Macronutrient content of cotton (Gossypium hirsutum) tissues across genotypes and environments: an indicator of resource quality for insect herbivores

Background/Question/Methods

Understanding the nutritional preferences and constraints on insect herbivores is integral to predicting when and where they will occur in ecosystems. Advances in the fields of nutritional ecology and physiology have shown that the balance of dietary macronutrients, particularly protein and carbohydrates, have significant impacts on insect performance. Despite this, information regarding plant macronutrient content is poorly documented, and data on the variability of these nutrients within individual plants and between species and environments are even more obscure. Elemental data on nitrogen (N) and carbon (C) content is common in the literature; however, it is difficult to accurately extrapolate protein and carbohydrate content from these data given that a large portion of plant N and C pools include non-nutritional compounds, such as cellulose/lignin, or toxic compounds, such as C- and N-containing allelochemicals. In this study protein and digestible carbohydrate content of cotton tissues (true leaves, terminal growth, squares, and bolls) were measured, as well as N and C, across several varieties in the field and greenhouse to: (1) determine the macronutrient content of different cotton tissues, (2) compare these concentrations across different varieties and environments, (3) and to determine how total N and C measures correspond to macronutrient concentrations. 

Results/Conclusions

Results show that there was a significant amount of variation in macronutrient content across tissues and environments; however, there were no differences in either nutrient across varieties. In both environments bolls had the highest carbohydrate content and the lowest protein content, a surprising result considering that many insect herbivores are thought to be more protein-limited and feed preferentially on fruiting bodies. In general, the foliar tissues had relatively high amounts of both carbohydrates and protein. Under greenhouse conditions terminal growth, true leaves, and squares had similar carbohydrate values, while true leaves and squares showed a significant reduction in carbohydrate content in the field. There was also a significant decrease in true leaf protein content in the field. Overall, there was greater variability in protein content across tissues in the greenhouse and greater variability in carbohydrate content across tissues in the field. These results show that cotton macronutrient content is highly variable across different tissues, providing a broad nutritional gradient for insect herbivores even within a single plant. These results also show that plant nutrient content is likely to be a large contributor to the environmental variability in these systems, which have important implications for insect performance and pest population dynamics.