植物吸收根的增殖和生长与养分变异的关系——臭椿、翠菊、加拿大一枝黄花分根实验的启示

构件理论认为植物根可以相对独立地吸收养分和对所处环境的养分条件做出响应。根据成本-收益理论, 单个根(构件)的生死、生长发育与其吸收的养分收益和自身建造、维持的消耗有关。基于此, 该文提出两个关于吸收根生死条件的假设: 1)当可利用养分低于低临界值, 根死亡在一段时滞(数天到几周)后发生; 2)当可利用养分高于高临界值并持续一段时间, 新的侧根产生。为了检验这两个假设, 用臭椿(Ailanthus altissima)、翠菊(Callistephus chinensis)、加拿大一枝黄花(Solidago canadensis)作实验物种, 设计了温室分根实验。每株植物选3个一级根, 分别引入3个不同养分水平的斑块: 0、20、200 μg N·g^-1。每4天将根暴露并拍照, 查数新根数并测量细根总长度和一级侧根长。由于高养分处理斑块内根的快速生长, 实验在开始后8天或12天结束。结果显示: 除臭椿在0养分处理外, 三物种在各养分处理下都有侧根产生, 总根长均有增加; 臭椿、翠菊、加拿大一枝黄花在不同观测时间和养分水平处理间的侧根数目和总根长差异显著, 而一级侧根长除臭椿外变异均较小; 整个过程中没有根死亡。研究结果部分支持两个假设。本研究还为进一步探究根模块构件增殖、生死过程机制提出新的建议, 即除需要更长的实验时间外, 还应该考虑: 1)多种资源各自及联合对根生长、生死过程的影响; 2)资源斑块和整个根系生长背景的资源丰度对比; 3)根构建和根维持的相对C消耗。

Proliferation and growth of plant fine roots and the influences from nutrient variation―implications from the split-root experiments of Ailanthus altissima, Callistephus chinensis and Solidago canadensis

Aims Modular theory of plants considers plant roots are relatively independent in resource absorbing and responding to heterogeneous soil environments, particularly resource environments. According to the cost-benefit theory, proliferation, growth and death of individual absorbing roots (modules) depend upon their resource uptake related to the carbon costs of their construction and maintenance, with a certain time-lag. Thus we hypothesized that: 1) a root will die when available nutrients are below a certain low threshold and last for a certain period and 2) new roots will emerge when available nutrients are above a certain high threshold and last for a certain period.
Methods We designed a greenhouse split-root experiment using three plant species: Ailanthus altissima, Callistephus chinensis and Solidago canadensis. The plants were grown individually in pots, and then three fine roots (uptaking roots) per plant were carefully sorted and placed in three plastic vessels of about 70 mL with one root per vessel. Three nutrient levels of 0, 20 and 200 μg N·g^–1 soil were applied in the three vessels. These roots were carefully exposed and photographed every four days, and the numbers of lateral roots, the length of 1st order laterals and the root length were evaluated. Repeated-measure ANOVA was used for statistical analysis.
Important findings The numbers of laterals and total root length differed significantly among the three species and under the three N levels. Both lateral numbers and total root length were the least in the 0 μg N·g^–1 treatment for A. altissima, and the highest in the 200 μg N·g^–1 level for S. canadensis. The length of 1st order laterals was less responsive than the other two measures. No fine roots were found dead during the experiment. These results demonstrated that the different species had different growth rates of fine roots under the same N treatments as expected, and indicated that different species may have different N thresholds. Results provided partial support for our hypotheses and hints for future experiments. We suggest that a sufficient examination of the hypotheses may require 1) a longer experiment period, 2) control of other vital resources such as water and other limiting nutrients and 3) consideration of the resource contrast between the treatment patches and the overall level. The relative contrast of C costs between root construction and root maintenance should also be considered.