Abstract
Glucosinolates are plant secondary metabolites used in plant defense. For insects specialized on Brassicaceae, such as the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), glucosinolates act as “fingerprints” that are essential in host plant recognition. Some plants in the genus Barbarea (Brassicaceae) contain, besides glucosinolates, saponins that act as feeding deterrents for P. xylostella larvae, preventing their survival on the plant. Two-choice oviposition tests were conducted to study the preference of P. xylostella among Barbarea leaves of different size within the same plant. P. xylostella laid more eggs per leaf area on younger leaves compared to older ones. Higher concentrations of glucosinolates and saponins were found in younger leaves than in older ones. In 4-week-old plants, saponins were present in true leaves, while cotyledons contained little or no saponins. When analyzing the whole foliage of the plant, the content of glucosinolates and saponins also varied significantly in comparisons among plants that were 4, 8, and 12 weeks old. In Barbarea plants and leaves of different ages, there was a positive correlation between glucosinolate and saponin levels. This research shows that, in Barbarea plants, ontogenetical changes in glucosinolate and saponin content affect both attraction and resistance to P. xylostella. Co-occurrence of a high content of glucosinolates and saponins in the Barbarea leaves that are most valuable for the plant, but are also the most attractive to P. xylostella, provides protection against this specialist herbivore, which oviposition behavior on Barbarea seems to be an evolutionary mistake.
Highlights
According to the optimal defense theory, the most valuable parts of a plant should be the most protected against herbivores [1,2]
In agreement with this theory, it has been found that, within a plant, different organs and leaves can contain different concentrations of defense compounds [4,5]. This the case for glucosinolates, plant secondary metabolites used for defense and found mainly in plants of the order Brassicales [6,7], which have been found in higher concentrations in younger compared to older leaves within the same plant [4,8,9,10,11]
A significant negative relationship was found between leaf size and content of glucosinolates for both G-type (y = 13.01–0.95x; n = 100; r = 0.39; F1,98 = 17.01; P#0.001) and P-type B. vulgaris (y = 14.81–0.86x; n = 20; r = 0.44; F1,19 = 4.20; P = 0.050) (Figs. 1A and 1B)
Summary
According to the optimal defense theory, the most valuable parts of a plant should be the most protected against herbivores [1,2]. Young leaves are supposed to be more valuable than older ones because they can make a higher contribution to the fitness of the plant as a result of having relatively higher photosynthetic potential [3]. In agreement with this theory, it has been found that, within a plant, different organs and leaves can contain different concentrations of defense compounds [4,5]. Saponin content changes over time at the whole plant level, often increasing with plant age, decreases with plant age and seasonal fluctuations have been recorded [21,22,23]. Br. (Brassicaceae), the only one in Brassicaceae where saponins have been found so far [24,25,26,27,28], seasonal fluctuations in saponin content seem to occur as inferred by changes in resistance to the flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae) [29,30]
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