Agriculture Reference
In-Depth Information
generally lower in this region, and the dif erence in day length becomes greater
with higher latitude (Table 2.1). The limit for the subtropics is the isotherm of
10°C average temperature for the coldest month. This 10°C isotherm excludes
the large land masses whose climates are temperate and includes almost half
of China, three-quarters of Japan, all of South Korea, the southern half of the
United States, all of the southern half of Australia, the North Island of New
Zealand and more than half of Argentina and Chile between 23° 30
′
and 40°
latitude.
Horticulturists who have spent their professional careers in regions where
the temperatures during the coldest month at sea level are rarely lower than
15-18°C fi nd it dii cult to accept the tropical classifi cation of regions with
winter temperatures down to 4-7°C and frost potential.
CLIMACTERIC FACTORS
Day length
The day length at the equator is about 12 h. At low latitudes in the tropics, the
increase in dif erence between the longest and shortest days is about 7 min per
degree (Table 2.1), increasing to 28 min per degree at latitudes between 50
and 60°. The dif erence in photoperiod (Table 2.1) is associated with the earth
being inclined on its axis by approximately 23° 30
′
; hence the solar equator
moves about 47° as the earth moves around the sun. The extremes are the
Tropic of Cancer (23°
30
′
N) to the Tropic of Capricorn (23°
30
′
S); within
this belt the sun's rays are perpendicular at some time during the year. At
the spring and autumn equinoxes, the lengths of the day and night are equal
everywhere over the earth.
Fruit trees such as mango, papaya, bananas, the annonas, avocado,
acerola and guava show no response to photoperiod and are capable
of fl owering at any season of the year. In equatorial Colombia, with
approximately 12-h day length, it is common to fi nd mango trees fl owering
during February-March and again in August. In the subtropics, fl owering is
more precise, occurring in the spring as a function of lower temperature and
moisture availability limiting growth. For guava, seedlings grown under 15-h
day length from germination to 140 days and fi eld transplanted produced fruit
within 376 days from sowing; the control seedlings under 10-h day length did
not fl ower. This result with guava refl ects the longer period available each day
for photosynthesis and not photoperiodism. Guava can also be forced to fl ower
by pruning.
Pineapple can fl ower naturally at any time of the year, depending upon the
size of the planting material, though it is a quantitative, but not an obligatory,
short-day plant. Interruption of the dark period by illumination suppresses
