Introgression of two chromosome regions for leaf photosynthesis from an indica rice into the genetic background of a japonica rice

Abstract

Increases in rates of individual leaf photosynthesis (P n) are critical for future increases of rice yields. A previous study, using introgression lines derived from a cross between indica cultivar Habataki, with one of the highest recorded values of P n, and the Japanese elite cultivar Koshihikari, identified four QTLs (qCAR4, qCAR5, qCAR8, and qCAR11) that affect P n. The present study examined the combined effect of qCAR4 and qCAR8 on P n in the genetic background of Koshihikari. The pyramided near-isogenic line NIL(qCAR4+qCAR8) showed higher P n than both NIL(qCAR4) and NIL(qCAR8), equivalent to that of Habataki despite being due to only two out of the four QTLs. The high P n of NIL(qCAR4+qCAR8) may be attributable to the high leaf nitrogen content, which may have been inherited from NIL(qCAR4), to the large hydraulic conductance due to the large root surface area from NIL(qCAR4), and to the high hydraulic conductivity from NIL(qCAR8). It might be also attributable to high mesophyll conductance, which may have been inherited from NIL(qCAR4). The induction of mesophyll conductance and the high leaf nitrogen content and high hydraulic conductivity could not be explained in isolation from the Koshihikari background. These results suggest that QTL pyramiding is a useful approach in rice breeding aimed at increasing P n.

Keywords: Hydraulic conductance; Oryza sativa; leaf nitrogen content; photosynthesis; quantitative trait locus; stomatal conductance..

Fig. 1.

Graphical genotypes of Koshihikari and the quantitative trait loci near-isogenic lines. Black bars indicate regions homozygous for Habataki alleles. Values in boxes indicate lengths of substituted regions.

Fig. 2.

Comparison of CO₂ assimilation rates at a photosynthetic photon flux density of 2000 μmol m^–1 s^–1 and an ambient CO₂ concentration of 370 μmol mol^–1 (A ₃₇₀) of Koshihikari, Habataki, and the quantitative trait loci near-isogenic lines in the paddy field. Measurements for each panels were conducted in different field plots. Data are mean±SD (n=3–5). Values above bars are percentages relative to Koshihikari. Different letters above bars indicate significant differences (P<0.05, Tukey’s test).

Fig. 3.

(A) Hydraulic conductance from roots to leaves (C _p), (B) root surface area (S _r), and (C) hydraulic conductivity (L _p) of plants grown in 3-l pots. C _p and S _r are expressed on a per stem basis. Values are mean±SD (n=4). Values above bars are percentages relative to Koshihikari. Different letters above bars indicate significant differences (P<0.05, Tukey’s test).

Fig. 4.

Relationships between leaf N content (LNC) and (A) CO₂ assimilation rate at an ambient CO₂ concentration of 370 μmol mol^–1 (A ₃₇₀) and (B) an intercellular CO₂ concentration of 280 μmol mol^–1 (A ₂₈₀) in flag leaves of Koshihikari (filled circles), Habataki (open circles), NIL(qCAR4) (triangles), NIL(qCAR8) (diamonds), and NIL(qCAR4+qCAR8) (squares) grown in 12-l pots. The solid and broken lines indicate the regression lines for Koshihikari and Habataki, respectively. Values are mean±SD (n=4–6).