1. Trang chủ >
  2. Khoa Học Tự Nhiên >
  3. Hóa học - Dầu khí >

VII. Major Constraints to Production

Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (19.23 MB, 371 trang )


2 99

and wireworms. Lentil fields in the Palouse can be devastated by aphid-vectored

pathogenic viruses (see subsequent section) and aphid-induced feeding damage

when pea aphid densities intermittently reach outbreak levels. The cowpea aphid

(black and smaller than the light green pea aphid) damages lentil through direct

feeding: its role in vectoring viruses is poorly understood. Several factors seem to

favor pea aphid outbreaks: (1) fall buildup of aphids on alfalfa and other perennial

host plants: (2) mild fall and early winter temperatures favoring abundant egg

laying by aphids, and thus a large overwintering population; (3) mild winter temperatures: and (4) spring conditions conducive to early movement of aphids from

overwintering hosts to lentils (Homan et al., 1991). Aphids have many natural

enemies, including lady bird beetles, parasitic wasps, lacewings, and syrphid flies,

but chemical control may be necessary if these insects do not combine to keep

aphids at subeconomic levels. Insecticide treatment for pea aphid control is considered when an economic threshold of 30 to 40 aphids are collected per 180”

sweep of a 38-cm-diameter insect net, with few natural enemies present, and aphid

numbers do not decline over a 2-day period (Homan et al., 1991).

Lygus bug feeding on the immature reproductive structures of lentil causes seed

and pod abortion as well as a serious seed quality problem known as “chalky

spot” in crops grown in northern Idaho and eastern Washington in some years

(Summerfield et al., 1982). Lygus bugs feed with piercing-sucking mouthparts

and inject toxic saliva into the immature seed. This action results in the formation

of a depression around the feeding area and a chalky blemish. Adult lygus bug

activity can be monitored during bloom and podding by making 25 of 180”sweeps

in at least five randomly selected places in a field. Chemical control is warranted

when 7 to 10 adult lygus bugs are collected per 25 sweeps (O’Keeffe et al., 1991).

The western yellow-striped armyworm is usually a late season pest. When heavy

infestation develops, larvae can defoliate plants and consume pods.


Some of the more serious disease problems of lentil include the following.

1. The Root Rotmilt Complex

Probably the most important disease problems of lentils worldwide are root rots

and wilts caused by P-ythium, Rhizoctonia, Sclerotinia, and Fusarium species

(Kaiser, 1987). Research is underway toward selection for resistance to the various

components of the root rot/wilt complex. Reports on the inheritance of resistance

to Fusarium wilt have been made in germ plasm from India (Kamboj eral., 1990).

Two other important diseases of lentil in many countries, especially in wetter

areas or during years with heavy rainfall, are rust and Ascochyta blight.

3 00


2. Rust

Caused by Uromyces viciae-fabae Pers., rust is a serious problem in areas

where mild temperatures and humid conditions favor development of the disease.

Some sources of resistance have been identified and progress toward developing

resistant cultivars is being made (Khare, 1981). Fortunately, rust of lentils has not

yet 'appeared in the Palouse region of the United States.

3. Ascochyta Blight

Blight caused by Ascochyta fabae Speg. f.sp. lentis Gossen et al., a seedborne

disease, causes severe damage in many cool, wet regions (Fig. 4). Work in several

countries has identified good sources of resistance and these lines are being incorporated into breeding programs. Ascochyta blight is becoming a major problem

in the United States and it continues to be an economic problem in the lentilproducing areas of Canada. Breeding programs have been initiated to introduce

into other lentil cultivars the resistance shown by the cultivar Laird (A. E. Slinkard, personal communication). Thiabendazole seed treatment can reduce the incidence of seedborne A. fabae f. sp. lentis, but the compound is not registered for

use on lentil in the United States (Kaiser, 1987).

