In the years immediately prior to World War II, the writer was a young graduate student in Canada working on storage and ripening of pears. At that time, it was customary for Canadian housewives to put fruits on sunny windowsills to ripen them. Because it seemed illogical that light should hasten ripening, I decided to put a row of unripe pears on the laboratory windowsill and cover half of them with a black cloth. Fortunately, I checked pulp temperatures: those under the black cloth were several degrees warmer. Then I tried shading with a white-painted board.
Better, but still quite a difference. By the time the next year’s pear crop came in, I was in uniform on the other side of the Atlantic. I never did return to the sunlightpear-ripening problem but have ever since been acutely aware that one way or another, temperature can be an interactant, wanted or not, in a great deal of plant research. A very useful concept for expressing heat units is “total day degrees”: that is, the accumulated number of days (or sometimes hours) above a certain base temperature.
Another version is the accumulated sum of diurnal maximum temperatures times the number of days. For the reverse (cold units), the usual figure is the total number of hours below a given temperature, such as 40°F or 5°C. The usefulness of such methods is not helped by overreliance on statistical analysis of findings based on an initial arbitrary decision. In the United States, for example, 40°F (4.4°C) and 45°F (7.2°C) have been common baseline temperatures for determining chilling hours.
As the Fahrenheit scale is abandoned in favor of Celsius, 5 and 7.5°C are more likely to be used. With such baseline variations, apparent fine statistical differences can be deceptive.