Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432, Ås, Norway.
Planta. 2021 Jan 3;253(1):14. doi: 10.1007/s00425-020-03525-9.
During desiccation, both apparent electron transport rate (ETR) and photosynthetic CO uptake peak when external water has evaporated. External water, causing suprasaturation, weakens the strong correlation between ETR and CO uptake. Lichens are poikilohydric organisms passively regulated by ambient conditions. In theory, apparent electron transport rate (ETR), estimated by photosystem II yield measured in light (Φ), is a proxy of photosynthetic CO uptake. Hydration level, however, is a complicating factor, particularly during suprasaturation that strongly reduces CO diffusion. Here, the cephalolichen Lobaria pulmonaria and two chlorolichens Parmelia sulcata and Xanthoria aureola were excessively hydrated before photosynthetic CO uptake and Φ using imaging fluorescence tools were simultaneously measured while drying at 200 µmol photons m s. CO uptake peaked when hydration had declined to a level equivalent to their respective internal water holding capacity (WHC) i.e., the water per thallus area after blotting external water. CO uptake and ETR in all species were highly correlated at hydration levels below WHC, but weaker at higher hydration (chlorolichens) or absent (cephalolichen). Yet, at a specimen level for the two chlorolichens, the correlation was strong during suprasaturation. The CO uptake-ETR relationship did not differ between measured species, but may vary between other lichens because the slope depends on cortical transmittance and fraction of electrons not used for CO uptake. For new lichen species, calibration of ETR against CO uptake is therefore necessary. At intrathalline scales, Φ during drying initially increased along thallus margins before reaching maximum values in central portions when hydration approached WHC. WHC represents the optimal hydration level for lichen photosynthesis. In conclusion, ETR is an easily measured and reliable proxy of CO uptake in thalli without external water but overestimates photosynthesis during suprasaturation.
在干燥过程中，当外部水蒸发时，表观电子传递速率 (ETR) 和光合 CO 吸收都会达到峰值。外部水导致过饱和，削弱了 ETR 和 CO 吸收之间的强相关性。地衣是由环境条件被动调节的变水生物。理论上，通过在光下测量的光系统 II 产量估算的表观电子传递速率 (ETR) 是光合 CO 吸收的替代物。然而，水合水平是一个复杂的因素，特别是在过饱和时，它会强烈降低 CO 的扩散。在这里，叶状地衣肺衣和两种壳状地衣 Parmelia sulcata 和 Xanthoria aureola 在进行光合 CO 吸收和 Φ 测量之前被过度水合，同时在 200 µmol 光子 m s 下干燥，使用成像荧光工具同时测量。当水合作用下降到相当于它们各自的内部水保持能力 (WHC) 时，即吸干外部水后的单位叶状体面积的水时，CO 吸收达到峰值。在 WHC 以下的水合水平下，所有物种的 CO 吸收和 ETR 高度相关，但在较高的水合水平（壳状地衣）或不存在（叶状地衣）时较弱。然而，在两种壳状地衣的标本水平上，在过饱和时相关性很强。所测量的物种之间的 CO 吸收-ETR 关系没有差异，但可能因其他地衣而异，因为斜率取决于皮层透射率和未用于 CO 吸收的电子分数。因此，对于新的地衣物种，有必要校准 ETR 与 CO 吸收的关系。在叶状地衣内部，在水合作用接近 WHC 时，干燥过程中 Φ 最初沿叶状体边缘增加，然后在中心部分达到最大值。WHC 代表地衣光合作用的最佳水合水平。总之，在没有外部水的情况下，ETR 是一种易于测量且可靠的 CO 吸收替代物，但在过饱和时会高估光合作用。
