Effects of Simulated and Insect Herbivory on Total and Protein Percipitable Phenolic Concentrations of Two Legumes

There were differences between simulated and grasshopper herbivory types. Although N was similar for both species, PPP and PB were different between herbivory types.

Nitrogen

Panicled tick-clover N concentration was similar between mechanical and grasshopper herbivory, although concentrations within herbivory types were different. Nitrogen concentration increased by 26% and 40%, respectively, across the 50% and 100% simulated defoliation treatments compared to the control but not across the grasshopper treatments (Table 1). While N concentration increased 14% between Intensity 1 and 2, it declined 9% from Intensity 2 to 3.

Nitrogen concentrations were also similar for SL between mechanical and grasshopper treatments. Sericea lespedeza N concentration decreased for most treatments compared to the control, but remained unchanged for the 100% simulated defoliation (Table 2). Within herbivory type, N concentration increased 12% between Intensities 1 and 2 but remained unchanged for Intensity 3. With repeated herbivory, N concentration increased for moderate herbivory (Intensities 1 and 2 and 50% defoliation) but decreased with greater herbivory (Intensity 3 and 100% defoliation).

The increase and subsequent decrease in N concentration with herbivory intensity is consistent with previous studies (Rooke et al. 2007; Schädler et al. 2007; Cooper et al. 2014). We agree with Cooper et al. (2014) that lower N concentration may be due to N mobilization for leaf production (Culvenor and Simpson 1991); greater herbivory intensities result in fewer leaves to photosynthesize, so the plant allocates resources to produce more leaves (Culvenor and Simpson 1991).

Alternatively, the reduction in N could be the result of nutrient stress (Bryant et al. 1993). Since the plants were grown in pots and unfertilized for the season, N was a limited resource outside of any N fixation, which may have been affected by a number of variables, such as exhausted soil nutrients such as P, photosynthetic rates, bacterial genotype, and environmental factors not accounted for by this study (West et al. 2005).

Similar trends between simulated and natural herbivory treatments are consistent with some studies (Lyytik?inen-Saarenmaa 1999) but not others (Baldwin 1990). Likewise, the tendency for SL N concentration to decline with increasing herbivory intensity was consistent with the findings of Cooper et al. (2014), with the exception of the 100% defoliation which was similar to the control. Lower N availability can limit feed efficiency by reducing urea recycling and microbial efficiency in ruminants (Reed 1995) and other forages (Mangan 1988).

Protein Precipitable Polyphenolics (PPP)

Panicled tick-clover PPP concentrations were lower for mechanical herbivory than grasshopper herbivory, with the exception of 50% defoliation which was similar to Intensity 1. Protein precipitable polyphenolic concentration decreased with increasing herbivory intensity (Table1). Within insect herbivory, PPP concentration increased 37% between Intensities 1 and 2 but decreased 12% between Intensities 2 and 3. Similarly, PPP decreased 20% between 50% and 100% defoliation. Protein precipitable phenolic concentration increased among all treatments with repeated herbivory, excluding 100% defoliation, which decreased 75%.

Sericea lespedeza PPP was similar between mechanical and grasshopper treatments for Harvest 1 but not 2. Protein precipitable polyphenolic concentration decreased an average of 20% as herbivory increased regardless of herbivory type (Table 2). Protein precipitable polyphenolic concentration decreased a maximum of 26% from control to Intensity 1 and a minimum of 15% each from Intensity 2 to 3 and from the control to 50% defoliation. Protein precipitable polyphenolics increased from Harvest 1 to 2 for all treatments except Intensity 1 and 50% defoliation, which decreased by 16 and 24%, respectively.

Panicled tick-clover PPP concentrations were inconsistent with a previous study. Cooper et al. (2014) found that PPP concentration increased from unclipped leaves to those submitted to 25% defoliation before declining for the remaining treatments. These results parallel what we observed following the second harvest, but not the first, suggesting that insect defoliation affects naive plants in a different way. Once-stressed regrowth never attained the intensity of the control plant, while repeatedly stressed regrowth remained above the intensity of the control with the exception of major defoliation (grasshopper Intensity 3 and 50% and 100% mechanical defoliation). Donnelly and Anthony (1983) reported greater intensities of tannins in plants repeatedly defoliated than in plants defoliated once. The reduction in PPP concentration with increased defoliation is consistent with a study observing Quercus spp., in which leaf PPP on previously defoliated branches decreased compared to control branches (Faeth et al. 1992).

