Agriculture Reference
In-Depth Information
fl owering. Though pineapple does not require low temperatures or diurnal
variations in temperature to fl ower, there is an interaction with temperature.
Temperatures lower than 17°C during short days can induce considerable
fl owering, even in small plants. No fl owers are produced on yellow passion
fruit (
Passifl ora edulis
f.
fl avicarpa
) vines under artifi cially induced short days
(8 h). Long days promote passion fruit vine growth and fl owering, while short
days promote vine growth only. This observation, however, could be due to the
amount of solar radiation received and not photoperiod.
Radiation
When compared to higher latitudes, the tropical latitudes have small seasonal
variation in solar radiation along with high intensity. The longer summer
day length at the higher latitudes means that these latitudes exceed the
daily amounts of solar radiation received in the tropics. The highest annual
energy input on the earth's surface, 12 MJ/m
2
/day, occurs in the more cloud-
free subtropical dry belt of 20-30° (Fig. 2.3). In the tropics, solar radiation
received is reduced by clouds and water vapour in the air, through refl ection
and absorption, to a minimum of ~7 MJ/m
2
/day at the equator. Over a large
portion of the tropics the average is 9 MJ/m
2
/day ± 20%.
In the tropics, atmospheric radiation transmissivity varies from 0.4 to
0.7, due largely to clouds and seasonal variation. The maximum recorded
irradiance under cloudless skies at noon in the tropics is 1.1 kW/m
2
, with a
daily total received of from 7 to 12 MJ/m
2
. About 50% of this energy is in the
0.4-0.7 μm waveband, which is known as photosynthetically active radiation
(PAR). In full sunlight, C
3
plants, including all fruit crops discussed in this topic
except pineapple, which is a CAM plant, are 'light' saturated. This saturation is
due to ambient CO
2
availability limiting the rate of photosynthesis.
High shade (3-5 MJ/m
2
/day) does not infl uence litchi fl owering, though
it does increase early fruit drop. Flowering in passion fruit is reduced once
irradiance falls below full sun. Irradiance is normally not a factor limiting
plant growth in the tropics except during heavy mist and cloud, and in shade
from other vegetation and mountains.
Temperature
Near the earth's surface, temperature is controlled by incoming and outgoing
radiation. Surface temperatures are modifi ed spatially and temporally
throughout the year by local factors more than radiation. The main factors are
continentality, the presence of large inland waterbodies, elevation modifi ed by
prevailing topography, and cloudiness. Highest diurnal temperatures occur
in dry continental areas, at higher elevations and in cloud-free areas. The
