Biomass production is dependent on carbohydrates fixed in photosynthesis.  Because of the importance of photosynthesis for plant growth, photosynthesis and its responses to environmental stress have been studied in much detail.  The efficiency with which these carbohydrates subsequently are converted into plant biomass is much less studied.  This carbon use efficiency can be defined as the net amount of carbon incorporated into plants (gross photosynthesis minus respiration) divided by the total amount of carbohydrates produced (gross photosynthesis).  Respiration at times has been seen as a wasteful process that reduces biomass production of plants.  This view has resulted in unsuccessful efforts to select for faster growing genotypes by selecting for lower respiration rates.  Approaching respiration as an inherently wasteful process neglects the fact that respiratory CO₂ losses are an essential and necessary part of plant metabolism.  A more nuanced view of respiration can be developed by dividing respiration into two components, 1) respiration directly related to processes involved in plant growth (growth respiration) and 2) other respiration, generally considered to be the metabolism needed to maintain existing plant tissues (maintenance respiration).  Carbon use efficiency is maximal when maintenance respiration accounts for a small component of the plant’s carbon balance. In general, the importance of maintenance respiration increases as plant size increases, and carbon use efficiency drops as a result.  Based on the efficiency of metabolic pathways in plants, the theoretical maximum for carbon use efficiency is about 0.70 (or 70%) when ‘typical’ plant tissues are produced and maintenance respiration is negligible. The remaining 30% of carbon is lost through growth respiration required to support the needed metabolism. A better understanding of how growth and maintenance respiration respond to environmental stresses can help in understanding how plant growth is affected by those stresses.