The Sky's the Limit: Based On the Radiation and Precipitation It Provides.

While transgenic plants offer exciting possibilities for restructuring the genetic base of crops, it is valuable to remember that fundamental environmental limits on crop growth and yield remain unchanged.  Identification of these limitations is not new as evidenced in some of the classical analysis developed more than fifty years ago by Prof. C.T. deWit.  The basic physical and physiological constraints are not altered by transgenic events.  Energy available in intercepted photosynthetically active radiation (PAR) imposes the ultimate limit on the amount of crop mass and yield that can be synthesized.  For example, the practical limit in using intercepted radiation to produce crop mass is about 2.0 g MJ-1 PAR for soybean, 2.8 g MJ-1 PAR for rice and wheat, and 3.8 g MJ-1 PAR for maize and sugarcane.  In the case where a total of 800 MJ PAR m-2 can be intercepted by the crop in a growing season and the harvest index is 0.5, the yield limits are 8 t ha-1 for soybean, 11.4 t ha-1 for rice and wheat, and 15.2 t ha-1 for maize.  Also, there is a quantitative requirement for water in the physical gas exchange of water vapor when stomata open for the acquisition of carbon dioxide.  The transpiration efficiency coefficient is about 5 Pa for soybean, 6 Pa for rice and wheat and 9 Pa for maize and sugarcane.  Assuming a growing season where the average vapor pressure deficit experienced during periods of transpiration is 2 kPa and the total water available for transpiration is 350 mm, the yield limits are 8.8 t ha-1 for soybean, 10.5 t ha-1 for wheat, and 15.7 t ha-1 for maize.  Since the PAR and water yield limits are consistent with current maximum yields, expectations for increasing yield limits with transgenic plants need to be consistent with basic, realistic analyses of “the sky’s the limit”.