Abstract
Simple SummaryLeaves can only toughen after they have finished growing and, as a result, many herbivorous insects specialize in newly developing leaves because softer leaves are easier to chew. The foliage of conifer trees is particularly tough, and so one would expect conifers to invest more defensive chemicals into soft vulnerable growing needles than into tough mature ones. We summarize the literature describing how chemical defenses of foliage change during the growing season in white spruce, an economically important conifer tree. We next report measurements of the toughness of white spruce buds as they swell, burst, and grow into young needles. As expected, buds soften as they swell in spring, but after budburst, needles become tougher until they are similar to previous-year foliage in mid-summer. Leaves grown in the sun are slightly tougher than leaves grown in the shade. However, there was no indication that trees invest more in chemical defense of growing leaves than of mature leaves.Changes during leaf ontogeny affect palatability to herbivores, such that many insects, including the eastern spruce budworm (Choristoneura fumiferana (Clem.)), are specialist feeders on growing conifer leaves and buds. Developmental constraints imply lower toughness in developing foliage, and optimal defense theory predicts higher investment in chemical defense in these vulnerable yet valuable developing leaves. We summarize the literature on the time course of defensive compounds in developing white spruce (Picea glauca (Moench) Voss) needles and report original research findings on the ontogeny of white spruce needle toughness. Our results show the predicted pattern of buds decreasing in toughness followed by leaves increasing in toughness during expansion, accompanied by opposite trends in water content. Toughness of mature foliage decreased slightly during the growing season, with no significant relationship with water content. Toughness of sun-grown leaves was slightly higher than that of shade-grown leaves. However, the literature review did not support the expected pattern of higher defensive compounds in expanding leaves than in mature leaves, suggesting that white spruce might instead exhibit a fast-growth low-defense strategy.
Highlights
Expanding leaves are generally less tough and more nutritious than mature foliage [1,2,3]. This is directly linked to the process of leaf growth: young leaves cannot accumulate lignin, cellulose, or other cell wall components until leaves have ceased growing [4], and as a result, they are both low in toughness and high in nitrogen content
The significant interactions between foliage type and Julian date imply that the trajectory of toughness over the growing season differs between swelling buds, current year foliage and previous year foliage and that main effects cannot be directly interpreted
The toughness of expanding buds showed a negative relationship with Julian date (Pearson correlation coefficient: −0.64; p < 0.0001), and decreased with water content (Pearson correlation coefficient: −0.506; p = 0.0016) as well
Summary
Ontogenetic changes in foliage quality limit the period when the foliage is suitable for herbivorous insects, promoting tight phenological host–herbivore relationships that define a window of opportunity [12,13,14]. The phenology and ontogeny of defensive compound expression in foliage has received little attention, despite the fact that many defoliators on white spruce, including the notorious spruce budworm (Choristoneura fumiferana (Clem.)), preferentially feed on expanding foliage [21,22,23]. We first summarize past work on ontogenetic progression of defense traits in expanding white spruce foliage and highlight knowledge gaps for future research. We measured white spruce leaf toughness during the growing season in both expanding and mature foliage in order to improve understanding of the ontogenetic trajectory of mechanical defense during the window of opportunity for herbivores. Leaf physical trait data from this boreal conifer will contribute to generalize understanding of ontogenetic shifts in plant defense syndromes during leaf development [1]
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