4. Seedborne Fungi

In the Palouse, reduced seed quality can result from infection of seeds by different pathogenic fungi, some of which are also pathogens of chickpea and pea

(Kaiser, 1992). The incidence of fungi associated with commercial lentil seeds in

the Palouse varies greatly from year to year and is influenced by weather conditions, particularly rainfall. The seedborne pathogens most frequently isolated from

discolored Palouse grown lentil seeds are Botrytis cinerea, Phoma medicaginis

var. pinodella (= Ascochyta pinodella), and two Fusarium species ( E acuminaturn and E avenaceum). The amount of rainfall during July, when the crop is

approaching maturity or is about to be harvested, appears to affect the incidence,

prevalence, and severity of seedborne pathogenic fungi. If excessive rainfall occurs during harvest or when plants are drying in windrows, lentils that remain on

or near the moist soil surface may have discoloration of their seeds resulting from

these conditions that favor colonization and infection of the pods and seeds by

several pathogenic and saprophytic fungi.

5. Viruses

Viruses are a major lentil disease problem in the Palouse. The viruses that infect

peas also infect lentil: alfalfa mosaic, bean (pea) leaf roll (BLRV), bean yellow


Figure 4.

Ascochyta blight of leaves (A), pods (B), and seeds (C) of lentil.


3 02


mosaic, pea enation mosaic (PEMV), and pea streak. These viruses are transmitted by pea aphids, generally from infected alfalfa and clover plants. Control of the

aphid vectors can reduce infection, but the economic thresholds are not known

and insecticides are often ineffective in preventing the spread of stylet-borne


In the Palouse region, PEMV and BLRV are the most important virus diseases

of lentil, but the crop is also a host of pea seedborne mosaic virus (PSbMV).

Indeed, PSbMV may cause stunting and malformation of leaves, stems, flowers,

and fruits. There may also be a reduction in yield and the production of smaller,

misshapen seeds. Fortunately, PSbMV of lentil has not been found under field

conditions in the Palouse. Sources of resistance to PSbMV have been identified

and are being used in the development of lentil cultivars (Kaiser, 1987). Sources

of tolerance to PEMV have been identified (F. J. Muehlbauer, personal observations) and are currently being incorporated into improved cultivars.



Drought is considered to be the major environmental stress that limits lentil

yields. Lentil crops are often grown in marginal areas where limited rainfall and

deficient soil moisture are encountered. The relegation of lentil to marginal lands

is a consequence of the increased area sown to more renumerative crops (e.g.,

wheat). It is not surprising then that average lentil yields have declined in those

regions. In general, drought and heat stress are commonly encountered by lentil

in the Middle East region and are experienced during the reproductive period (Erskine, 1985a). Drought-tolerant cultivars are required to stabilize lentil production

and also to extend lentil cultivation into those areas that receive less rain.

As with other crops, success in screening for drought tolerance is not yet possible because of the lack of efficient screening techniques and knowledge of what

to screen for. Late planting has been used to simulate drought and heat stress;

however, meaningful comparisons between late and normally planted lentil might

be difficult because of the difference in plant growth duration. A line source irrigation system was used to create a moisture gradient in an area with otherwise

insufficient rainfall (Karaki, 1986). Under the line source system, there was considerable variation among 10 lentil genotypes in response to moisture stress which

gives encouragement for further work.

Another approach to drought resistance screening is to select for drought avoidance through early maturity. This could be a suitable approach when the major

moisture stress occurs toward the end of the growing season. However, in seasons

that receive adequate rainfall, early genotypes may not have the ability to respond

to above-average moisture if and when available.

Agents that simulate moisture stress have been tested for use in drought resistance screening. Haddad et al. (1987) screened 40 lentil genotypes by growing



1-week-old seedlings in different concentrations of polyethylene glycol (PEG) solutions that created different degrees of moisture stress. These genotypes were

grouped into strongly resistant, moderately resistant, and susceptible types.

Landraces are potential sources of drought tolerance because they have demonstrated their ability to survive under extremely stressful environments. The wild

species such as L. culinaris ssp. orientalis might also have considerable drought

tolerance. These potential sources of drought tolerance need to be considered by


Hot weather is thought to cause aborted seeds, empty pods, and reduced yields

in Brazil (Manara and Manara, 1983). Elsewhere, cold tolerance and salt tolerance, if found, could be used by breeders to extend the range of adaptation of the

crop. Jana and Slinkard (1979) found differences in salt tolerance in the cultivated

lentil, but the level of tolerance was too small to be of any breeding value.



