Kadeem Jamal Gilbert, David W Armitage, Ulrike Bauer, Kenji Fukushima, Laurence Gaume, Rachel Love, Qianxi Lin, Sukuan Liu, Sylvie Martin-Eberhardt, Jonathan Millett, Tanya Renner, Mathias Scharmann, Chris Thorogood

Abstract
• Background
Leaf economic theory holds that physiological constraints to photosynthesis have a role in the coordinated evolution of multiple leaf traits, an idea that can be extended to carnivorous plants occupying a particular trait space that is constrained by key costs and benefits. Pitcher traps are modified leaves that may face steep photosynthetic costs: a high-volume, three-dimensional tubular structure may be less efficient than a flat lamina. While past research has investigated the photosynthetic costs of pitchers, the exact suite of constraints shaping pitcher trait variation remain under-explored—including constraints to carnivorous function.
• Scope
In this review, we describe various constraints arising from the dual photosynthetic and carnivorous functions of pitchers arising from developmental, functional, budgetary, and environmental factors. In addition, we identify the data required to establish the leaf economic spectrum (LES) for carnivorous pitcher plants (CPPs), and — owing to the multifunctional roles of pitcher leaves — discuss difficulties in placing pitchers onto existing frameworks.
• Conclusion
Because pitcher traps serve multiple functions, both photosynthesis and nutrient acquisition (carnivory), they are difficult to place in the context of the LES, especially in light of a current lack of trait data. We describe a spectrum across the independent CPP lineages in approaches to balancing carnivory-photosynthesis tradeoffs. Future efforts to collect relevant data can clarify the forces that shape observed pitcher trait variation, and increase understanding of principles that may be ultimately generalized to other plants.