©R.A.T. George 2009. Vegetable Seed Production, 3rd Edition (R.A.T. George) 129 Although this family contains a large number of genera, there are relatively few that are of major importance as cultivated vegetables. The main vegetables that are produced from seed are:
Lactuca sativa L. Lettuce Cichorium endiva L. Endive
Cichorium intybus L. Chicory, witloof chicory
There are many species of local importance which are used as leafy vegetables and include: Tragopogon porrifolius L., salsify or vegetable oyster plant, which is cultivated as a minor root vegetable in some temperate areas including north- ern Europe; Taraxicum officinale Wigg., dandelion (which is also a common weed in parts of the world, including Europe); and Lactuca indica L., culti- vated as a leafy vegetable in Asia, especially China, Indonesia and The Philippines.
1. Lactuca sativa var. capitata L.: the cabbage or head lettuce, which is gen- erally subdivided into crispheads (iceberg) and butterheads. The crispheads have firm hearts produced by the close overlapping of course-veined brittle leaves with prominent midribs.
The butterheads have relatively soft textured leaves with a greasy appear- ance. As in the crispheads, the leaves of mature plants form a heart, although less firm than the crispheads.
2. Lactuca sativa var. longifolia Lam.: the cos or Romaine lettuce. The upright, relatively narrow, crisp textured leaves form a closed head.
3. Lactuca sativa var. crispa L.: the ‘leaf’ or curled lettuce does not form a heart but has a loose head of leaves. Some of the cultivars have very curly and fringed leaves.
4. Lactuca sativa var. asparagina Bailey (syn. var. angustana Irish): all the forms of this group have typical fleshy stems which are the main culinary attrac- tion, especially in Asia. The group may also be referred to as ‘stem lettuce’, a typical cultivar is Celtuce. Some of the members of this group have a light-grey leaf colour.
There is an increasing interest and demand in some markets for lettuce marketed as’ baby leaves’, especially for the pre-packed salad trade.
Systems for the classification and identification of lettuce cultivars have been documented by Bowring (1969) and Rodenburg and Huyskes (1964). Classifications according to seedling characters only have been based on work by Rodenburg (1958), and while extremely useful for ‘Distinctness, Uniformity and Stability’ tests, the characters are useful for selection or roguing only if observations are made before about the fifth leaf stage. Some of the seedling characters such as absence or pres- ence of anthocyanin cannot be observed after the first true leaf has emerged.
Clarkson and George (1985) have examined the application of seed and seedling characters up to the third leaf for the identification of lettuce cultivars, examples of the third leaf stages of six cultivars are shown in Fig. 8.1.
Fig. 8.1. Third leaf of six lettuce cultivars showing range of morphological types.
Tom Thumb Paris White Continuity Lollo Rossa Oak Leaf Salad Bowl
Cultivar description of lettuce
● Seed: colour white, yellow or black.
● Morphological head type: butterhead, cos or crisp; hearting or leafy.
● Season of use: summer, outdoor or protected cropping, winter hardiness.
● Seedling or young plant characters at 3–4 leaf stage (see Fig. 8.1).
● Anthocyanin pigmentation: leaf colour and form.
● Mature plant: leaf lustre, degree of blistering.
● Time to maturity, relative duration mature plant holds for market before bolting.
● Resistance to specific races of Bremia lactucae (downy mildew).
● Resistance to Lettuce Mosaic Virus (LMV).
(A detailed Test Guideline 013 is obtainable from UPOV, see Appendix 2.)
Soil pH and nutrition
This crop should be grown in a soil which has a pH of at least 6.0. Because lettuce is susceptible to calcium deficiency, it is preferable to adjust the pH to 6.5 by adding a suitable liming material during site preparation.
The lettuce seed crop responds to the same pattern of nutrients as the market crop and seed producers generally use a base dressing of 3:2:2 of N:P:K. Nitrogen plays an important role in total seed yield which responds to up to 80 kg nitrogen/ha. However, excess nitrogen tends to produce a loose atypical head which is difficult to confirm as true to type when roguing or selecting. Nitrogen can be applied during seedbed preparation but supplemen- tary top dressings or foliar sprays (using urea) should be applied during the crop’s growth. This is especially useful if there is likely to have been significant amounts of leaching. Extra applications of nitrogen when the plants have com- menced bolting will increase seed yield.
Sowing and young plant establishment
The seed is drilled into rows 50–60 cm apart at a rate of 1.5–2 kg/ha. The young plants are thinned to between 25 and 30 cm apart within the rows.
Plants raised in modules are planted 25 cm apart with 50–60 cm between the rows.
Irrigation
The lettuce seed crop responds directly to irrigation. In addition to increasing total plant weight in the vegetative stage, a satisfactory supply of water assists in the increase of total seed yield, although there is a tendency for seed crops receiving frequent irrigation to be up to 5 days later in maturity. The significant
gain in seed yield is generally considered to outweigh the possible disadvantage of a slight delay to optimum harvest time of the seed.
