It remained a problem for onion growers to get optimum seed yield in Southern region of Punjab due to some barriers. Being a cotton belt, abundant application of sprays caused increase in mortality rate of pollinators. Non availability of pollinators during the flowering period of onion caused only 16.88% fruit setting and free availability of pollinators increased fruit setting upto 72.07% (Rao et al., 1989). Rao and Lazer (1983) recorded 93.5% fruit setting with 3.6 g seed per umbel in free availability of pollinators and 9.8% fruit setting with out pollinators with 0.3 g seed per umbel. In open pollinated field, seed yield was higher than controlled environment (Orlova et al., 1981; Sihag, 1985). More honey bees caused more seed setting (Kubisava et al., 1986).
However, seed setting in onion depends upon kind of pollinators (Currah and Ockendog, 1983) and varieties of onion. Because trend of pollinators remained differential in different varieties (Kumer, 1985). This is because, some varieties are more attractive due to more sugar concentration of the nectar, varying from 67 to 75%. (Rao and Lazar, 1983). Sometimes crops grown aside of the onion caused more attraction to pollinators and consequently they preferred those crops instead of onion. Sprays at flowering also caused distraction of pollinators, which ultimately resulted in substantial reduction in seed setting of onion.
Materials and Methods
Four varieties viz, Phulkara, Dark red, Desi and Rubina were randomized in triplicate Randomized Complete Block Design with plant to plant 30 cm and row to row 75 cm spacing. Bulbs having 300-400 g were sown on 15.10.2001 at Vegetable Research Sub-Station, Multan on one side of ridges. Fertilizers at the rate of 125 kg DAP and 125 potash per hectare were applied as basal dose during land preparation and 250 kg urea were added during apparent flowering and umbel formation of crop in two split doses.
Possible efforts were taken to complete spraying schedule before on set of flowering to minimize the errors. However, last sprays were applied at evening time, after proper visiting of pollinators. At crop termination, data of 10 plants was recorded in each variety. Average data for number of flowers per umbel, number of fertilized flowers per umbel, seed setting %age, average umbel weight and average seed weight of umbel were calculated for all varieties. Umbel weight was recorded before threshing and seed weight of umbel was measured after proper threshing and winnowing. Statistical analysis was carried out by Steel and Torrie (1981).
Results and Discussion
Analysis of variance showed significant differences (P<0.01) for all characters except seed setting %age (Table 1).
It is obvious from the Table 2 that varieties, Desi and Rubina
showed highest mean value for all characters. Desi showed highest number of
flowers per umbel (973.4) number of fertilized flowers (613.9), seed setting
%age (61.07%) flowered by Robina showing 868.2, 531.2 and 60.07% value for these
traits, respectively. The variety Robina got highest umbel weight and seed weight
per umbel and Desi remained second having 11.52 g for umbel weight and 5.54
g seed weight per umbel. The reduction in seed setting was 40 to 50%. This reduction
could be compensated by increasing the number of pollinators and decreasing
the effect of side grown crops and number of sprays during flowering. For example,
Kumar et al. (1989) recorded three times more yield in open pollinated
field than controlled environment (cages) due to non availability of pollinators.
Similarly Rao and Lazer (1983) observed 93.5% seed setting in onion in open
pollinated trial and 9.8% in cages and seed weight per umbel was 3.6 g in open
pollinated field and 0.3 g in cages.
||Mean squares of fertilized flowers per umbel, total number
of flowers per umbel, %age of seed setting, total umbel weight and seed
weight per umbel in open pollinated field
|| Mean data for five characters of four varieties of onion
This substantial reduction in seed setting and seed weight was due to absence
or very low visiting of pollinators (Orlova et al., 1981). He observed
that even 2-3 days delay in introduction of honey bees caused reduction in seed
yield as much as 15%.
In this experiment varieties did not show significant differences for seed setting %age, which may be due to similar attraction of varieties to pollinators. Because Orlova et al. (1981) concluded that amount of sugar secreted by the nectaries varied from 0.49 to 4.8 mg per flower of onion, which is major substance for attraction. However in these four varieties similar response was observed, perhaps it is due to similar concentration of sugar in flowers.
Proper visiting of pollinators could enhance seed production upto 50% by minimizing the side grown attractive crops and completing the spraying schedule before onset of flowering. Response of varieties for setting remained similar. Application of pollinators preventive insecticides at the evening time could enhance seed setting by retarding mortality rate of biological agents.