Lentil flowers are cleistogamous and naturally self-pollinated, with extremely

infrequent natural cross pollination (estimated to be less than 0.8%)which is presumed to be caused by small insects such as thrips (Wilson and Law, 1972).

Hybridization of lentil flowers with hand emasculation and pollination is difficult because they are small and delicate and must be handled with care. However,

the selection of flowers in the correct development stage followed by careful

emasculation and pollination can lead to a high percentage of successful cross

pollinations. To obtain successful hybridizations it is essential that the reproductive system crossing techniques and environmental conditions be fully understood.



Environmental factors play a major role in the degree of success in lentil hybridization. Flowering and seed set, for example, are improved by a photoperiod

(16 hr or longer) and good irradiance. Flowers usually open before 10.00 hr on

cloudless days, but when the sky is overcast, they may not open until 17.00 hr.

The corolla petals fade within 3 days, and pods are visible 3 or 4 days later

(Meimandi-Nejad, 1977; Muehlbauer et al., 1985; Summerfield et al., 1985).

Lentils are considered to be either long-day or day-neutral plants (Shukla, 1955;

Moursi and Ab El-Gawad, 1963; Saint-Clair, 1972; Summerfield et al., 1985).

Under greenhouse and growth chamber conditions, day temperatures of about

25" C and night temperatures of about 18"C are reasonable combinations for good

3 04


plant growth and seed set. Seed set can also be improved by maintaining relative

humidity at about 50% (Wilson, 1972).



The equipment used to hybridize lentil consists of magnifying glasses (usually

7 or 10 X ), sharply pointed forceps, small tags, and 95% alcohol. Persons with

keen eyesight can hybridize lentils without the aid of magnification. The forceps

used for emasculation and pollination are immersed in the alcohol between crosses

to prevent contamination by unwanted pollen. Tags are used to record the parents,

the date of the cross, and sometimes the initials of the person who made the cross.



Emasculation is necessary to prevent self-pollination.Flowers in which the corolla has reached a length equivalent to about 75% of that of the sepals (Fig. 5A)

are generally at the proper stage for emasculation. At this stage of development,

Figure 5. Cross-pollination in lentil.



the anthers are intact and the stigma is receptive. When the lengths of the corolla

and sepals are equal, the flowers generally will have pollinated.

The flower bud selected for emasculation is held between the thumb and forefinger so that the suture of the keel is accessible. The peduncle holding the flower

is weak and tender and must be handled with care. The sharply pointed forceps

are used to remove the sepals closest to the keel. The wings and the standard are

folded back and held between the thumb and forefinger. The keel is then split

longitudinally to expose the staminal column and stigma. While carefully holding

the flower, the stamens are removed by grasping the filaments with the forceps

and breaking them free from the staminal column to complete the emasculation

(Fig. 5B).


Pollination should be performed immediately after emasculation for best results

(Wilson, 1972; Malhotra et al., 1978; Muehlbauer et al., 1980; Muehlbauer and

Slinkard, 1981). Flowers in which the corollas have elongated to a length about

equal with that of the sepals are selected as sources of pollen (Fig. 5C). The flowers at this stage of development are open or slightly so and have anthers that have

very recently dehisced. Viable pollen suitable for transfer is identified by its bright

orange-yellow color and is contained in flowers in which the keel and wing petals

are turgid. Pollen retains its viability for several days if flowers are collected immediately after anthesis, placed in petri dishes, and stored in the dark at 4- 10”C.

The flower chosen as a source of pollen is held between the thumb and forefinger and the standard and wings are folded back and held. At this point, two

methods of gaining access to the pollen can be used and are equally successful.

