In breeding for high ascorbic acid we were using a plan of procedure similar to the convergent improvement, or double back-cross, technique of Richey and Sprague. Two divergent series of selections have been established by backcrossing to both parents from F1 the cross 2958 x P6 (high weight low ascorbic, 3-way disease resistant x low weight high ascorbic disease susceptable--obtained from Yeager). Between the F1 (first convergence) and the secondary F1 (second convergence) the best judged single plant in each divergent selection was self-pollinated for two successive generations. Second convergence was made in a screenhouse where multiple cross pollination was made between those plants in each of the two series which performed best with the characters of the non-recurrent parent.
Since it is possible to raise almost three tomato crops annually, and seasonal variation in fruit size and ascorbic acid content is high it is difficult to know exactly how much progress is being made on the different convergences. However, the two parents to the original cross are planted with each test and when comparison is made between the performances if the derivative lines with the parents from step to step, it is apparent that considerable progress is being made in the direction of combining commercial size with disease resistance and increased yield of ascorbic acid concentration. Trends observed between the second and third convergences are indicated in the table attached.
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Yield at |
second |
converg- |
ence |Yield from progenies backcrossed to the two
(January | divergent parents. (August l950)
1950) |
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Weight Ascorbic| wt. Ascorbic
grams acid | Total Fruit per acid
per mg/100gm|nature of Yield per fruit mg/
fruit |backcross (gm.) plant (gm.) 100gm
___________________|__________________________________________
15 1 45.0 35.6 5-1 x T6 plant#116 1317 37 35.6 48.6
X= 1010 32 31.7 36.6
5-1 x 2958 (Lost)
10-7 x P6 plant#20 1384 49 28.2 48.6
X10 7 38.7 40.2 plant#25 1372 35 39.2 48.2
plant#178 1440 49 29.4 46.4
X 35.9 34.6 X= 889 29 30.4 39.3
10-7 x 2958
plant#83 2744 36 76.2 33.4
plant#84 4741 53 89.4 35.8
X= 1361 24 56.4 29.1
P6 27.4 40.0 P6-2-n X= 1185 46 25.8 43.4
2958 61.9 26.9 2958-10-n X= 1383 21 65.9 26.1
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x designates selected plant
X desigmates mean value of population