The influence of water stress at different lettuce growth stages was investi- gated by Izzeldin et al. (1980). They reported that the highest seed yield was obtained when the mother plants were subjected to a moderate water stress during the vegetative stage but followed by adequate available water during anthesis. Water stress during anthesis resulted in a reduction in seed yield.
In areas where the total water requirement is applied through overhead irrigation systems, growers generally maintain a drier regime when seed ripen- ing has commenced. Overhead irrigation is detrimental once seed has begun ripening because the impact of the water droplets causes ripe seed to fall. In some areas, or conditions, irrigating late in the life of the crop encourages weed growth which can make harvesting difficult and increase contamination of the harvested seed with weed seeds.
Flowering and pollination
The physiology of lettuce has been reviewed by Wien (1997b), who reported that lettuce is a quantitative long-day plant and that it can also be seed vernal- ized. The vernalization requirements of some lettuce cultivars and other Lactuca species have been investigated by Prince (1980). As there are types which heart in response to day length, it is important to grow individual cultivars under the conditions prescribed by the breeder in order to confirm the morpho- logical characters. Waycott (1993) has discussed the effect of photoperiod on the transition to flowering in lettuce genotypes.
The inflorescence of lettuce, which is called a capitulum, contains approxi- mately 24 florets. These are highly developed in favour of self-pollination and the crop is therefore largely self-fertilized. However, some cross-pollination can take place between lettuce cultivars and also between cultivated lettuce and some wild Lactuca species. For example, the cultivated lettuce Lactuca sativa is cross-compatible with Lactuca serriola, which is a wild plant and fence-line weed, especially in countries around the Mediterranean and parts of Asia. If mechanical contamination of the lettuce seed crop with seeds of wild Lactuca species occurs, it presents insurmountable problems during the subsequent seed processing.
Isolation
For the production of certified seed a period of 3 years should have elapsed from a previous lettuce seed crop produced on the same site or 2 years from a previous market crop. Some authorities make an exception to this minimum isolation period when the soil or substrate has been effectively fumigated or partially sterilized. This exception is especially useful when lettuce seed is being produced in protected structures or glasshouses.
Although up to 5% cross-pollination has been observed in lettuce in some areas, most authorities regard it as a self-pollinating crop and only specify a physical barrier (e.g. adjacent sections of greenhouses) of a minimum of 2 m between different cultivars.
Roguing and selection
There are three main stages for roguing and selection, which are:
1. The young plant during the four- to six-leaf stage.
2. The mature plant at time of heading.
3. After bolting has commenced.
There is an additional earlier stage when the plant is still a seedling, up to the third leaf. In practice this can only be used prior to planting, when the plants have been raised in modules or if relatively few seedlings are being examined (see Fig. 8.1).
The most important roguing stage is at the time of heading; in practice this is usually the only stage at which commercial seed crops are rogued.
The features used in the assessment of trueness to type of lettuce seed crops are mainly based on the morphological characters observed during the vegetative stages of plant growth up to and including hearting (or head- ing in the case of non-hearting cultivars) and are listed above under cultivar description.
Seed stalk emergence
The seed stalk does not always emerge readily from the lettuce heart. Although this is not a major problem with the butterhead types, it can be with the crisp- heads because many of the cultivars in this group form very firm heads which are mechanical barriers to the emerging inflorescence. The poor emergence results in the development and elongation of basal shoots, relatively few flowering shoots on the main axis which becomes distorted and a reduction of the potential seed yield. The general tardiness of bolting of firm-headed lettuce also results in an increase in the mature plant’s exposure to pathogens such as Botrytis cinerea.
Improving seed stalk emergence
Traditionally, several mechanical methods have been used to assist the emergence and elongation of the flower shoots. These have included deheading, slashing and quartering of the heads with special tools to assist the breakage of the solid heart without damaging the unexposed growing point; some lettuce seed producers refer to this operation as ‘racing’. Another simple method is to give a sharp blow with the palm of the hand to the top of the lettuce head. This fractures the leaf bases and, following removal of the detached head, the growing point remains
unharmed. An alternative is to manually peel off the leaves around the heart, although this is only really practical when relatively few plants are to be used for seed production. The timing of the mechanical or manual operations is important;
if left too late, the extended flowering shoot inside the heart will be damaged.
Growth-regulating chemicals have been used to promote bolting in lettuce.
Gibberellic acid has been applied before hearting to promote early bolting, thus avoiding the problems associated with flower shoot emergence. The applica- tion of aqueous solutions of gibberellic acid at concentrations between 20 and 500 ppm have been used for the butterhead types (Harrington, 1960). However, the application of chemicals to lettuce before the hearts have been completely formed in order to assist flower-stem emergence does not enable the seed producer to confirm the plant’s morphological characters at normal market maturity and can therefore adversely affect the selection or roguing efficiency.