One approach is to grasp the keel with the forceps and remove it from the flower

to expose the pollen laden stigma. This pollen laden stigma can then be used as a

“brush” to apply pollen to the emasculated flower to complete the crossing procedure (Fig. 5D). Another method is to puncture one side of the keel with a single

prong of the forceps and peel the keel away from the stigma. This approach often

leaves more pollen on the stigma compared to the former approach. Pollination of

the emasculated flower is performed as described earlier. Sufficient pollen should

be applied to the emasculated flower so that it is readily visible on the stigma; this

may require several male flowers. The petals of the pollinated female flower are

then carefully returned to their original position to protect the stigma. Pollinated

flowers seldom need protection from foreign pollen.

After pollination has been completed, a tag is fixed to the internode directly

below the pollinated flower to identify the cross. An alternative method of identifying crosses is to use color-coded thread tied to the peduncle of the flower or to

the internode directly below.


3 06

Successful pollinations and seed setting vary according to individual skill and

prevailing environmental conditions. Wilson (1972) reported up to 82% successful manual pollinations in a greenhouse environment. Success in that study was

achieved with adequate irradiance, about 50% relative humidity and temperatures between 15 and 25°C. Pollination success greater than 80% was obtained

by Slinkard in controlled environment chambers with dayhight temperatures of

21"/15"C (Muehlbauer and Slinkard, 1981).

The percentage of successful pollinations decreases under field conditions, especially during hot weather with dry air. Malhotra et al. (1978), for example,

reported 27 to 43% success under field conditions in India. Mera and Erskine

(1982) found that enclosing plants in perforated polyethylene bags improved the

success rate compared to unbagged controls. They also discovered that pollinations of the first flowers on a plant had a 44.8% success rate compared to only

17.3% when later-formed flowers were used.



A reliable genetic marker for the identification of F, hybrid seed is red cotyledon color, a trait controlled by a single dominant gene. In using this marker, F,

hybrid seeds from crosses between yellow cotyledon female parents and red cotyledon male parents can be identified by their red cotyledons. Seeds with yellow

cotyledons can be discarded as selfs. The gs gene for stem coloration is also a

useful marker to identify F, hybrids where the male parent has Gs for purple stems

and the female parent has gs for green stems.

Because of the differences in seed size between lentil accessions, it is advisable

to use large-seeded parental lines as the female parents. This reduces losses from

shattering since the pod of the large-seeded parent can accommodate the hybrid

seed while that of the small-seeded parent often cannot. Erskine (personal communication) uses the reverse because the smaller-seeded parent will often yield

two hybrid seeds when used as the female. However, timely harvest is necessary

to prevent losses due to shattering. Pod and seed development can be observed

within 3 days of pollination. When cross-pollination is successful, ovary development is rapid and the swelling seeds in the pod are obvious. But, when crosspollination is unsuccessful, the ovary often enlarges but aborts later even when

the developing pod may have reached as much as 50% of its potential size.

Harvesting and separating the seeds from the pods are always done by hand, as

soon as the pods become yellow-brown, so as to avoid seed losses from pod

dehiscence. Harvested pods should be allowed to dry completely in envelopes or

bags before removing the seeds. Hybrid seeds can usually be kept at room temperature provided they are to be sown in a reasonable period of time. For long-



term storage, lentil breeding material may be stored at about 10"C with 30% relative humidity or in a freezer at - 20" C.



Germ plasm resources for lentil improvement programs are maintained at a

number of places, including the USDA-ARS Regional Plant Introduction Station

located at Pullman, Washington (Table I). Large collections are also maintained

by ICARDA and by the Indian Agricultural Research Institute in New Delhi,

India. Smaller collections are maintained by programs in several countries. [For

more information on germ plasm collection, curators, and addresses, see Bettancourt et al. (1 989).] These germ plasm collections comprise the landraces and, in

the case of the larger collections, the related wild species (Muehlbauer, 1992).

