Climate and soil
- Temperature, light is the important climatic factors, influence the germination, growth and development and flowering.
- Yield and quality of lettuce seed is mainly depends on the temperature prevail during maturation and development.
Land and isolation
Land
- Land to be used for seed production shall be free of volunteer plants.
Isolation
- Lettuce is mainly self pollinated crop but one to six per cent cross pollination due to insects have been reported.
- Seed fields must be separated from fields of other varieties, and fields of same variety not conforming to varietal purity requirements of certification, at least by 50 metres for foundation seed production and 25 metres for certified seed production.
Sowing season
- In Tamil Nadu sowing is done during September – October, whereas it is in March – April in areas of severe winter.
Seed rate
- Direct sowing: 1-1.5 kg ha-1
- Transplanting: 400-500 g ha-1
Pre-sowing seed management
- The lettuce seed germination is strongly temperature dependant. As temperature rises even to 2 or 3°C above the optimum, lettuce germination can sharply decline from 100% to nearly 0% due to the phenomenon known as thermoinhibition.
- This inhibition of germination upon imbibition at a super optimal temperature is not permanent.
- If the temperature returns to an appropriate degree for germination, the seeds are able to resume germination.
- As expanded period of imbibition at supra optimal temperatures may induce a secondary dormancy called thermo dormancy.
- In this case, the seed become dormant and will not germinate even if they are returned to favourable temperatures for germination.
- The ability of lettuce seed to germinate at high temperature is termed as thermo tolerance.
Nursery preparation
- Seed beds are sterilized with formalin to protect from soil borne diseases or drenched with 0.2% Brassicol or captan. Seeds are sown in July either in line or broadcast. For transplanting one hectare, about 60-70 sq.m. area of seed bed is sufficient. Soon after sowing, seed bed is irrigated with rose can. Beds are irrigated on alternate days and seedlings are sprayed with 0.2 per cent Dithane M-45 or Difoltan at 10-15 days interval. After 5-6 weeks of sowing seedlings are ready for transplanting.
Main field preparation
- Land should be prepared by ploughing thoroughly although it is not necessary to pulvarise the seedbed to a fine texture, since a granular surface results in better germination than a powdery one.
Manures and fertilizer
- Application of 25-50 kg N and 100-150 kg P2O5 per hectare are recommended, depending upon the soil fertility.
- In Tamil Nadu blanket recommendation of 30:60:60 kg NPK ha-1 is recommended.
Spacing
- Plant growth, seed yield and seed quality attributes were significantly influenced by spacing.
- Normally seedlings are transplanted at a distance of 30-45 x 20-30 cm.
Seed sowing
- Seeds are sown either on raised beds (50- 55 cm wide) or in rows spaced equally across the field.
- Dept of sowing is also very important in lettuce shallow (< 10 mm) sowing.
- The sowing can take up in dry soil and then the field can be irrigated.
- Another procedure is pre irrigating the beds, so that the seed is placed in moist soil a few days after irrigation.
Weed management
- Weed caused heavy loss in yield.
- After 2-3 weeks of crop emergence, removal of weeds is essential. For the control of weeds, hand weeding is a common practice and 3-4 hand weedings are adequate throughout the crop period.
Irrigation
- Frequent and light irrigation have been found more effective for high quality seed production.
- Life irrigation should be given on 3rd day.
- Under semiarid conditions, the crop should be irrigated at the time of thinning and again after deheading, followed by another irrigation at the time of flowering.
Deheading
- The important operation in lettuce seed production is deheading.
- So the seed stalk is free to bolt instead of being trapped within the tight head, where it often breaks and rot.
- More common method of deheading is to peel back the leaves on each plant by hand to expose the growing point.
- This method does not cause injury to the stem, but it is laborious and costly.
- If gibberellic acid is applied properly bolting might occur before the plant from heads, making deheading unnecessary.
Plant protection measures
Aphid
- Spraying of any systematic insecticide can be used.
Damping off
- Seed treatment with 2% cereson
Downy mildew
- Spraying of 0.2% Dithane Z-78 can be used.
Roguing
- Off type plants should be removed as soon as they are detected.
- Trueness of type is especially important in the heading varieties of lettuce.
- Plants are rogued based on uniformity of maturity, head size, leaf cover, foliage colour, and leaf type.
- Rogued plants should be destroyed completely by cutting atleast 2.5cm below the ground surface. S
- ince shoots may develop on plants cut off. just below the lower leaf, causing contamination if they produced seed.
- The practice of harvesting a seed crop after removing a crop of market lettuce, followed by some growers, is not desirable.
- If plants are not carefully rogued before the heads are harvested for market the percentage of off type plants producing seed may be very large.
Field standards
| Factors
|
Foundation
|
Certified
|
| Off types
|
0.10%
|
0.20%
|
Maturity and harvesting
- In lettuce it is difficult to determine the precise time for harvest because flowering of the plant continues for a period of 70 days. More than 90% of the total seed yield came from the first 2 flowering peaks, which occurred during the first month. Seed produced from the first 2 flowering peaks were heavier than those produced later in the season.
- The seed coat colour changes from white to black on the eight to ninth day after anthesis. Dry seed weight continues to increase until 14-16 days after anthesis and then start decline. Lettuce seeds start to germinate 9-10 days after anthesis.
- Germination reached the maximum at 14 days after anthesis. The seed weight increased towards maturity and maximum weight was reached when the seeds and the pappus protrude out of the capsule, which was 14-17 days after anthesis.
- There are two ways of harvesting lettuce seed. One is the ‘shake method’, at the time of maturity, the plant is bent over and the seed head shaken in to container so that the ripe seed fall off. This procedure has the advantage of minimizing loss of seed. It also allows the separation of seeds into lots based on time of maturity. Another method is cutting the plants by hand, which is carried out carefully keeps shattering losses of seed to a minimum. Stems are cut several inches above the ground and then the plants are placed in small piles. Crops are usually harvested early in the morning when plants are still moist with dew, which to keep shattering to a minimum.
Threshing
- The seeds should be threshed as soon as the plants are dried sufficiently, which may require 3-4 days in hot dry weather.
- The best quality seeds were produced by harvesting without cutting off the stem followed by Pneumatle grading.
Yield
- Seed yield: Head type - 225-450 kg ha-1
Seed processing
Grading
- Seed polymorphism is a common phenomenon which affects uniformity of crop stand. Therefore, the optimum seed size needs to be determined to ensure better field performance.
- After threshing, lettuce seed usually require cleaning. Normally the seeds first go through a farming mill followed by separator. Indent separators are also used and care should be taken to avoid mechanical admixture and contact of seed with moisture.
Seed treatment and storage
- Lettuce seed is orthodox in nature and can be stored for longer period under controller conditions.
Seed certification standards
- A minimum of three inspections shall be made
Seed standards
| Factor
|
Standard for each class
|
| Foundation
|
Certified
|
| Pure seed (minimum)
|
98%
|
98%
|
| Inert matter (maximum)
|
2%
|
2%
|
| Other crop seed (maximum)
|
None
|
None
|
| Weed seeds (maximum)
|
5 no. / kg
|
10 no. / kg
|
| Other distingulshable varieties (maximum)
|
10 no. / kg
|
20 no. / kg
|
| Germination (maximum)
|
70%
|
70%
|
| Moisture
|
8%
|
8%
|
| For vapour proof container (maximum)
|
6%
|
6%
|
|
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