Difference in planting dates would bring about a variation in growth and yield of stevia. So, selection of suitable time of planting is necessary to maximize the growth and biomass yield in stevia.
1.1 Growth parameters
Dates of planting did not exhibit significant differences with respect to dry leaf yield.
Irrespective of nitrogen levels, January planting produced higher dry leaf yield (3.11 t/ha)
compared to October planting (2.59 t/ha). Assessment of dry matter production and
distribution to various plant parts is important for determination of total yield of crop Donald, 1962). The reasons for higher dry leaf yield in case of January planting may be attributed to higher leaf dry matter accumulation. The leaf dry matter accumulation was higher in case of January planting (29.61 g/plant) and it was lower in case of October planting (26.11).
The variation in leaf dry matter may be related to the variation in number of leaves
per plant, number of branches per plant and plant height. The number of leaves per plant was higher in case of January planting (391.36) compared to October planting (370.33). The plants planted in January produced higher number of branches per plant as compared in October and November, which produced minimum number of branches per plant. The plants of January planting, which experienced congenial long-day conditions with high temperature and light intensity during vegetative growth resulted in vigorous growth, which enabled them to produce more branches. October and November plants experienced unfavouarble climatic conditions (short-days and low temperature) and as a result had poor growth and in turn had less branches production.
The variation in number of leaves per plant may be due to difference in number of branches per plant, it was highest (48.04) in case of January planting compared to November planting (47.42) and lowest in case of October planting (46.82). The plant height was higher (54.72 cm) in case of January planting and it was lower (50.43 cm) in case of October planting. This increased number of leaves in early planting dates could be directly correlated to the fact that early planting had increased number of leaf bearing points in terms of increased number of branches per plant. The crop planted in October and November produced minimum number of leaves, due to the fact that they did not experience favourable growing conditions during their grand vegetative growth period. The results obtained in the present study are in confirmity with the findings of Kiyatkin (1975) in chrysanthemum.
The reasons for higher dry leaf yield in case of January planting, probably due to higher leaf area (4825.73 cm2/plant) produced in case of January planting compared to October planting (4656.36 cm2/plant) January planting recorded higher LAI (4.84) compared to October planting (4.59). Leaf area in stevia differed significantly due to different dates of planting. The leaf area per plant was highest in plants planted in January. Leaf area was minimum in plants planted during October and November. This reduced leaf area in later planting dates might be due to production of less number of leaves, which could be due to reduction in plant height and number of branches.
Increased leaf area in the plants of January planting could be attributed to the production of more number of leaves per plant which could be directly correlated to the increased plant height and number of branches.
The higher dry leaf yield and growth parameters in case of January planting may be
due to dense canopy compared to October panting. Economical yield is part of the total
biological yield of the crop and hence the dry matter production is an important eterminant of the economic yield (Donald, 1963). The total dry matter production was higher (108.78 g/plant) in case of January planting compared to October planting (100.96 g/plant).
The higher fresh leaf yield (11.58 t/ha) was obtained in case of January planting than
in October planting (9.73 t/ha). However, there was non-significant differences in fresh leaf yield between the dates of planting. The reasons for marginally higher fresh leaf yield with January planting may be attributed to more number of leaves per plant. This increased number of leaves in early planting dates could be directly correlated to fact that early planted plants had increased number of leaf bearing points in terms of increased number of branches per plant.
In initial stages of growth (30 DAP) October planting had putforth good growth with
respect to plant height and number of branches per plant, though there were no statistical differences between them. But after 45 DAP, January planting performed better compared to October planting by providing higher plant height, number of branches and leaves per plant. The reasons may be due to (day length) favourable soil physical conditions offered for the penetration and distributing roots.
1.2 Flowering
The plants of October and November plantings were early to initiate flower buds,
early to flower and early to reach for first flowering while the plants of January were late for the same. Early flowering in October and November plants could be due to exposure of plants to unfavourable climatic conditions during the vegetative growth period as a result they entered early into the reproductive phase as they experienced short days and low temperature which favours flowering in stevia. The earliness in flowering due to short days may be attributed to an earlier morphological differentiation of flowers The earlier cessation of vegetative phase immediately after planting as observed in the plant height and number of leaves per plant at early stages of growth in the treatments of October and November plantings should have also contributed to the earliness in flowering in these treatments. Earliness in flowering due to short-day conditions has been reported by previous workers in chrysanthemum (Bareman et al., 1993 and Meher et al., 1999b). On the other hand, the January planting experienced congenial climate (long days and optimum temperature) and remained sufficiently longer in vegetative phase. The delay may be attributed to the floral inhibitors produced by the leaves under long day conditions, which in turn affects the apical differentiation as suggested by Tanaka (1968).
Plants of later plantings (October, November), which experienced lower light intensity
and duration, lower night temperature during their vegetative growth period, remained dwarf and had less number of branches and leaves and as a result had decreased flowering duration. Similarly, Nagaraju (2001) reported wider flowering duration in China aster when planted during May months.
1.3 Interaction effects nitrogen levels and dates planting on growth and
yield
The interaction effects between dates of planting and nitrogen levels did not exhibit
significant differences with respect to dry leaf yield. However, the higher dry leaf yield (4.01 t/ha) was recorded in case of January planting when 105:30:45 kg NPK per ha was applied. Whereas, the lowest dry leaf yield (1.8 t/ha) was obtained in case of October planting with 60:30:45 kg NPK per ha.
The differences in dry leaf yield due to dates of planting and nitrogen levels
interaction may be attributed to variations in growth parameters due to variations in
temperature and light intensity. January planting which received 105:30:45 kg NPK per ha recorded maximum number of leaves per plant at harvest (468), whereas lower number of leaves per plant (295.78) was recorded in case of interaction between October planting and 60:30:45 kg NPK per ha.
The number of branches per plant at harvest was maximum (49.39) in case of
January planting which received 105:30:45 kg NPK per ha. Whereas, lower number of
branches per plant (45.78) was obtained due to combination of October planting and 60:30:45 kg NPK per ha.
The higher plant height (59.31 cm) was recorded in case of January planting when
105:30:45 kg NPK ha was applied, whereas, lowest plant height (48.00 cm) was recorded in case of October planting received 60:30:45 kg NPK per ha.
At harvest, dry matter accumulation was maximum (131.63 g/plant) in case of
January planting when 105:30:45 kg NPK ha was applied compared to that of lowest (80.60 g/plant) dry matter accumulation in case of October planting received 60:30:45 kg NPK per ha. Similar results with respect to maximum leaf dry matter production (35.82 g/plant) was recorded in the same treatment combination.
The maximum dry matter accumulation due to higher levels of nutrients may be due
to the fact that for initial impetus for plants to grow, sufficient nutrient is very much essential. The plants will have good vegetative growth which in turn helps to utilize light better.
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