Cucurbit Genetics Cooperative Report 15:67-68 (article 25) 1992
Cold Resistance in Accessions of Watermelon from Zimbabwe
Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456
Watermelon seedlings grown at low temperatures (< 20 C
˚ ) usually develop a foliar mottle and stunting. Cotyledons appear to be whitish green, while the first leaves develop a 'mosaic', consisting of scattered irregular white-yellow patches. A persistent low temperature is conducive to more prominent foliar symptoms, malformation, and growth retardation. Varietal reaction, however, is also an important factor, hence, some cultivars appear more affected than others by this disorder. Due to this sensitivity, temperature is a major factor in determining watermelon planting dates in warm. as well as temperate regions of the world.
During the winter of 1988, while analyzing for viral resistance a collection of 57 plant introductions (PI's) of Citrullus lanatus from Zimbabwe (3, 4), we noted that plants of a large number of these lines (67%) were prominently affected by mosaic-like symptoms. Under the same conditions, plants of the remaining lines (33%) developed normal growth,. To confirm the apparent resistance to cold temperature (15-20
˚ C), which prevailed at the beginning of our tests, additional seedlings of the same collection and domestic cultivars were exposed to two sets of temperatures, using greenhouses and growth chambers. at 28 - 30
˚ C., plants of every line grew normally, but at 15 - 20
˚ C, those of 38 PI lines and 15 domestic and foreign cultivars developed varying degrees of foliar variegation, malformation, and stunting. Under the same conditions, 19 PI lines yielded mostly normal plants.
The availability of cold-resistant watermelons offered the opportunity to elucidate simultaneously the genetics of the mosaic-like disorder and cold resistance. Inheritance studies were based on crosses between the cold-sensitive cultivar New Hampshire Midget (NHM) with the sold-resistant line PP261-1. This was a single-plant selection from PI 482261, which we previously reported to be resistant to the Florida strain of zucchini yellow mosaic virus (ZYMV-FL), and poses sing the gene for the isozyme Pgi-2b (1, 3). Using similar genetic populations we had also found that the resistance to ZYMV-FL in PP261-1 was conferred by the single recessive gene zym, not linked to Pgi-2b (3). cold tests were conducted in both an insect-free greenhouse and a growth chamber at 15 - 20
˚ C. Although complete data will be presented at a later date, our findings can be summarized as follows:
a) All plants of F1 (PP266-1 x NHM) and F1 (NHM x PP261) were normal.
b) Resulting F2 plants of these crosses segregated in a ratio of 3 normal: 1 cold-sensitive.
c) Plants of F1 x NHM were all cold-sensitive, whereas those of F1 x PP261-1 segregated 1 normal : 1 cold sensitive.
d) There was no detectable linkage between the factor for cold resistance and Pgi-2b or zym. However, Pgi-2b ( = Pgi-22 ) (2) was a valuable marker in determining whether some of the populations resulted from true crosses.
From our findings, it is evident that the mosaic or leaf variegation affecting many watermelon lines is not cytoplasmically inherited. This disorder is induced by the expression of a single recessive nuclear gene, which is activated by temperatures below 20˚C. In the absence of such a factor, plant growth is unaffected by temperature. Because this cold-resistance is inherited dominantly, it can be easily exploited for the development of commercial F1 hybrids.
The watermelon collection from Zimbabwe offers a range of plant sizes, leaf types, fruit characteristics, and maturity. Most of the cold-resistant lines can be considered as primitive forms (landraces) of cultivated watermelon. They develop very large and vigorous plants, producing fruits with a firm yellowish flesh, and low sugar content. In Zimbabwe, these fruits are used mostly for cooking and for animal feeding. Due to their unusual firmness and low sugar, these fruits can be stored for many months. Young plants of these lines can be used as rootstocks, because of their well developed root system and apparent resistance to soil-borne diseases.
The introduction of cold resistance into commercial cultivars can have great economic advantage for the production of early crops. They could be planted one or two months before the usual planting dates, particularly where heavy frost occurs rarely. Although none of the cold resistant lines is frost-resistant, in two consecutive years, they were only moderately damaged by the first heavy frosts in western New York.
- Benscher, D. and R. Provvidenti. 1991 Allozyme diversity at the pgi-2 locus in landraces of Citrullus lanatus from Zimbabwe. Cuc. Genet. Coop. 14:104-106.
- Henderson, W.R. 1991. Gene list for watermelon. Cuc. Genet. Coop. 14: 127-137.
- Provvidenti, R. 1991. Inheritance of resistance to the Florida strain of zucchini yellow mosaic virus in watermelon. HortScience 26:407-408.
- Toll, J. and V. Gwaranzimba. 1983. Collecting in Zimbabwe. Plant Genet. Resources Nwsl. ACP:PGR 53:2-5.