Young, H. W.
Two Lycopersicon esculentum breeding lines, Alabama 10-1 and a crack susceptible strain of Marglobe differentiated by genetic "marker" genes for unripe fruit color, ripe fruit color and plant growth habit and characters not known to be controlled by qualitative genes such as fruit cracking, number of fruit per plant, number of locules per fruit and frait diameter were crossed. A total of 2525 plants of the P1, P2, F1, F2, F3, BC to P1 and BC to P2 generations were observed in the years 1951, 1952, and 1953 in randomized plots at the Horticultural Farm, The Ohio State University, Columbus, Ohio. The plants were pruned to one stem and subjected to fluctuating moisture conditions by use of overhead irrigation.
Length and depth measurements were made of each fruit crack and notation made as to whether the crack was of the radial, concentric or bursting type. In addition to cracking, data were also taken on all the other characters listed above, which differentiated the two parent lines. Data were taken daily and each fruit was harvested at the red ripe stage of maturity.
Radial crack resistance was concluded to be recessive and controlled by two major genes designated as cr and ra located in linkage groups IV and III respectively. A radial crack index system was used to evaluate more critically radial crack resistance. Radial crack resistance was associated with low incidence of concentric cracks, pink fruit color, large number of fruit per plant, low number of locules per fruit, small fruit diameter and semi-determinate plant growth habit. In view of the association of radial crack resistance with few locules per fruit, it was necessary to postulate a pair of genes, designated as Lo lo for locule number, located in linkage group IV, to explain the inheritance of locule number in the cross studied.
A mild type of concentric cracking found in the cross studied exhibited a heterosis effect and increased concentric cracking in the F1 and segregating progenies. The incidence of this type of cracking was associated with indeterminate plant growth habit, uniform unripe fruit color, few fruit per plant, radial crack resistance, cracking resistance of the burst type and small fruit diameter. However, a precise mode of inheritance of concentric cracking was not postulated.
Bursting resistance was found to be recessive and was concluded to be controlled by two major genes designated as Bt (with greater effect) and bg (with lesser effect) located in linkage groups III and VII respectively. Low incidence of bursts were associated with uniform unripe fruit color and low incidence of concentric cracks. It was suggested that the burst type of cracking might be a source of difficulty in tomato lines if they were not screened to eliminate this character before the lines are used in breeding for radial crack resistance.
Because of the recessive nature of the radial and burst types of cracking and the association of radial and concentric cracking with certain undesirable characters such as few locules per fruit and small fruit diameter, it is suggested that large F2, F3 and Backcross populations will be needed to isolate the cross-over types. It is further suggested that selection should be made in F2, F3 and Backcross populations of a cross involving parents with resistance to all types of cracking, in addition to other good horticultural characters to increase the probability of combining crack resistance and acceptable commercial qualities.
The following table summarizes the proposed genotypes of the parents used in this experiment:
P 1 - Alabama 10-1 P 2 - Marglobe
Plant habit sp sp (Semi-)determinate indeterminate
Radial cracking cr cr resistance susceptible
Locule number lo lo lesser number greater number
Unripe fruit color u u uniform color green shoulder
Bursting bg bg smaller percentage larger percentage
of burst fruit of burst fruit
Bursting Bt Bt larger percentage smaller percentage
of burst fruit of burst fruit
Ripe color y y colorless epidermis yellow epidermis
Radial cracking ra ra resistance susceptibility
Locule number Lc Lc lesser number greater number