Improving yield potential of tropical rice: Achieved levels and perspectives through improved ideotypes

Improving the genetic yield potential (YP) of tropical, irrigated rice varieties is a priority objective of rice breeding programs worldwide in the interest of achieving food security and maintaining political stability. But YP has stagnated at about 10 Mg ha−1 since the Green Revolution. We present a survey of researchers’ current top yields across different environments and countries, experimentally investigate YP-related traits and radiation use efficiency (RUE) of 12 elite materials, and use a simple model to explore traits that would raise the yield ceiling. The survey indicated that maximal grain yield is between 5 and 12 Mg ha−1 depending on radiation during flowering and grain filling. The experiments conducted in several environments in the Philippines indicated that (1) different morphologies in terms of panicle number and size and leaf size lead to similar YP due to trait–trait compensation, and (2) differences in RUE are partly attributable to variation in terminal senescence which is strongly environment dependent. Simulations thus focused on post-floral physiological processes, namely dynamics of light interception, carbon assimilation and maintenance burden. Scenarios of different degree of stay-green indicated that terminal senescence is essential to limit N requirements and maintenance burden, but partial stay-green would strongly benefit RUE and YP, particularly if accompanied with increased leaf photosynthetic capacity. The need to increase pre-floral C and N reserves for grain filling is discussed, resulting in a concept to refine current ideotypes such as IRRI’s New Plant Type and China’s Super Hybrid Rice. In conclusion, current best tropical breeding products do not have higher YP than some varieties dating 30–40 years, and new concepts are needed in rice breeding. Breeding for such plants should be done under high N inputs.