Development-dependent effects of UV radiation exposure on broccoli plants and interactions with herbivorous insects

The responses of plants to stress can highly depend on their developmental stage and furthermore influence biotic interactions. Effects of outdoor exposure to different ambient radiation conditions including (+UV) or excluding (−UV) solar ultraviolet radiation were investigated in broccoli plants (Brassica oleracea L. convar. botrytis) at two developmental stages. Plants either germinated directly under these different outdoor UV conditions, or were first kept for three weeks in a climate chamber under low radiation before outside exposure at +UV and −UV. Access of herbivores to the plants was possible under the outdoor conditions. Plants of both groups protected their tissue against destructive UV by increasing concentrations of phenolic compounds (flavonoids and hydroxycinnamic acids) after +UV exposure. But only plants that germinated under +UV conditions kept smaller than plants grown under −UV conditions, indicating certain costs for production of phenolics or for other potential metabolic processes specifically in young, growing plants. In contrast, growth of plants transferred at a later stage did not differ under both UV conditions. Thus, plants responded much more sensitive to the environment they experienced at first growth. Glucosinolates, the characteristic secondary compounds of Brassicaceae, as well as proteinase inhibitors, remained unaffected by UV in all plants, demonstrating independent regulation pathways for different metabolites. Plant infestation by phloem-feeding insects, Aleyrodidae and Aphididae, was more pronounced on +UV exposed plants, whereas cell content feeders, like Thripidae were more abundant on plants under the −UV condition. Choice experiments with the cabbage whitefly Aleyrodes proletella L. (Aleyrodidae), commonly found on Brassica spp., revealed that the key environmental cue navigating their behaviour seems to be the radiation composition, rather than plant quality itself. In conclusion, stress mediated changes of plant chemistry and morphology depend on the plant life cycle stage and are not necessarily mirrored in the behavioural responses of herbivorous insects.