Berkowitz G A, Gibbs M
Institute for Photobiology of Cells and Organelles, Brandeis University, Waltham, Massachusetts 02254.
Plant Physiol. 1983 Apr;71(4):905-11. doi: 10.1104/pp.71.4.905.
Addition of sorbitol, which facilitated reductions in reaction medium osmotic potential from standard (0.33 molar sorbitol, -10 bars) isotonic conditions to a stress level of 0.67 molar sorbitol (-20 bars), inhibited the photosynthetic capacity of isolated spinach (Spinacia oleracea) chloroplasts. This inhibition, which ranged from 64 to 74% under otherwise standard reaction conditions, was dependent on reaction medium inorganic phosphate concentration, with the phosphate optimum for photosynthesis reduced to 0.05 millimolar at the low osmotic potential stress treatment from a value of 0.25 millimolar under control conditions.Stromal alkalating agents such as NH(4)Cl (0.75 millimolar) and KCl (35 millimolar) were also found to affect the degree of low osmotic potential inhibition of photosynthesis. Both agents doubled the rate of NaHCO(3)-supported O(2) evolution under the stress treatment, while hardly affecting the control rate at optimal concentrations. These agents also reduced the length of the lag phase of photosynthetic O(2) evolution under the stress treatment to a much greater degree. The rate-enhancement effect of these agents under the stress treatment was reversed by sodium acetate, which is known to facilitate stromal acidification.The reaction medium pH optimum for photosynthesis under the stress treatment was higher than under control conditions. In the presence of optimal NH(4)Cl, this shift was no longer evident.Internal pH measurements indicated that the stress treatment caused a 0.43 and 0.24 unit reduction in the stromal and intrathylakoid pH, respectively, under illumination. This osmotically induced acidification was not evident in the dark. The presence of 0.75 millimolar NH(4)Cl partially reversed the osmotically induced reduction in the illuminated stromal pH. It was concluded that stromal acidification is a mediating mechanism of the most severe site of low osmotic potential inhibition of the photosynthetic process.
添加山梨醇可促使反应介质的渗透势从标准（0.33摩尔山梨醇，-10巴）等渗条件降低至0.67摩尔山梨醇（-20巴）的胁迫水平，但会抑制离体菠菜（Spinacia oleracea）叶绿体的光合能力。在其他标准反应条件下，这种抑制作用在64%至74%之间，它取决于反应介质中的无机磷酸盐浓度，在低渗透势胁迫处理下，光合作用的最佳磷酸盐浓度从对照条件下的0.25毫摩尔降至0.05毫摩尔。还发现基质碱化剂如NH(4)Cl（0.75毫摩尔）和KCl（35毫摩尔）也会影响低渗透势对光合作用的抑制程度。在胁迫处理下，这两种试剂都使NaHCO(3)支持的O(2)释放速率加倍，而在最佳浓度下几乎不影响对照速率。这些试剂还在更大程度上缩短了胁迫处理下光合O(2)释放的延迟期。胁迫处理下这些试剂的速率增强作用被已知可促进基质酸化的醋酸钠逆转。胁迫处理下光合作用的反应介质pH最佳值高于对照条件。在存在最佳NH(4)Cl的情况下，这种变化不再明显。内部pH测量表明，胁迫处理在光照下分别使基质和类囊体腔内的pH降低了0.43和0.24个单位。这种渗透诱导的酸化在黑暗中不明显。0.75毫摩尔NH(4)Cl的存在部分逆转了光照下基质pH的渗透诱导降低。得出的结论是，基质酸化是光合过程中低渗透势抑制最严重部位的介导机制。
