Photosynthetic light and CO2 utilization and C4 traits of two novel super-rice hybrids

Characteristics of photosynthetic light and CO2 use efficiency from seedling to heading stage, and C4 pathway enzyme activities in both flag leaves and lemma were compared between two newly developed super-rice hybrids (Oryza sativa L.), Liangyoupeijiu and Hua-an 3, and a traditional rice hybrid, Shanyou 63. At seedling and tillering stages, Liangyoupeijiu and Hua-an 3 had higher net photosynthetic rates (Pn) and light saturated assimilation rates (Asat) than did Shanyou 63, at both normal (360 micromol mol(-1)) and doubled (720 micromol mol(-1)) CO2 concentrations. At the heading stage, the flag leaves of all three rice hybrids had similar Pn and Asat. However, the two super-rice hybrids had higher apparent quantum yield (AQY) and carboxylation efficiency (CE) during all three typical developmental stages, and higher quantum yield of CO2 fixation (PhiCO2) at the tillering and heading stages. In addition, Liangyoupeijiu showed significantly higher activities of the C(4) pathway enzymes in both flag leaves and lemmas than did Shanyou 63. As a result, flag leaves of the two super-rice hybrids had higher Pn at morning, noontime and late afternoon during the daily cycle. Since most of the grain yield of rice comes from the photosynthesis of flag leaves, the similar Asat and much higher AQY, CE and PhiCO2 at heading stage of the two super-rice hybrids indicates that higher photosynthetic efficiency rather than higher photosynthetic capacity may be the primary factor contributing to their higher grain yields.