Stigma and glume characteristics synergistically determine the stigma exsertion rate in thermo-photo sensitive genic male sterile wheat

Abstract Background: Wheat hybrids have been widely demonstrated to have remarkable heterosis or hybrid vigor in increasing yield potential and stability since the 1960s. However, commercial application of hybrid wheat is still hindered by higher seed costs mostly resulting from lower yield in hybrid seed production. Stigma exsertion has been verified as a decisive factor in increasing rice's hybrid seed yield but has been nearly ignored in hybrid wheat breeding and seed production. Results: In this study, four thermo-photo sensitive genic male sterile lines K41S, K64S, K66S and K68S with different stigma exsertion rate (SER) were used to compare their differences in floral architecture relative to stigma exsertion over two growing seasons. K41S and K64S exhibited relatively higher SER at 21.87% and 22.81%, respectively. No exserted stigma was observed in K66S, and K68S had only 0.82% SER. The study found that stigma length, glume width and the length-width ratio of the glume were significantly correlated with SER, with the correlation coefficients of 0.46, -0.46 and 0.60, respectively. Other stigma features such as branch angle, stretch width and hairbrush length, as well as glume length, also had a weakly positive correlation with SER (r = 0.09-0.27). For K41S and K64S, the SER was significantly affected by the differences in stigma branch angle and stigma stretch width among florets. A cross-pollination survey showed that the outcrossing ability of florets with exserted stigma was about 3 times as many as that of florets with non-exserted stigma. As a result, the stigma exserted florets that accounted for 21.87% and 22.81% of the total florets in K41S and K64S, produced 46.80% and 48.53% of the total cross-pollinated seeds in both sterile lines. Conclusions: These findings suggest that the SER in wheat sterile line is synergistically determined by specific stigma and glume characteristics. Longer stigma combined with slender glume appears to be the basic floral feature of stigma exsertion. It is expected that breeding and utilizing sterile lines with higher SER would be a promising solution to cost-effective hybrid wheat seed production.