Agriculture Reference
In-Depth Information
rate of decrease in temperature with elevation (adiabatic lapse rate) varies
with cloudiness, hence season, and between night and day. The normal rate is
about 5°C per 1000 m under cloudy conditions and can range from 3.1 to 9°C
per 1000 m.
The human sensitivity to temperature is modifi ed by the rate of
evaporation. Evaporation from human skin is primarily infl uenced by
humidity, wind speed and response to sunshine. A human can endure high
temperature if the humidity is low; hence the discomfort felt in the humid
tropics is associated with high temperatures and humidity (>25°C and >80%
R.H.). These conditions are also favorable to growth of microorganisms and
insects. The problem of controlling plant diseases and insect pests in the
tropics is compounded by the absence of a cold winter and aridity to limit their
adult development.
Tropical fruit crops such as mango, guava, acerola, papaya, pineapple,
some annonas and others originated in the warm, lowland tropics. Others
such as the litchi, Mexican and Guatemalan races of avocado, cherimoya
and purple passion fruit are subtropical fruits by virtue of their origin in
the subtropics or at higher elevations in the tropics. Man, in his attempt to
commercialize tropical fruit crops, has extended production into subtropical
regions beyond the Tropics of Cancer and Capricorn and has generated
considerable knowledge on the range of temperature adaptability of these
crops. Data on threshold temperatures and durations of exposure for various
stages of plant development of tropical fruit trees are often unavailable;
minimum temperatures in the coolest month that support survival,
commercial or best production have been approximated (Table 2.3). The
minimum temperature criterion takes into account dif erences in elevation and
latitude. In regions subjected to marginal winter temperatures, site selection
becomes a paramount consideration, such as southern-facing slopes in the
northern hemisphere and northern-facing slopes in the southern hemisphere.
Young plant growth may also be inhibited when soil temperature exceeds
35°C, a common condition in the tropics. Leaf temperature can exceed air
temperatures by 20°C; for example, pineapple fruit and leaf temperature have
been recorded in excess of 50°C in the fi eld. Hence, maximum temperatures
in the orchard microclimate need to be considered in site selection. Three
crops (mango, litchi and avocado) illustrate this temperature adaptability and
the varietal and race adaptation at various stages of development. Mature
mango trees have been found to withstand temperatures as low as −16°C for
a few hours with some injury to leaves. Flowers and small fruits may be killed
when temperatures less than 4.5°C occur for a few hours during the night.
Mango varietal dif erences for cold resistance have not been observed. Mango
responds to cool night temperatures (10-14°C) with profuse fl owering. Ideally,
day temperatures during this period should be warm (21-27°C). When winter
night temperatures are mild (16-18°C), fl owering is more erratic.
 
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