Abstract
Respiration is an essential link between assimilation of carbon and energy in photosynthesis and subsequent growth and yield of all crops. It provides usable energy and biochemical building blocks (carbon skeletons) required for growth and maintenance, releasing CO2 and energy as byproducts in the process. Field data indicate that from about 30% to well over 50% of photosynthetically assimilated carbon (net of photorespiration) is released as respiratory CO2 over a season, varying between crops, developmental stages, and environments. Unstressed crops have a relatively small CO2-based respiration/photosynthesis (R/P) ratio during early vegetative growth, but it can markedly increase with ontogeny, especially during reproductive growth in oilseed crops, and with stress. It is suggested that considerations of energy transformations, in addition to CO2 exchanges, provide meaningful insights into respiration-photosynthesis-yield relationships. Both rate and efficiency of respiration are important, with efficiency of ‘growth respiration’ relatively well understood, and perhaps near potential maximum values in crops. Conversely, a minimum crop ‘maintenance respiration’ requirement remains unclear. The contribution that inadvertent selection for favorable respiration made to past yield gains is unknown but may have been significant in maize at least. The quantitative importance of respiration to crop carbon and metabolic energy budgets raises the questions, Is a fraction of present crop respiration inefficient or nonessential, is there genetic variation associated with that fraction that can be used in selection, and can directed enzyme evolution improve efficiency of respiration or curtail any unproductive components? To the extent that maintenance processes such as protein turnover and active transport to counteract leaks across membranes can be reduced without detriment, they are targets for modification to increase yield. Technologies now available to quantify crop respiration, at least of individual organs, can be put into action for high-throughput screening, and advances in bioengineering imply that modifications to respiration need not be limited by existing genetic variation in crops and their wild relatives. In short, yield gains via directed respiratory changes are possible, though prospects for success may require a moderate shift in research priorities from photosynthesis to respiration as a path to crop improvement.
Published Version
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