Background
The authors write that, across the world, "the daily minimum temperature (Tmin) increased at a faster rate than the daily maximum temperature (Tmax) in the 20th century," and they say that the warming of the Tmin "is nearly twice as large as that of the Tmax (Karl et al., 1993; Easterling et al., 1997; Houghton et al. (2001)." In China, in fact, they state that the warming of Tmin has been 2-3 times that of Tmax, citing Ren et al. (2005).

What was done
Working at the Gucheng Agrometeorological Experiment Center of China's Meteorology Administration in Dingxing County, Hebei Province, and taking winter wheat as a test crop, the five researchers conducted field experiments where infrared heaters were used to increase night temperature by about 2.5°C in contrast to the normal night temperature in two whole growing seasons of winter wheat: 2008-2009 (a warmer year) and 2009-2010 (a colder year).

What was learned
Fang et al. found that, in the warmer year, winter wheat yield was unaffected in the high-night-temperature treatment by the extra warming directed towards the surface of the soil and crop. In the colder year, on the other hand, winter wheat yield was increased by more than 30% in response to the nighttime warming, largely due to the fact, as they describe it, that "the higher temperatures extended the growth period before over-wintering, and significantly increased the number of tillers and effective panicles."

What it means
It would appear that the greater warming of daily minimum temperatures, as compared to daily maximum temperatures, has been a great boon to winter wheat production in Northern China.

References
Easterling, D.R., Peterson, T.C. and Karl, T.R. 1997. Maximum and minimum temperature trends for the globe. Science 277: 364-367.

Houghton, J.T., Ding, Y., Griggs, D.J., Eds. Climate Change 2001: The Scientific Basis. Cambridge University Press, Cambridge, United Kingdom.

Karl, T.R., Jones, P.D., Knight, R.W., Kukla, G., Plummer, N., Razuvayev, V., Gallo, K.P., Lindseay, J., Charlson, R.J. and Peterson, T.C. 1993. A new perspective on recent global warming: Asymmetric trends of daily maximum and minimum temperature. Bulletin of the American Meteorological Society 74: 1007-1023.

Ren, G.Y., Chu, Z.Y., Zhou, Y.Q., Xu M.Z., Wang, Y., Tang, G.L., Zhai, P.M., Shao, X.M., Zhang, A.Y., ... Zhang, L., Bai, H.Z., Liu, X.F. and Tang, H.Y. 2005. Recent progresses in studies of regional temperature changes in China. Climatic and Environmental Research 10: 701-716.