Data from: The effect of nitrogen availability and water conditions on competition between a facultative CAM plant and an invasive grass

Abstract Plants with crassulacean acid metabolism (CAM) are increasing their abundance in drylands worldwide. The drivers and mechanisms underlying the increased dominance of CAM plants and CAM expression (i.e., nocturnal carboxylation) in facultative CAM plants, however, remain poorly understood. We investigated how nutrient and water availability affected competition between Mesembryanthemum crystallinum (a model facultative CAM species) and the invasive C3 grass Bromus mollis that co-occur in California's coastal grasslands. Specifically we investigated the extent to which water stress, nutrients, and competition affect nocturnal carboxylation in M. crystallinum. High nutrient and low water conditions favored M. crystallinum over B. mollis, in contrast to high water conditions. While low water conditions induced nocturnal carboxylation in 9-week-old individuals of M. crystallinum, in these low water treatments, a 66% reduction in nutrient applied over the entire experiment did not further enhance nocturnal carboxylation. In high water conditions M. crystallinum both alone and in association with B. mollis did not perform nocturnal carboxylation, regardless of the nutrient levels. Thus, nocturnal carboxylation in M. crystallinum was restricted by strong competition with B. mollis in high water conditions. This study provides empirical evidence of the competitive advantage of facultative CAM plants over grasses in drought conditions and of the restricted ability of M. crystallinum to use their photosynthetic plasticity (i.e., ability to switch to CAM behavior) to compete with grasses in well-watered conditions. We suggest that a high drought tolerance could explain the increased dominance of facultative CAM plants in a future environment with increased drought and nitrogen deposition, while the potential of facultative CAM plants such as M. crystallinum to expand to wet environments is expected to be limited.

摘要 具备景天酸代谢（crassulacean acid metabolism, CAM）途径的植物，其在全球旱地的种群丰度正逐步提升。然而，学界对CAM植物优势度提升的驱动因素与机制，以及兼性CAM植物体内的CAM表达（即夜间羧化作用）仍缺乏深入认知。本研究以加利福尼亚沿海草原中同域分布的模式兼性CAM物种冰叶日中花（Mesembryanthemum crystallinum）与入侵性C3草本绒毛雀麦（Bromus mollis）为研究对象，探究养分与水分可利用性对二者种间竞争的影响。本研究还专门考察了水分胁迫、养分供给与种间竞争对冰叶日中花夜间羧化作用的影响程度。与高水分条件相反，高养分、低水分的生境更有利于冰叶日中花而非绒毛雀麦的生长。尽管低水分条件可诱导9周龄的冰叶日中花个体产生夜间羧化作用，但在这些低水分处理组中，整个实验期间将养分供给降低66%，并未进一步增强其夜间羧化活性。在高水分条件下，无论养分水平如何，单独生长或与绒毛雀麦伴生的冰叶日中花都不会进行夜间羧化作用。由此可见，高水分条件下，冰叶日中花的夜间羧化作用会因与绒毛雀麦的激烈种间竞争而受到抑制。本研究为兼性CAM植物在干旱条件下相较于草本植物的竞争优势，以及冰叶日中花在水分充足条件下利用光合可塑性（即转换为CAM代谢途径的能力）与草本植物竞争的能力受限，提供了实验证据。我们推测，较强的耐旱性可解释未来干旱加剧与氮沉降背景下，兼性CAM植物优势度提升的现象；而诸如冰叶日中花这类兼性CAM植物向湿润生境扩张的潜力则较为有限。