| Water
The
Ka'a He'ê area is characterised for being semi-humid,
where the average annual rainfall is 1500 millimetres. This
plant can withstand little to moderate draught, for which
special irrigation systems must be implanted for its growth.
Especially during the months between December and February
which are characterised by extreme dry spells, or during
the months of July and August when due to the prevailing
cold winds from the South.
Water
is just as important for the final dry matter produce and
for which irrigation is a must in all company undertakings.
At the small planters level (small scale), in regions with
a good distribution of rainfall, from 1500 to 1700 millimetres
annually, on soils with good retention of humidity, as are
the cases in the Provinces of Itapua, Alto Paraná,
San Pedro and Caaguazú, it would be quite feasible
to omit extra irrigation.
According
to Sumida (5), the plant is resistant to humidity but not
to draught for this harms the appearance of the leaves,
which will show, with an increase of humidity, that they
not only grow better but also have the tendency of growing
bigger. This author, whilst undertaking research in order
to study the resistance of the plant to draught, proved
that the lack of water in the ground during the initial
stages of growth did not produce shrivelling until they
reached the minimum level of humidity. Nevertheless, draught
can produce large-scale effects in the following stages
of the plants growth. The leaves shrivel when exposed to
draught in their state of vigorous growth, which could result
in a lessening of dry material. At the start of their growth
period, the leaves do not shrivel easily, but the plants
growth ceases thus causing a strong reduction in the final
production of dry material when compared to other states
of growth.
Temperature
Ka'a
He'ê is highly resistant at low temperatures. It has
been observed that its growth during the winter season is
less vigorous than during the other seasons. The optimal
seed germination temperature is 20ºC and when the temperature
is between 25º and 30ºC this causes sensibility
to light in the seeds germination.
Regarding
high temperatures the plants are sensible to the dry heat
of Paraguay, causing screwing of the terminal sprouts of
the higher leaves but bettering as the heat decreases. This
reaction is characteristic in the plants where there exists
an unbalance between the absorption and evaporation through
transpiration. It is for this reason that it is convenient
to protect de upper parts in order to diminish the effect
of transpiration when transplanting is done in hot periods.
According
to Shock (4) the extreme temperatures in areas of origin
can be between 60ºC and 43ºC, whereas the annual
average is 23º C and 9ºC. According to studies
undertaken by Miyazaki (15) and his collaborators, the plant
can withstand a temperature of 3º C if maintained at
a minimum temperature of 5ºC thus enabling it to support
the winter without undue problems. Besides, it is very resistant
at low temperatures and may even flower during this period
although its growth will be less vigorous than during the
other seasons.
Sumida
(1980) undertook specific studies trying to determine the
influence of the low temperatures on Stevia nodes. He found
that the lowest temperature they can support is that of
15ºC. In his studies he describes the resistance that
the young nodes present at low temperatures during short
periods, the young nodes had a height of 5 cm. They were
formed by more than 10 leaves, which supported temperatures
of 5ºC during 70 minutes.
Kawatani
(16), when visiting growers of Ka'He'ê in the South
east of Asia, found that the height of the plants is greater
in places nearer the Tropics but that the leaf produce per
hectare is smaller in proportion of the stems in relation
to leaves observed in these latitudes. Based on these results
the sub-tropical latitudes are preferable, as is the geographical
situation of Paraguay, the original centre of the Stevia.
Ground
Does
best in light to medium textured soils. Moderate soil fertility
is best (soil in the Upper Parana), and that having a good
content of organic matter; soils with a good capacity to
retain humidity and soils with good internal drainage. Nevertheless,
they can also prosper in soils with low retention of humidity.
Sumida
(1981) observed high productivity under conditions of excessive
soil humidity. This was due to the roots breathing not decreasing
on average under conditions of low oxygen content because
the average oxygen consumption of its roots were less than
that of other crops found in higher lands, as the Soya for
example. Therefore it can be considered a crop that can
be undertaken on flat land that were previously used for
raising rice.
Kawatani
(16) observed that there exist answers to the application
of complete fertilisers compared to that observed without
fertiliser, in so far as the production of dry leaves (open
air-drying). Flowering was early when applying complete
fertilisers because of an increase in the nitrogen dose.
He observed an increase in growth, number of nodes, thickness
of stem and number of branches.
Photoperiods
The
Stevia is a plant of short days and according to Kudo (17)
the flowering under photoperiods of 11 hours, this occurs
at 46.4 days after sowing; with photoperiods of 12.5 hours,
these occur after 92.6 days. The photo period also has an
influence in the number of nodes; so that in 11 hours of
photo periods one obtains 13.3 nodes; with 14 hours of photo
periods one obtains 34 nodes, from which one can deduct
that the greater the photo period, higher will the plant
grow.
Valio
and Rocha (18) investigated the relation between flowering
and photoperiods and concluded that the Stevia is a short
day's plant with critical photoperiods between 13 and 14
hours.
Metivier
and Viana (19) studied the relation between production and
photo period, and proved that by comparing the photo period
of 8 hours with that of 16 hours, the glycoside content,
proteins and steviosides increased in the leaf during the
second period and that the steviol synthesis increased by
45%.
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