-Towards the development of high-yielding varieties suitable for cultivation under increased CO2 concentration-
Introducing a gene that increases the number of grains of a high-yielding rice variety into the conventional variety Koshihikari by cross breeding resulted in a significant increase in yield under conditions of high CO2 concentration. This research may contribute to the development of high yielding varieties suitable for cultivation under increased atmospheric CO2 concentrations predicted in the near future.
The atmospheric CO2 concentration has increased 1.4 times in about 250 years since the second half of the 18th century and is predicted to increase further in the future. It is known that rice plants increase photosynthetic products and increase yield and growth when atmospheric CO2 concentration rises. However, the traits which the rice varieties should possess for increasing the yield under future high CO2 concentration conditions has not been specified so far.
Therefore, NARO constructed an environment of high CO2 concentration (about 1.5 times of present value) which is predicted 50 years later, using an Free-air CO2 enrichment (FACE) facility that can realize high CO2 concentration outdoors, and studied the yield and growth of cultivated lines by introducing genes that increase the number of grains possessed by a high yielding rice variety to Koshihikari by artificial hybridization. Under high CO2 concentration, it has been clarified that high yield can be obtained as increased photosynthetic products can be transferred to the larger spikelets.
Genes that increase the number of spikelet of high-yielding varieties can be easily introduced by artificial cross-breeding to existing varieties such as Koshihikari. Hence this research may contribute to the development of high-yielding varieties suitable for cultivation under increased atmospheric CO2 concentrations as predicted in the near future.
Nakano H. et al. (2017) Sci. Rep. 7: 1827. doi:10.1038/s41598-017-01690-8
Overview of the Tsukubamirai free-air CO2 enrichment (FACE) experiment facility (A, B) and tested varieties and lines (C)
A, B: The atmospheric CO2 concentration in the high concentration CO2 test area was set to 578 ppm (2012) and 576 ppm (2013). Normal atmospheric CO2 concentrations were 383 ppm (2012) and 383 ppm (2013). Experiment is conducted with four fields with one high-CO2 test area (a) and one control zone (b). In the high CO2 test area (a), the tubes filled with CO2 are arranged in a regular octagonal shape.
C: The "NIL-APO1" with genes of spikelet of a high-yielding variety "Takanari" that can increase the number of grains produced more grains as compared to "Koshihikari" which does not possess that gene.
Fig. 1 : Grain yield (A) and yield components (B, C) of NIL-APO1 and Koshihikari under high and normal atmospheric CO2 concentrations
A: Grain yield of NIL-APO1 hardly increased under normal atmospheric conditions but increased significantly under high CO2 conditions as compared to Koshihikari.
B: NIL-APO1 produced more grains under both normal atmospheric condition and high CO2 conditions as compared to Koshihikari.
C: NIL-APO1 showed slight decrease in the ripening rate under normal atmospheric conditions, but hardly decreased under high CO2 conditions as compared to Koshihikari. The ripening ratio was expressed as the ratio of the number of brown rice (a) to the total number of spikelet (a to d).
Fig. 2. Amount of nonstructural carbohydrates (stem NSC) (A) of photosynthetic products in stems at maturity under high and normal atmospheric CO2 concentrations and mechanism for increased yield in NIL-APO1 (B).
A: The amount of stem NSC in NIL-APO1 under normal atmospheric conditions does not vary much from Koshihikari but 14% less under high CO2 condition.
B: Under high CO2 concentration, Koshihikari has limited number of spikelet so the increased photosynthetic products due to high CO2 concentration remain as stem NSC. In contrast, NIL-APO1 has large number of spikelet so photosynthetic products that increased due to high CO2 concentration are transferred to the spikelet leading to significant yield increase.