The adaptive value of young leaves being tightly folded or rolled on monocotyledons in tropical lowland rain forest: an hypothesis in two parts

Abstract

In tropical lowland rain forest, we find that species with the leaves tightly folded or rolled until they reach at least 50% of final length occur in 10 of the 15 monocot families with >100 species, and in 12 of the 24 monocot families with <100 species, but in only seven of the 212 dicot families (eudicots and magnoliids). Earlier researchers have described how examples of tightly folded and rolled leaves develop, but most have not considered the potentially adaptive value of this pattern of growth. We hypothesize that it is a protection against herbivorous invertebrates. For tropical and temperate dicots, the young leaves have been found to suffer much smaller losses to herbivores while folded and rolled than after they are unfolded or unrolled. Being folded or rolled until a late stage involves an 'opportunity cost' in the loss of photosynthesis. Among dicots, defences involving such a cost (notably late development of photosynthetic systems in pendent soft young leaves) are typical of shade-tolerant species, which have longer-lived leaves than light-demanders. In contrast, among monocots late folding and rolling are found in both shade-tolerators and light-demanders. We hypothesize that late folding and rolling bring a net advantage to monocots in general, whether shadetolerant or light-demanding, despite the opportunity cost, because they mostly have fewer leaves per plant of a given size, and therefore an individual leaf is relatively more valuable to the plant. As a coda, we suggest that the 'sleep movements' of some tropical plants, and the circinate vernation of ferns and some cycads, provide protection against invertebrate herbivores through the apposition of two or more layers of leaf.

What Defenses Do Plants Use?

“…Indeed, an examination of species, where young leaves remained tightly folded or rolled until they had reached more than 50% of their mature length, showed that 56% of monocots but only 3.3% of dicots fell into this category (Grubb & Jackson, 2007). Indeed, an examination of species, where young leaves remained tightly folded or rolled until they had reached more than 50% of their mature length, showed that 56% of monocots but only 3.3% of dicots fell into this category (Grubb & Jackson, 2007).…”

The role of ducking stems in plant anti-herbivore defense and its exploitation by herbivorous insects

“…Quercus alba trees with few leaves touching each other suffer less damage from leaftying caterpillars than do those with many touching leaves (Marquis et al 2002). Furthermore, tightly folding or rolling young leaves of monocotyledons in tropical lowland rainforests may deter insect herbivory (Grubb and Jackson 2007).…”

“…The roles of the architectural traits of plants in herbivory resistance have received less attention than have chemical and physical traits (Lawton 1983;Grubb and Jackson 2007), but have recently attracted more interest (Rudgers and Whitney 2006;Wise et al 2010a). As ducking stems are widely distributed among various plant taxa, growth forms and regions (Table 1), this architectural resistance is likely to be universal.…”

Herbaceous monocot plant form and function along a tropical rain-forest light gradient: a reversal of dicot strategy

“…To date interspecific comparative analyses of tropical monocots focusing on leaf traits suggest that herbaceous monocots may have a gradient in leaf form and function along light gradients that is Email: nswenson@oeb.harvard.edu the opposite of that found in dicot trees (Cooley et al 2004, Rundel et al 1998. Unfortunately, these results arise from separate analyses of leaf traits from taxa spanning multiple plant orders and there exist few detailed comparative analyses of leaf and stem function across these gradients (Dominy et al 2008, Grubb & Jackson 2007. The above studies have also generally not taken phylogenetic relatedness into account in their design and analyses (but see Dominy et al 2008).…”