Perennial wheat and perennial rye are high yielding, perennial hybrids of annual wheat and rye with close wild relatives, which have been developed as alternative food crops. These species occupy an unusual ecological niche as they combine high resource allocation (ca. 25-50% harvest index) with a perennial life history. These levels of reproductive allocation are almost nonexistent among naturally occuring herbaceous perennials, and raise the question of how they manage to balance allocation to large, competing resource sinks: reproduction and survival. In a 2011 study of perennial wheat and perennial rye, comparing both against their annual relatives, we experimentally adjusted the sink/source ratio at multiple points during the growing season, to alter the resource status of the plants. Sink/source manipulations were carried out by covering up leaf area or removing florets, and physiological and life history responses were studied.
In general, perennial cereals responded to sink/source manipulations through several strategies. Photosynthetic rates were elevated or lowered (by around 10-25%) in response to higher or lower sink strength, while carbohydrate depletion in stem and leaf tissues also depended on sink strength. Perennial species (in contrast to annuals) maintained relatively stable seed size across all the sink/source manipulation treatments, showing that they were able to buffer the effects of changing sink strength. Finally, the more resource depleted perennial plants failed to flower a second time in the fall, and showed lower photosynthetic rates; they recovered completely from the effect of the treatments by the following year, however, and showed no long term effect. Thus perennial wheat and rye appear to respond to changes in resource status and sink strength by a combination of photosynthetic, biochemical and life history adjustments, which allow them to maintain stable seed size and to survive year to year. Perennial cereals appear to have more capacity to respond to changes in resource status/sink strength than annuals, and more capacity to maintain stable seed size, which could indicate that the annual species are currently operating close to their maximal photosynthetic rates. These results are both indicative of the capacity of perennial cereals to be feasible crops for the future, and also help us better understand how highly productive perennials balance survival vs. reproduction tradeoffs.