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�. � �