Recent years have seen increasing attention paid to the idea of developing perennial agricultural crops as a means of producing needed food, fiber, forage and other agricultural products while maintaining higher levels of ecosystem services (less nutrient leaching, more carbon sequestration, healthier symbioses with fungi and insects). Challenges have been raised to the idea of perennial agriculture on the grounds that plant investment in seed yield and investment in longer term survival are incompatible processes because they compete for the same carbohydrate pool. In contrast it has been argued that plants can meet increased reproductive demand for carbohydrates through sink-regulation of photosynthesis, reproductive photosynthesis, and the increased total growth potential in the second year due to belowground investment in the first year.
We tested these hypotheses with hybrid perennial wheat plants, Triticum aestivum x Thinopyrum intermedium. Our guiding hypothesis was that moderate alterations to sink size (clipping reproductive seed heads) and to source size (removing leaf area) would cause compensatory changes in photosynthetic rates and carbohydrate storage. These hypotheses were generally supported: perennial wheat plants showed greater photosynthesis in source-reduced plants, and lower photosynthesis in sink-reduced plants, and showed no effect of moderate defoliation on seed yield. Research is ongoing to determine whether changes in total nonstructural carbohydrate levels (TNC) are mechanistically related to the changes in photosynthesis. This ability to match photosynthesis to carbohydrate demand suggests that perennial agricultural plants should, under selective breeding, be able to compensate for the increased carbon demand from reproductive and vegetative sinks, and that moderate increases in yield can come without necessarily reducing the ability for longer-term survival. In partial support of this hypothesis, second year perennial wheat plants due appear to be competitive with annual wheat under certain conditions, though first year plants do have lower yield at present.