Quantity over quality: light intensity, but not red/far‐red ratio, affects extrafloral nectar production in Senna mexicana var. chapmanii

Abstract

Extrafloral nectar (EFN) mediates food‐for‐protection mutualisms between plants and insects and provides plants with a form of indirect defense against herbivory. Understanding sources of variation in EFN production is important because such variations affect the number and identity of insect visitors and the effectiveness of plant defense. Light represents a potentially crucial tool for regulating resource allocation to defense, as it not only contributes energy but may help plants to anticipate future conditions. Low red/far‐red (R/FR) light ratios can act as a signal of the proximity of competing plants. Exposure to such light ratios has been shown to promote competitive behaviors that coincide with reduced resource allocation to direct chemical defenses. Little is known, however, about how such informational light signals might affect indirect defenses such as EFN, and the interactions that they mediate. Through controlled glasshouse experiments, we investigated the effects of light intensity, and R/FR light ratios, on EFN production in Senna mexicana var. chapmanii. Plants in light‐limited conditions produced significantly less EFN, and leaf damage elicited increased EFN production regardless of light conditions. Ratios of R/FR light, however, did not appear to affect EFN production in either damaged or undamaged plants. Understanding the effects of light on indirect defenses is of particular importance for plants in the threatened pine rockland habitats of south Florida, where light conditions are changing in predictable ways following extensive fragmentation and subsequent mismanagement. Around 27% of species in these habitats produce EFN and may rely on insect communities for defense.

Changing light conditions in pine rockland habitats affect the intensity and outcome of ant–plant interactions

“…, Yamawo & Hada , Izaguirre et al . ), including S. chapmanii (Jones & Koptur 2015b). Yamawo and Hada (), for example, found that EFN production in Mallotus japonicus was increased at high light intensities.…”

“…For each plant, combined nectar volume from the five most recently matured leaves on the chosen branch was measured using 1‐, 2‐, and 10‐ μ l micropipettes. Nectar concentration was measured using a handheld refractometer, and total sugar production was estimated from the combination of these measurements (see Jones & Koptur ,b). In the results, EFN is expressed as total sugar production.…”

“…). We have shown that light intensity significantly affects EFN production in S. chapmanii (Jones & Koptur 2015b). Here we sought to investigate how changing light conditions might affect ant–plant interactions in S. chapmanii , as a model for the potential effects on the many other EFN producing species in the pine rocklands.…”

“…Extrafloral nectar production has been observed to increase in response to high light intensity (Yamawo & Hada , Jones & Koptur 2015b). Furthermore, the induction of EFN production, either through treatment with jasmonic acid (Radhika et al .…”

“…chapmanii (hereafter referred to as Senna chapmanii ) (Fabaceae), is a perennial low‐lying shrub native to the pine rocklands of south Florida and the Caribbean. The species bears single globe‐shaped EFNs on the rachis between the first pair of leaflets, as well as on the pedicels, which are commonly patrolled by ants (Jones & Koptur ,b, Koptur et al . ).…”