Photoperiod, also called light duration and day length (or daylength), refers to the length of the light period as contrasted to darkness within a day. Day length controls or influences several plant growth and development processes that determine or affect crop yield.
The difference in day length can have a marked influence on the establishment and yield of certain crops. As already mentioned, giant potatoes and cabbages have been produced in Alaska (about 61°N) during summer when light is intense and almost continuous.
In the corn belt region of the United States which lies about 44°N, day length in June to July exceeds 16 hours and solar radiation is at maximum. As a result, yield of a 180-day corn crop can go as high as 22 tons per hectare. In contrast, the yield potential of hybrid corn in the Philippines is only up to 13 tons/hectare (Lantican 2001).
On the other hand, some temperate crops may perform differently when grown in the tropics due to pronounced difference in light duration. But for cauliflower, cabbage, beets, and some other temperate crops, it seems that the temperature during the growing period is the most limiting factor (Vergara 1978).
The duration of the light period varies according to such factors as geographic location, distance from the equator, and time of the year.
On March 21 and September 21, the sun is directly over the equator and rises and sets exactly in the east and west, respectively. As such, 12 hours of light and 12 hours of darkness occur at all places on the surface of the earth. On December 21, the sun is farthest south of the equator. This means that in the northern hemisphere all locations have the shortest daylength and longest dark periods. This is reversed on June 21 where the sun is farthest north of the equator. This time the longest light and the shortest dark period occur in all places in the northern hemisphere and the shortest light and longest dark period occur in the southern hemisphere (Edmond et al. 1978).
With some exceptions, the Northern Hemisphere refers to all countries north of the equator. It includes all of North America, the northern parts of South America, about two-thirds of Africa, all of Asia excluding parts of Indonesia, and all of Europe. The Southern Hemisphere includes most of South America, one-third of Africa, all of Antarctica, a small portion of Asia (parts of Indonesia), and all of Australia/Oceania which are located to the south of the equator (http://www.worldatlas.com/aatlas/imageh.htm, March 27, 2011).
Edmond et al. (1978) further explained that between northern and southern locations in the northern hemisphere, the light duration is longer in the northern locations from March 21 to September 21, and the day length is longer in the southern locations from September 21 to March 21. For example, day length increases from 12.7 hours to 16.4 hours at 10° to 50° latitude north of the equator, respectively, from June 20-22 while it decreases from 11.7 hours to 8.1 hours at 10° to 50° latitude south of the equator.
Photoperiod can also vary with different locations within the same country. This is true to the Philippines as illustrated by Lantican (2001). The Philippines lies between latitudes 4°40’N up to 21°10’N of the equator at longitude 116°40’ to 126°34’E. General Santos City is located 6°N of the equator. On June 21 it has a day length of 12 hours and 28 minutes and, on December 22, 11 hours and 46 minutes with a difference of 42 minutes. In contrast, Laoag, Ilocos Norte, which lies 18°N of the equator has a day length of 13 hours and 13 minutes on June 21 and 11 hours, 3 minutes on December 22, or a difference of 2 hours and 10 minutes.
(Ben G. Bareja May 2011)