Erda F G, Bloemen J, Steppe K
Laboratory of Plant Ecology, Ghent University, Gent, Belgium.
Plant Biol (Stuttg). 2014 Jan;16(1):43-8. doi: 10.1111/plb.12009. Epub 2013 Apr 29.
In studies on internal CO2 transport, average xylem sap pH (pH(x)) is one of the factors used for calculation of the concentration of dissolved inorganic carbon in the xylem sap ([CO2 ]). Lack of detailed pH(x) measurements at high temporal resolution could be a potential source of error when evaluating [CO2] dynamics. In this experiment, we performed continuous measurements of CO2 concentration ([CO2]) and stem temperature (T(stem)), complemented with pH(x) measurements at 30-min intervals during the day at various stages of the growing season (Day of the Year (DOY): 86 (late winter), 128 (mid-spring) and 155 (early summer)) on a plum tree (Prunus domestica L. cv. Reine Claude d'Oullins). We used the recorded pH(x) to calculate [CO2*] based on T(stem) and the corresponding measured [CO2]. No statistically significant difference was found between mean [CO2*] calculated with instantaneous pH(x) and daily average pH(x). However, using an average pH(x) value from a different part of the growing season than the measurements of [CO2] and T(stem) to estimate [CO2*] led to a statistically significant error. The error varied between 3.25 ± 0.01% under-estimation and 3.97 ± 0.01% over-estimation, relative to the true [CO2*] data. Measured pH(x) did not show a significant daily variation, unlike [CO2], which increased during the day and declined at night. As the growing season progressed, daily average [CO2] (3.4%, 5.3%, 7.4%) increased and average pH(x) (5.43, 5.29, 5.20) decreased. Increase in [CO2] will increase its solubility in xylem sap according to Henry's law, and the dissociation of [CO2*] will negatively affect pH(x). Our results are the first quantifying the error in [CO2*] due to the interaction between [CO2] and pH(x) on a seasonal time scale. We found significant changes in pH(x) across the growing season, but overall the effect on the calculation of [CO2*] remained within an error range of 4%. However, it is possible that the error could be more substantial for other tree species, particularly if pH(x) is in the more sensitive range (pH(x) > 6.5).
在关于植物体内二氧化碳运输的研究中，木质部汁液平均pH值（pH(x)）是用于计算木质部汁液中溶解无机碳浓度（[CO2*]）的因素之一。在高时间分辨率下缺乏详细的pH(x)测量数据，可能是评估[CO2*]动态变化时潜在的误差来源。在本实验中，我们对一棵李树（Prunus domestica L. cv. Reine Claude d'Oullins）在生长季不同阶段（一年中的天数（DOY）：86（冬末）、128（仲春）和155（初夏））进行了二氧化碳浓度（[CO2]）和茎干温度（T(stem)）的连续测量，并在白天每隔30分钟补充测量一次pH(x)。我们根据T(stem)和相应测量的[CO2]，利用记录的pH(x)来计算[CO2*]。用瞬时pH(x)计算的平均[CO2*]与每日平均pH(x)之间未发现统计学上的显著差异。然而，使用与[CO2]和T(stem)测量不同生长季部分的平均pH(x)值来估算[CO2*]会导致统计学上的显著误差。相对于真实的[CO2*]数据，误差在低估3.25±0.01%到高估3.97±0.01%之间变化。测量的pH(x)没有显示出显著的每日变化，与[CO2]不同，[CO2]在白天升高，夜间下降。随着生长季的推进，每日平均[CO2]（3.4%、5.3%、7.4%）升高，平均pH(x)（5.43、5.29、5.20）下降。根据亨利定律，[CO2]的增加会增加其在木质部汁液中的溶解度，并且[CO2*]的解离会对pH(x)产生负面影响。我们的结果首次量化了在季节时间尺度上由于[CO2]与pH(x)相互作用导致的[CO2*]误差。我们发现整个生长季pH(x)有显著变化，但总体而言对[CO2*]计算的影响仍在4%的误差范围内。然而，对于其他树种，误差可能更大，特别是如果pH(x)处于更敏感的范围（pH(x)>6.5）。
