Foliar bacteria mediate leaf traits and photosynthetic performance of tropical tree seedlings in deeply shaded understory habitats
One of the world’s largest and least-explored habitat is the surface of leaves, the phyllosphere. The phyllosphere hosts an array of bacterial and fungal epiphytes that may be benign, mutualistic, or pathogenic. Bacteria can be particularly abundant both on and inside of leaves. The degree to which these bacteria either enhance or inhibit physiological performance in situ is unexplored. Moreover, any impact of these bacteria is likely to be mediated by the supply rate of soil nutrients that are often co-limiting to plants even in deeply shaded understories. We tested two hypotheses: 1. The net effect of reducing the abundance of bacteria both on and inside plant leaves will enhance physiological performance and decrease leaf defensive traits; 2. Increasing the supply rate of co-limiting soil resources will mitigate the negative effect of bacteria on seedling physiological performance. We tested these two hypotheses by spraying seedlings of five common tree species with antibiotics or water (control) every 10-14 days for nearly three years. Our seedlings were nested within a factorial nutrient fertilization experiment where nitrogen, phosphorus, and potassium have been added to mature lowland Neotropical forest in Panama for over 15 years. We measured leaf toughness, thickness and specific leaf area, as well as photosynthetic performance at the end of the experiment.
The antibiotic treatment reduced bacterial abundance and diversity both inside and on the surface of leaves measured each of four times through the course of the experiment. Antibiotic treatment increased photosynthetic performance and caused the development of thicker but less tough leaves. Thus the net effect of reducing bacteria was to enhance physiological performance strongly suggesting that the bacteria were pathogenic. There were important species by antibiotic treatment interactions for both morphological and physiological traits. This suggests that some species are more vulnerable to bacteria than others and that bacteria may mediate long-term plant persistence in the understory. In addition, we found that soil resources often limited plant performance and that bacteria likely were key in mediating these responses. Our results are some of the first to demonstrate that bacteria reduce plant performance in natural habitats and may be particularly important in the understory of tropical forests where these bacteria are abundant and diverse.