed three patterns of transformation with sustained water loss. These were: 1) peak kinetic pattern (at the early stage of dehydration fluorescence enhanced and quenched subsequently, representing a normal physiological state). 2) plateau kinetic pattern (with sustained water loss fluorescence enhanced continuously but quenching became slower, finally reaching its maximum). 3) Platform kinetic pattern (fluorescence fell and the shape of kinetic curve was similar to plateau kinetic pattern). A critical water content (CWC) could be found and defined as the percentage of water content just prior to the fluorescence drop and to be a significant physiological index for evaluation of plant drought tolerance. Once thallus water content became lower than this value the normal peak pattern can not be recovered even through rehydration, indicating an irreversible damage to the thylakoid membrane. The CWC value corresponding to different marine species were varied and negatively correlated with their desiccation tolerance, for example. Laminaria japonica had the highest CWC value (around 90%) and the lowest dehydration tolerance of the three. In addition, a fluorescence "burst" was found only in red algae during rehydration. The different fluorescence parameters F-o, F-v and F-v, F-m were measured and compared during water loss. Both F-o and F-v increased in the first stage of dehydration but F-v/F-m. kept almost constant. So the immediate response of in vivo chlorophyll fluorescence to dehydration was an enhancement. Later with sustained dehydration F-o increased continuously while F-v decreased and tended to become smaller and smaller. The major changes in fluorescence (including fluorescence drop during dehydration and the burst during rehydration) were all attributed to the change in F-o instead of F-v This significance of F-o indicates that it is necessary to do more research on F-o as well as on its relationship with the state of thylakoid membrane.

玉米种子脱水耐性与其线粒体完整性及抗氧化系统的关系

本文以正常性玉米种子‘农大108’(Zea Mays L. ‘Nongda 108’) 种胚为实验材料，研究了玉米种子发育过程中脱水耐性的变化规律，细胞匀浆以及线粒体水平上活性氧清除酶活性与种子脱水耐性/敏感性的关系，以及线粒体结构和功能完整性在发育过程中不同阶段对脱水的应答，以期在亚细胞分区水平上，针对活性氧产生的源头位点 (线粒体) 探明种子细胞脱水耐性/敏感性与抗氧化系统运转的关系。结果表明： 玉米种子在发育过程中先获得萌发能力后获得脱水耐性，并且脱水耐性的获得是一个渐进的过程。人工授粉后26天 (Days after pollination, DAP) 之前的种胚不具有脱水耐性，26 DAP时开始获得脱水耐性，到34 DAP后种胚完全获得脱水耐性。 在发育过程中，种胚线粒体的呼吸速率逐渐降低，并且对脱水的敏感性也逐渐下降。脱水会降低脱水敏感性种胚线粒体的结构完整性；脱水同时会降低线粒体功能的完整性，包括线粒体能量产生的速率和效率，以及三羧酸循环关键酶的活性。但当种胚获得脱水耐性后，脱水将不再影响种胚线粒体结构和功能的完整性。 玉米种胚发育过程中脱水耐性的变化与细胞中的抗氧化系统有关。在细胞匀浆水平上，脱水过程中脂质过氧化产物的积累与细胞脱水耐性的关系不明显；但是在线粒体水平上脱水会明显导致脱水敏感性种胚线粒体膜质过氧化程度的升高。脱水导致脱水敏感种胚细胞中几个重要的抗氧化酶活性的下降，但是与细胞匀浆水平相比，在线粒体水平上抗氧化酶系统对脱水更加敏感。 总之，发育早期玉米胚对脱水之所以敏感有两方面的原因，一方面是发育早期线粒体具有较高的代谢速率因而产生过多的活性氧，另一方面是由于脱水导致各抗氧化酶活性的显著降低，失去了抗氧化保护功能。而在发育晚期，早期本来很活跃的许多代谢随之关闭，呼吸速率降到很低，因而产生的活性氧减少，同时由于抗氧化系统对脱水的耐受性，所以脱水不会对线粒体的结构和功能造成伤害。与细胞匀浆水平相比，线粒体水平上抗氧化系统的运转与种胚在发育过程中脱水耐性的获得的关系更加密切。