Sericea lespedeza PPP concentrations following grasshopper herbivory differed from what Cooper et al. (2014) reported for mechanical harvests. Cooper et al. (2014) observed a decrease in leaf PPP concentration between 0 and 25% defoliation followed by an increase under more intense herbivory. This suggests differences in plant response between low intensity natural and simulated herbivory.

An additional simulated herbivory treatment for both species would be required in this study to directly compare my simulated results with Cooper et al. (2014). The decrease in PPP for both species from 50% to 100% defoliation, however, is similar, suggesting that there may be distinctive responses in leaf PPP accumulation for herbivory types. This phenomenon has been supported by other studies (Kessler and Baldwin 2002), including other physiological processes (Baldwin 1988; Kessler and Baldwin 2002) and herbivores (Ward and Young 2002). However, it has also been contradicted by other studies, such as those addressed in the meta-analysis by Nyk?nen and Koricheva (2004).

Protein bound by PPP (PB)

Panicled tick-clover PB was much lower for simulated herbivory than grasshopper herbivory, and 50% defoliation was similar to the control for Harvest 1. Panicled tick-clover PB increased by 17% between grasshopper Intensities 1 and 2 but declined by 14% between Intensities 2 and 3 (Table 1). Simulated treatments also caused a 100% decrease in PB, from the first to the second harvest.

Protein bound by PPP was also lower for SL simulated herbivory treatments than grasshopper herbivory, although all treatments were at least 50% greater among herbivory treatments compared to the control (Table 2). There were no differences in PB between the simulated treatments for the first harvest; concentrations of PPP in leaves submitted to 100% mechanical defoliation, however, declining by 48%, twice as much as for the 50% defoliation.

Panicled tick-clover PB dynamics as a result of herbivory were consistent with those reported by Cooper et al. (2014) for simulated herbivory of PTC and by Alba-Meraz and Choe (2002) for Melanoplus herbivory of other plant species. Protein binding affinity affects ruminant nutrition since it prevents microbial degradation in the rumen (Silanikove et al. 2001; Min and Hart 2003; Pawelek et al. 2008). Panicled tick-clover PB declined following 100% defoliation in this study as well as in that reported by Cooper et al. (2014) corresponding to the costly requirements of making protein-binding compounds (Coley et al. 1985; Kein?nen et al. 1999), since polyphenolic production is reliant on photosynthesis to fix carbon (Bryant et al. 1983; McDonald et al. 1999). These results do not support some hypotheses which predict that more resources are spent on defenses in high intensity herbivory conditions (Janzen 1974; Coley et al. 1985). McDonald et al. (1999) found that increased CO₂ availability increases polyphenolic concentrations in certain trees. Starch, a product of photosynthesis (Sharkey 1985), is also positively correlated with certain types of tannins, including CT (Lawler et al. 1997; McDonald et al. 1999). Even in nutrient-rich conditions, CT were much lower in low light (thus low photosynthetic activity) conditions for Eucalyptus spp., possibly mirroring conditions with little to no leaves (Lawler et al. 1997), although severe defoliation increases polyphenolic response in some species (Bryant et al. 1993).

Compared to PTC, SL PB changes as a result of herbivory were inconsistent, with no apparent trends across treatments. For example, both harvests types increased leaf PB concentration between the control and Intensity 1 and decreased between Intensity 1 and 2, but were similar in simulated treatments in Harvest 1 while they decreased between simulated treatments in Harvest 2. Cooper et al. (2014) found that defoliation resulted in less leaf PB than in undefoliated plants, in contrast to responses to both herbivory types in this study. This may be due at least in part to differences in how simulated herbivory treatments were applied (Baldwin 1990). Cooper et al. (2014) removed leaves a pre-measured distance from the top of the plant to simulate ruminant herbivory, while we harvested equally from all heights to simulate grasshopper herbivory as observed in a pilot study at my greenhouse.