Vu J C, Allen L H, Bowes G
Department of Agronomy, University of Florida, Gainesville, Florida 32611.
Plant Physiol. 1987 Mar;83(3):573-8. doi: 10.1104/pp.83.3.573.
Soybean (Glycine max [L.] cv Bragg) was grown at 330 or 660 microliters CO(2) per liter in outdoor, controlled-environment chambers. When the plants were 50 days old, drought stress was imposed by gradually reducing irrigation each evening so that plants wilted earlier each succeeding day. On the ninth day, as the pots ran out of water CO(2) exchange rate (CER) decreased rapidly to near zero for the remainder of the day. Both CO(2)-enrichment and drought stress reduced the total (HCO(3) (-)/Mg(2+)-activated) extractable ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity, as expressed on a chlorophyll basis. In addition, drought stress when canopy CER values and leaf water potentials were lowest, reduced the initial (nonactivated) RuBPCase activity by 50% compared to the corresponding unstressed treatments. This suggests that moderate to severe drought stress reduces the in vivo activation state of RuBPCase, as well as lowers the total activity. It is hypothesized that stromal acidification under drought stress causes the lowered initial RuBPCase activities. The K(m)(CO(2)) values of activated RuBPCase from stressed and unstressed plants were similar; 15.0 and 12.6 micromolar, respectively. RuBP levels were 10 to 30% lower in drought stressed as compared to unstressed treatments. However, RuBP levels increased from near zero at night to around 150 to 200 nanomoles per milligram chlorophyll during the day, even as water potentials and canopy CERs decreased. This suggests that the rapid decline in canopy CER cannot be attributed to drought stress induced limitations in the RuBP regeneration capability. Thus, in soybean leaves, a nonstomatal limitation of leaf photosynthesis under drought stress conditions appears due, in part, to a reduction of the in vivo activity of RuBPCase. Because initial RuBPCase activities were not reduced as much as canopy CER values, this enzymic effect does not explain entirely the response of soybean photosynthesis to drought stress.
大豆（Glycine max [L.] cv Bragg）在室外可控环境箱中，于每升330或660微升二氧化碳浓度下种植。当植株生长至50日龄时，通过每天傍晚逐渐减少灌溉量来施加干旱胁迫，使得植株在后续每一天更早出现萎蔫。在第9天，随着花盆缺水，二氧化碳交换速率（CER）在当天剩余时间迅速降至接近零。以叶绿素为基础计算，二氧化碳富集和干旱胁迫均降低了总（HCO₃⁻/Mg²⁺激活的）可提取核酮糖-1,5-二磷酸羧化酶（RuBPCase）活性。此外，在冠层CER值和叶片水势最低时的干旱胁迫，与相应的非胁迫处理相比，使初始（未激活的）RuBPCase活性降低了50%。这表明中度至重度干旱胁迫降低了RuBPCase的体内激活状态，同时也降低了总活性。据推测，干旱胁迫下的基质酸化导致了初始RuBPCase活性降低。来自胁迫和非胁迫植株的激活RuBPCase的K(m)(CO₂)值相似，分别为15.0和12.6微摩尔。与非胁迫处理相比，干旱胁迫下的RuBP水平低10%至30%。然而，即使水势和冠层CER下降，RuBP水平在夜间从接近零增加到白天每毫克叶绿素约150至200纳摩尔。这表明冠层CER的快速下降不能归因于干旱胁迫诱导的RuBP再生能力限制。因此，在大豆叶片中，干旱胁迫条件下叶片光合作用的非气孔限制部分原因似乎是RuBPCase体内活性的降低。由于初始RuBPCase活性的降低幅度不如冠层CER值大，这种酶促效应并不能完全解释大豆光合作用对干旱胁迫的响应。
