Cucurbit Genetics Cooperative Report 15:1-3 (article 1) 1992
E. Van Wann
U.S.D.A. Agricultural Research Service, South Central Agricultural Research Laboratory, Lane, Oklahoma 74555
One of the objectives of the U.S. National Plant Germplasm System (NPGS) is to evaluate and develop an automated descriptive inventory of all accessions (plant introductions) in the system. The Germplasm Resources Information Network (GRIN) was established to receive and enter descriptive data into the automated system as it becomes available. The Crop Advisory Committee (CAC) responsible for each crop species develops a list of descriptors and provides oversight for much of the evaluation work. This evaluation of cucumber (Cucumis savitus L.) accessions (PIs) for tolerance to soil moisture deficit was supported in part by funds provided by the NPGS as approved by the CAC for cucurbit crops.
The U.S. cucumber germplasm collection consists of approximately 800 accessions. They have diverse origins, representing about 45 countries and most of the continents of the world (1, 3). Although the cucumber collection is small compared to other major crop species, it has been described as marginally adequate for economically important traits, such as disease resistance and tolerance to environmental stresses (2). Genetic diversity is critical to the development of improved cultivars. Also important to plant breeders is the availability of identified sources of genetic traits that may be desired in breeding programs. Stress tolerance, especially drought tolerance, has not been studied extensively in cucumber, and the amount of genetic diversity for that trait is unknown. This project was undertaken to determine the extent of genetic diversity for response to soil moisture deficit and to provide evaluation data to GRIN.
To date, 649 accessions from the cucumber collection that is maintained by the Regional Plant Introduction Station at Ames, Iowa have been evaluated for their response to soil moisture deficit. A segment of the collection was evaluated in 1989, 1990 and 1991; with 300 the first year, 200 the second year and 149 the third year. The evaluations were made in field plantings using a randomized complete block design with 4 replications. Each plot consisted of 2 rows 1.5 m long spaced 0.4 m apart on beds 1.5 m wide. Plots were over-seeded and thinned to 20 plants per plot after seedling emergence.
Soil moisture was maintained by irrigation so that soil tensiometer readings did not exceed 20 kPa until plants began to flower. By withholding irrigation at that time, the soil tensiometers were allowed to reach approximately 55 kPa. During the period in which the tensiometers were between 35 and 55 kPa, plots were rated visually on a scale of 1 to 9. The scale was established based on the percentage of leaves in a plot that showed wilting, where 1=+ 100% wilting, 3 = 75%, 5 = 50%, 7 = 25%, and 9 = 0% of the leaves wilted. The interim values were used when the percentage of wilted leaves was judged to be between the 25% intervals.
In each of the three segments of the germplasm collection, we found a full range of responses to the moisture deficit. The frequency distribution of individual accessions showed a bell-shaped curve (Fig. 1). The similarities of the three frequency distributions indicated that evaluations could be compared across segments (years).
The most tolerant accessions in each segment of the germplasm collection are listed in Table 1. In 1989, 4 accessions had a mean rating of 7.7 and above (2% of the population) which was considered to have strong tolerance to soil moisture deficit. In 1990 and 1991, the frequency of tolerant accessions was 4.0 and 4.7% respectively. In each of the tests, there were accessions at the extreme end of the rating scale that showed high susceptibility to wilting when subjected to soil moisture deficit. Those accessions amounted to approximately 20% of all the accessions tested.
Some of the accessions that were rated highly tolerant had unique plant and leaf characteristics. Two had a short internode (dwarf) plant type and another had upright, glabrous leaves. However, others had normal plant and foliage characteristics typical of mos commercial cultivars.
In a separate study, the visual rating technique was compared to several other parameters of cucumber plant response to soil moisture deficit (Wann and Staub, unpublished). Visual rating was shown to be a reliable technique for evaluating cucumber plant response to soil moisture deficit. In that study, the Scheduler Plant Stress Monitor (The Carborundum Company, Solon, OH 44139) was used to determine the crop water stress index (SWSI). There was a significant negative correlation (r = -0.59, P = 0.01) between the visual ratings and the CWSI. However, neither rating system would be expected to distinguish among plant mechanisms that might account for their tolerance to a soil moisture deficit.
Table 1. The most tolerant accessions in each segment of the germplasm collection with ratings are listed.
Year |
PI accession |
Origin |
Visual Ratingz |
| 1989 test | 200815 | Burma | 7.7 |
| 1989 test | 263079 | Soviet Union | 9.0 |
| 1989 test | 308915 | Soviet Union | 9.0 |
| 1989 test | 308916 | Soviet Union | 9.0 |
| 1989 test | 344445 | Iran | 8.1 |
| 1989 test | 422181 | Czechoslovakia | 7.7 |
| 1990 test | 164433 | India | 9.0 |
| 1990 test | 211962 | Iran | 7.6 |
| 1990 test | 279468 | Japan | 7.6 |
| 1990 test | 292012 | Israel | 8.0 |
| 1990 test | 344438 | Iran | 8.1 |
| 1990 test | 426629 | Pakistan | 8.5 |
| 1990 test | 525075 | Mauritius | 8/1 |
| 1991 test | 169392 | Turkey | 7.7 |
| 1991 test | 169395 | Turkey | 8.1 |
| 1991 test | 176519 | Turkey | 7.8 |
| 1991 test | 204568 | Turkey | 7.8 |
| 1991 test | 211984 | Iran | 7.7 |
| 1991 test | 211985 | Iran | 7.9 |
| 1991 test | 249550 | Iran | 8.0 |
z Mean of 4 observations taken from each of 4 replications.
Fig. 1. Frequency distribution of cucumber accessions based on visual rating of plant response to imposed soil moisture deficit. Ratings were made on a scale of 1 to 9 where 1 = severe stress (100% of leaves wilted) and 9 = no visible stress response. Tests were run in 3 years: 1989 (300 accessions), 1990 (200 accessions) and 1991 (149 accessions).
Literature Cited
- Knerr, L.D., J.E. Staub, D.J. Holder and B/P. May. 1989. Genetic diversity in Cucumis sativus L. assessed by variation at 18 allozyme coding loci. Theor. Appl. Genet. 78: 119-128.
- Staub, J.E., J. Bachzynska, D. van Kleinwee, M. Palmer, E. Lakowska and A. Dijhuizen. 1989. Evacuation of cucumber germplasm for six pathogens. Proc. Cucurbitaceae 89: 149-153. Charleston, SC.
- Staub, J.E. and Alina Krasowska. 1990. Screening of the U.S. cucumber germplasm collection for heat tolerance. Cucurbit Genet. Rpt. 13: 4-7.