Wurr et al. (1986) included the use of gibberelin 4 plus 7 in their studies of the effects of seed production techniques on seed characteristics, subsequent seedling growth and crop performance of the crisp lettuce cultivar ‘Pennlake’.
They concluded that the major differences between commercial seed lots of lettuce result from differences in the environment rather than the seed produc- tion technique.
Seed development and harvesting
The succession of inflorescences during anthesis subsequently provides a steady sequence of ripe seed. The length of time from flowering to ripe seed produced on an individual capitulum is 12–21 days, depending on environment. High temperatures increase the rate of development and ripening.
The influence of post-flowering temperature on seed development and subsequent performance in crisp lettuce, cv. Saladin, was investigated by Gray et al. (1988b). They reported that seed yields per plant increased from a mean of 15–27 g, and fell to 20 g with progressive temperature increases (in 16 h day and 8 h night) of 20/10°C, 25/15°C and 30/20°C; they also included evalua- tions of resulting seed quality. In general, seed produced at 25/15°C exhibited a greater variation in numbers of seeds per floret, cell numerical volume den- sity, seed weight, times of seedling emergence and seedling and mature head weight than seed produced at the lower temperatures.
If the seed producer waits for the development and ripening of the seeds from the later inflorescences, the earlier ripened ones will probably have been shed and lost. It is, therefore, general practice to cut and harvest the seed when an estimated 50% of seed heads are ready on a typical sample. The stage of ripeness at which the pappus is fully developed and dry is referred to as ‘feath- ering’, as illustrated in Fig. 8.2.
The standing lettuce seed crop can be hand- or machine-cut and left in windrows. A machine which minimizes shattering should be used for the cut- ting process. There is less shattering if the plants are cut with the dew on them.
The cut material is normally left in windrows for up to 5 days, but in very arid areas the seed can be extracted the same day as cutting. After windrowing the
seed is extracted in stationary threshers or passed through a combine. If the seed is combined direct from the standing crop, many small pieces of wet plant debris, such as fragments of leaf bract, will be mixed with it, causing an undesir- able increase in seed moisture unless the material is quickly dried.
In production areas, with plenty of available labour, the seed can be har- vested from single plants by shaking their heads into a canvas bag or sack. If this is done every 2 or 3 days the maximum amounts of seed are collected.
Seed cleaning
Pre-cleaning vegetable seeds of Asteraceae is an important operation because the weight of chaff in mechanically harvested seed can be as high as one-and- a-half times the weight of the seed itself. The initial cleaning can be done with an air-screen machine. There is usually some plant material such as small pieces of flower stem remaining in the sample which can be removed by passing the seed through a disc separator or an indent cylinder.
Seed yield and 1000 grain weight
A satisfactory seed yield for hearting lettuce under good conditions is between 0.5 and 1 t/ha.
The 1000 grain weight of lettuce is from 0.6 to 1.0 g according to the cultivar.
Fig. 8.2. Lettuce with approximately 50% of seed ready for harvest.
Indexing for lettuce mosaic virus (LMV)
The macroscopic symptoms of LMV are observed as a clearing between the veins when a portion of leaf from an infected plant is held up to the light. Plants which become infected relatively early in their life are usually stunted and fre- quently fail to heart. When seeds are saved from infected plants, up to 15% of the seeds carry the virus in their embryos. Plants which are infected as a result of seed transmission act as reservoirs from which sap-sucking insect vectors transmit the virus to other plants in the same or neighbouring crops. The aphid Myzus persicae is especially important as a vector of this virus.
In addition to ensuring satisfactory isolation from other lettuce crops it is important to rogue out infected plants as soon as the symptoms are observed.
Basic seed stocks can be produced in insect-free structures or, in some parts of the world such as Australia and California, at temperatures too high for aphid attack. There are other viruses of lettuce that cause similar mosaic-like symp- toms but they have not been found to be seed-borne.
Mosaic-indexed seed is produced under strictly controlled conditions which incorporate satisfactory isolation, roguing and virus vector control. The result- ing seed is then subject to ‘mosaic indexing’. In the standards for seed health of selected vegetable crops for certification in the UK, this involves growing on a sample of 5000 seeds derived from the seed crop in a cool insect-proof green- house or structure and noting the incidence of LMV. A basic or certified lettuce seed stock which has been found to produce ‘nil infection’ from this screening is said to be ‘mosaic-indexed’. This is an example of how the seed producer can assist the grower to avoid problems arising from seed transmitted pathogens.
Pathogens
The main seed-borne pathogens of Lactuca sativa with the common names of the diseases they cause are listed in Table 8.1.