Study on the mechanism of quality deterioration of agricultural products through cell membrane permeability and surface nature of cell wall

测定了菠菜叶细胞的2种水分渗透系数、扩散渗透性Pd和渗透渗透性Lp，以及调节水分跨膜运动的水通道蛋白PM28A的表达随时间的变化。通过核磁共振法测定Pd值，测定了2℃下储存1d和6d时的水交换时间和扩散渗透率，结果表明，水交换时间和扩散渗透率分别为71.3 ms ~ 55.4 ms和5.1x10^<-5> ~ 6.6x10^<-5> ms^<-1>。这从定量上表明，随着贮藏时间的延长，细胞膜的透水性增加。通过将原生质体置于不同渗透压的溶液中，测定了原生质体的渗透性。测定了2℃和20℃条件下菠菜叶细胞的Lp含量。在20℃下贮藏至少72h后，Lp有所增加，但与初始值基本保持一致。Lp的增加是由于水通道蛋白的表达增加，加速了水的运输和/或由于磷脂双分子层的松散包装而导致膜性质的变化。由于本研究中测定的水通道蛋白之一PM28A的表达量在储存2d后下降到近一半，因此磷脂双分子层的恶化被认为是储存72h后Lp升高的主要原因。综上所述，本研究为在收获后1 ~ 2 d通过适当的胁迫降低水通道蛋白的表达来抑制水分流失提供了线索。

As indices of water permeability of spinach leaf cells, 2 kinds of water permeability coefficient, diffusional permeability Pd and osmotic permeability Lp were measured and the change in expression of aquaporin PM28A that regulates water movement across membrane with time was also examined.Through the measurement of Pd by NMR method, the water exchange time and diffusional water permeability were determined at the end of 1d and 6d storage under 2℃, The results showed that the water exchange time and the diffusional water permeability varied from 71.3 ms to 55.4 ms and from 5.1x10^<-5> to 6.6x10^<-5> ms^<-1> respectively. This indicated the increase in water permeability of cell membrane as storage time quantitatively.The osmotic water permeability Lp was also measured by newly developed method that makes a protoplast expose to solutions with different osmotic pressure. Lp was determined for the spinach leaf cells stored at 2℃ and 20℃. While Lp increased after at least 72h storage at 20℃, it was kept almost same as the initial value. The increase in Lp is caused by the increase in the expression of aquaporin that accelerates the water transport and/or the change in membrane properties due to loose packing of phospholipid bilayer. As the expression of PM28A, one of aquaporins, measured in this study decreased to nearly half in 2d storage, the deterioration of phospholipid bilayer is considered to be a major cause of the increase in Lp after 72h storage.With the information above, this study gives the clue to suppress water loss in 1 to 2 d after harvest by applying an appropriate stress to agricultural products to decrease the expression of aquaporin.