Agriculture Reference
In-Depth Information
shown tolerance of light frost down to
−
2°C, with fl ower damage by even light
frost. The Mexican-Guatemalan hybrids, such as 'Fuerte' have shown wider
tolerance of cold than the Guatemalan cultivars. Temperatures of 12-13°C
during fl owering can prevent growth of pollen tubes and embryos, leading to
production of unfertilized, underdeveloped fruits.
Rainfall
Temperature determines agricultural activity in the temperate regions of the
mid-latitudes, while rainfall is the crucial factor in the tropics. The seasonal
and diurnal distribution, intensity, duration and frequency of rainy days
vary widely in the tropics, both in space and in time (Fig. 2.2). The maximum
rainfall occurs near the equator, with no dry season. Surrounding this
equatorial zone in Africa and South America are areas with two rainy seasons
alternating with two dry seasons; rarely are the seasons of the same duration
or intensity. Further from the equator is a region of minimum rainfall at
20-30° latitude, associated with the subtropical high pressure area, with one
rainy season, frequently due to the monsoons. Topography can signifi cantly
modify the generalized rainfall pattern; examples include the western coast
of India and Borneo, and the coastal areas of Sierra Leone, where monsoonal
winds are forced to rise because of mountain ranges. Trade winds can bring
considerable rainfall and are subjected to the forced rise by topographical
features. Other factors infl uencing rainfall include changing and slowing
down of wind speed as it approaches the equator and continentality, such as
in south-west and central Asia. The above factors lead to complicated rainfall
patterns, with broad generalization possible while remembering that there is
considerable variation (Fig. 2.2).
Tropical fruit production is normally limited by available soil moisture. The
stage of growth or development at which water stress occurs greatly af ects the
fi nal yield and quality. Many factors infl uence the amount of rainfall available
to plants, including evaporation and transpiration rates, surface runof , soil
water-holding capacity and percolation through the soil profi le beyond the
rooting area. Using the average tropical daily net radiation of 9 MJ/m
2
and
the latent heat of vaporization for water (2.45 MJ/kg), an evaporation rate of
ca
4 mm per day can be calculated. This evaporation rate is similar to that in
a temperate summer. Higher rates of 10-15 mm per day occur for irrigated
crops in the semi-arid tropics due to the advection of hot, dry air. Rainfall and
irrigation need to make up this evaporative loss, and a mean monthly rainfall
of 120 mm (~4 mm per day) would be required.
Excessive rainfall causes major problems with fl owering, pests, diseases and
fruit quality. Many trees, such as mango and litchi, require a dry (or cold) period
to stop vegetative growth and induce fl owering. Mango and litchi originated
in areas with a monsoon climate that provides distinct wet and dry seasons.
