Turner D P, Van Broekhuizen H J
Mantech Environmental Technology, Inc., US EPA Environmental Research Laboratory, 200 S.W. 35th, Corvallis, Oregon 97333, USA.
Environ Pollut. 1992;75(3):259-63. doi: 10.1016/0269-7491(92)90124-s.
Limited evidence to date suggests that acidic precipitation promotes leaching of nutrient cations from conifer foliage. In order to evaluate the relative contribution of the apoplast cation exchange complex and symplast nutrient pools to the leached ions, the magnitude of foliar leaching in response to acidic solutions was compared to foliar apoplast cation exchange capacity (CEC) for two conifer tree species (Pseudotsuga menziesii and Picea engelmanii). Leaching potential was determined by immersing detached needles in acidic solutions (pH5.6, 3.1, 2.1) over a range of time intervals (1, 4, 8, 24 h) and foliar apoplast cation exchange capacity was determined by potentiometric titration. Leaching increased with decreasing pH and increasing time of immersion. At pH values 2.1 and 3.1, equivalents of H+ depleted from the acidic solutions approximated equivalents of base cations gained by the solutions. Maximum amounts leached were less than 40 microeq g(-1) dry weight of needles for the combination of calcium, magnesium and potassium. Measured foliar apoplast CEC for these species was approximately 120 microeq g(-1) dry weight of needles. These relative magnitudes indicate that the apoplast rather than the symplast provided the leached ions. Because the apoplast foliar cation exchange capacity appears to be large relative to the rates of H+ diffusion across the cuticle, it may help to insulate the symplast nutrient reservoir from exposure to the H+ of acidic solutions bathing the foliage.
目前有限的证据表明，酸性降水会促进针叶树叶片中营养阳离子的淋溶。为了评估质外体阳离子交换复合体和共质体营养库对淋溶离子的相对贡献，将两种针叶树（花旗松和恩氏云杉）叶片在酸性溶液作用下的淋溶量与叶片质外体阳离子交换容量（CEC）进行了比较。通过将离体针叶在一系列时间间隔（1、4、8、24小时）内浸泡在酸性溶液（pH5.6、3.1、2.1）中来测定淋溶潜力，并通过电位滴定法测定叶片质外体阳离子交换容量。淋溶量随pH值降低和浸泡时间延长而增加。在pH值为2.1和3.1时，从酸性溶液中消耗的H⁺当量近似于溶液获得的碱性阳离子当量。钙、镁和钾的最大淋溶量小于40微当量/克（干重）针叶。这些物种的实测叶片质外体CEC约为120微当量/克（干重）针叶。这些相对大小表明，提供淋溶离子的是质外体而非共质体。由于相对于H⁺穿过角质层的扩散速率而言，叶片质外体阳离子交换容量似乎很大，它可能有助于使共质体营养库免受浸泡叶片的酸性溶液中H⁺的影响。
