Artificial light at night (ALAN) has been shown to alter aspects of plant growth, but we are not aware of any studies that have examined whether the effects of ALAN on plants depend upon the backdrop of variation in other abiotic factors that plants encounter in field populations. We conducted a field experiment to investigate whether ALAN affects the growth and anti-herbivore defenses of common milkweed, Asclepias syriaca, and whether the effects of ALAN are influenced by plant density or soil moisture content. Artificial light at night, soil moisture, and plant density were manipulated according to a split-plot factorial design. Although increasing soil moisture by watering had no significant effects on latex exudation, attributes of plant growth generally responded positively to watering. The basal stem diameter (BSD) and height of plants were affected by ALAN × soil moisture interactions. For both of these variables, the positive effects of ALAN were greater for plants that were not watered than for plants that were. Basal stem diameter was also affected by an ALAN × plant density interaction, and the positive effect of ALAN on BSD was greater in the low-density treatment than in the high-density treatment. Our results demonstrate that the effects of ALAN on plant growth can be altered by soil moisture and plant density. Consequently, the effects of ALAN on plants in nature may not be consistent with existing frameworks that do not account for critical abiotic variables such as water availability or biotic interactions between plants such as competition.

夜间人造光（ALAN）已被证明能改变植物生长的各个方面，但我们尚无任何研究检查ALAN对植物的影响是否取决于植物在田间遇到的其他非生物因素的变化背景。人口。我们进行了一项野外实验，以研究ALAN是否会影响普通马利筋（Asclepias syriaca）的生长和抗草食动物防御以及ALAN的影响是否受植物密度或土壤水分含量的影响。根据裂区因子设计对夜间的人造光，土壤湿度和植物密度进行了控制。尽管通过浇水增加土壤水分对乳胶的渗出没有显着影响，但是植物生长的属性通常对浇水有积极的反应。植物的基茎直径（BSD）和高度受ALAN×土壤水分相互作用的影响。对于这两个变量，未浇水的植物对ALAN的积极影响要大于未浇水的植物。基茎直径也受ALAN×植物密度相互作用的影响，在低密度处理中，ALAN对BSD的积极作用大于在高密度处理中。我们的结果表明，土壤水分和植物密度可以改变ALAN对植物生长的影响。因此，ALAN对自然植物的影响可能与现有框架不一致，该框架没有考虑关键的非生物变量（例如水的可利用性）或植物之间的生物相互作用（例如竞争）。