低渗膨胀对菠菜完整叶绿体光合作用的影响

Effect of Hypotonic Swelling on Photosynthesis in Spinach Intact Chloroplasts

菠菜离体完整叶绿体需要合适的介质渗透压（约０．９ＭＰａ）以保持其较高的光合作用速率。当渗透压因降低介质中山梨醇浓度（从０．３３ｍｏｌ／Ｌ至０．１７ｍｏｌ／Ｌ）而降低时，叶绿体的完整率保持不变。低于临界渗透压（约０．５ＭＰａ），叶绿体被膜就发生破裂．并丧失ＣＯ２同化能力。在轻度低渗条件下，虽然叶绿体被膜未破，但依赖ＣＯ２的放氧速率已受抑制。渗透压在０．９ＭＰａ与０．５ＭＰａ之间，叶绿体依赖ＰＧＡ的放氧抑制，可由加入山梨醇至正常浓度（０．３３ｍｏｌ／Ｌ）而解除。膨涨叶绿体的ＡＴＰ合成水平与正常叶绿体相同，而ＮＡＤＰＨ形成速率则明显降低。利用能透过被膜的不同电子受体ＮＣ２、ＰＧＡ和ＯＡＡ发现，在膨胀叶绿体中，ＮＯ２的还原不受形响，而ＰＧＡ及ＯＡＡ的还原明显被抑制。我们推测，低渗膨胀叶绿体中光合作用的抑制，至少有一个原因是Ｆｄ－ＮＡＤＰ氧化还原酶作用的受阻。

Isolated intact chloroplasts ofspinach need a medium with a suitableosmotic pressure (about 0. 9 MPa) tomaintain their high rate of photosynthesis. When the osmotic pressure ofthe medium was decreased by loweringsorbitol concentration from 0. 33 mol/L to 0. 17 mol/L, the Percentage ofintact chloroplasts remained unchangeduntil a critical osmotic pressure ofabout 0. 5 MPa was reached. Below0. 1 MPa the chloroplast envolope mpture occurred and CO2 fixation capacitylost. In slightly hypotonic conditions,the CO2-dependent O2 evolution wasinhibited, though the chloroplasts werestill intact(Fig. 1 ). Between 0. 9 MPaand 0. 5 MPa, the inhibition of PGAreduction in hypotonically swollenchloroplasts could be overcome byadding sorbitol to normal concentration(0. 33 mol/L ) (Fig. 3 ). To examinethe site of inhibition we measuredlight-dependent changes of ATP andNADPH levels in indict chloroplasts.The ATP level of the swollen chloroplasts ramained almost the same as thatof the normal chloroplasts(Fig. 4 ), butthe NADPH level was significantlylowered by swelling (Fig. 6). By usingdifferent penetrable electron acceptorsNOd, PGA and OAA, it was foundthat swelling of chloroplasts had no effect on nitrite reduction which used reduced ferredoxin as electron donor. Onthe other hand, PGA or OAA reduction which required NADPH as electron donor was significantly inhibitedby hypotonic swelling (Fig. 5 ). Fromthe above results, it may be deducedthat one of the reasons for the photosynthesis inhibition in hypotonicswelling chloroplasts is the inhibition ofFd-NADP oxidoreductase.