The ICARDA collection is by far the largest and comprises over 6000 accessions, including wild species from 53 countries. The USDA collection numbers

Table I

Lentil Germ Plasm Collections











Ethiopian Genebank/Addis Ababa,


ICARDA/Aleppo, Syria

National Seed Storage Laboratory/

Fort Collins, CO

Pakistan Agricultural Research


Regional Plant Introduction Station/

Pullman, WA

Vavilov Institute of Plant Industry/

St. Petersburg, Russia

ZG Kulturpflanyenforschung/Gatersleben, Germany

Institute of Crop Germplasm ResourceslBeijing, Peoples Republic

of China

Indian Agricultural Research Institute/

New Delhi, India

Number of













over 3500 accessions and includes more that 150 accessions of related wild Lens

species. The collection maintained at the Indian Agricultural Research Institute in

New Delhi exceeds 3000 accessions. Samples for breeding and other research

purposes are available on request.

An evaluation of 4500 accessions from ICARDA’s collection suggested the

presence of considerable variation for important agronomic traits such as grain

yield, straw yield, 100 seed weight, seed number per pod, time to 50% flowering,

time to maturity, plant height, height of lowest pod, and pod number per peduncle

(Erskine, 1985b). Thus, significant variation for important traits is available for

breeding purposes.



All of the recognized wild Lens species have been collected and are being maintained in germ plasm repositories, including L. culinaris ssp. orientalis, L. ervoides, L. nigricans, and L. odemensis (Ladizinsky, 1979c; Muehlbauer, 1981). The

wild material has been the basis of a series of genetic and cytogenetic studies

(Ladizinsky, 1979a,b). Recent efforts have concentrated on collection and preservation to ensure that the wild relatives are available for future breeding. Even

though collections have been made in southern Europe, the Middle East, and Asia

Minor, wild species are still relatively underrepresented in the germ plasm collections (Muehlbauer, 1981; Erskine, 1985b). It is believed that the wild species will

contribute to the improvement of pest resistance, seed quality, and tolerance to

environmental stress.









Lentil genetic information has progressed from relatively few characterized

genes in 1981 (Muehlbauer and Slinkard, 1981) to the characterization of additional genes, isozyme loci, restriction fragment length polymorphisms (RFLPs),

and random amplified polymorphic DNAs (RAPDs) (Zamir and Ladizinsky, 1984;

Havey and Muehlbauer, 1989; Simon et al., 1993). A genetic linkage map for

these loci now contains nearly 100 loci (Simon et al., 1993). The map (Fig. 6) has

shown considerable conservation with the linkage map for pea (Weeden et al.,

1994). The described genes of lentil are listed in Table 11.














PM H l 1l c








Dashed lines indicate regions showingsimilar

linkage in pea. Regions have been situated to

correspond to placement on the pea map. with the

following exceptions.

A: Linkage region found on chromosume5 or pea

B: Linkage region found on chromosome 3 of pea




[=Ten centimorgans


Figure 6.

suspected arrangement based on the pea map

The lentil genetic linkage map.

Table I1

List of Gene Symbols in Lentils























Number of flowers per inflorescence

Plant growth habit

Epicotyl color

Cotyledon color

Cotyledon color (synonymous with Yc)

Gray seed coat ground color

Tan seed coat ground color

Flower color

Pod indehiscence

Resistance to pea seedborne mosaic virus

Seed coat spotting

Flower color

Gill and Malhotra (1980)

Ladizinsky (1979b)

Ladizinsky (1979b)

Slinkard (1978b)

Singh (1978)

Vandenberg and Slinkard (1990)

Vandenberg and Slinkard (1990)

La1 and Srivastava (1975)

Ladizinsky (1979b)

Haddad et al. ( 1978)

Ladizinsky (1979b)

La1 and Srivastava (1975);

Wilson and Hudson (1978)

Wilson and Hudson (1978)

Slinkard (1978b)

Vandenberg and Slinkard (1989)

Vandenberg and Slinkard (1989)

Vandenberg and Slinkard (1989)

Vandenberg and Slinkard (1989)

Flower color

Cotyledon color

Glabrous pod

Green pod color

Tendril-less leaf

Chlorina chlorophyll mutant

Xem Thêm
Tải bản đầy đủ (.pdf) (371 trang)