Spatio-Temporal, Genotypic, and Environmental Effects on Plant Soluble Protein and Digestible Carbohydrate Content: Implications for Insect Herbivores with Cotton as an Exemplar.

Plant soluble protein and digestible carbohydrate content significantly affect insect herbivore fitness, but studies reporting plant protein and carbohydrate content are rare. Instead, the elements nitrogen and carbon often are used as surrogates for plant protein and digestible carbohydrate content, respectively. However, this is problematic for two reasons. First, carbon is found in all organic molecules, which precludes strong correlations with ecologically important dietary macronutrients (e.g., digestible carbohydrates, the primary energy source for most insect herbivores). Second, some elements (e.g., nitrogen) are present in both macronutrients (e.g., protein) and non-nutritive secondary compounds (e.g., alkaloids, protease inhibitors); in these cases N values would greatly overestimate protein available for an insect herbivore. Thus, the objective of this study was to explicitly document plant protein-carbohydrate content and assess its variation in cotton (Gossypium hirsutum and G. barbadense), which is a nutritional resource for a number of insect herbivores. We did this by measuring plant soluble protein (P) and digestible carbohydrate (C) content across seven plant tissues, five varieties, and two growing environments. Significant differences in P and C concentration, total macronutrient content (P + C), and P:C ratio were observed across plant tissues, plant age and environment; smaller differences were seen across plant genotype. Foliar tissues had higher total P + C content compared to reproductive tissues, except for developing seeds and developing flowers, which contained twice the total P + C content; these two tissues also had the highest P content. Our data show that even agricultural monocultures offer a highly heterogeneous protein-carbohydrate landscape for insect herbivores. Characterizing plant resources using nutritional currencies (e.g., protein and carbohydrates) that are ecologically and physiologically-relevant to insect herbivores can be used to enhance our understanding of plant-insect interactions.