Cucurbit Genetics Cooperative Report 10:10-11 (article 9) 1987
R. W. Robinson
Horticultural Sciences Department, New York State Agricultural Experiment Station, Geneva, NY 14456
Cucumber plants normally have alternate leaves, with a single leaf per node at 180° angle from leaves at adjacent nodes. The Lemon cultivar is heterogeneous for leaf arrangement, with some plants having alternate leaves and others two opposite leaves per node, borne at 90° angle from the pair of leaves at the next node. In a population of 930 ‘Lemon’ plants, 17% had opposite leaves.
Opposite leaf arrangement is unstable. All plants with opposite leaves at the first nodes of their main stem eventually convert to alternate leaf arrangement. The number of nodes with opposite leaves on 18 opposite-leaved ‘Lemon’ plants varied from one to ten, with a mean of 5.4. The change from opposite to alternate leaf arrangement is usually abrupt, with all nodes on the main stem above the point of transition having alternate leaves and those below opposite leaves. Occasionally, a plant has a node with a single leaf between nodes with opposite leaves. Internodes are often longer after the change from opposite to alternate leaf arrangement.
Opposite-leaved plants do not breed true for that trait. Progeny of over 100 self-pollinated ‘Lemon’ plants with opposite leaves all segregated for alternate vs. opposite leaf arrangement.
Opposite leaf arrangement is recessive. All F1 plants of alternate x opposite or the reciprocal cross had alternate leaves.
There is no evidence of a cytoplasmic factor being involved in the inheritance of opposite leaf arrangement; similar ratios were obtained in the F2 generation of reciprocal crosses between alternate- and opposite-leaved plants. The proportion of seedlings with opposite leaves was significantly less than 25% in each of 26 F2 populations. The combined segregation ratio was 875 alternate to 86 opposite.
Tkachenko (1) concluded that at least three genes are required to produce opposite leaves. An alternate explanation is that inheritance is simple, but the single recessive gene has incomplete penetrance. Evidence agreeing with, though not conclusively proving the latter hypothesis, was obtained when linkage was detected between opposite leaf arrangement and two genes known to be on the same chromosome. Genes of ‘Lemon’ for sex expression (m) and five fruit locules (l) are linked (2), and were associated with opposite leaves in segregating generations. In coupling phase F2 populations there were 302 alternate +: 51 alternate m: 31 opposite +: 22 opposite m (contingency X2 = 22.9, p > .001) and 139 alternate +: 29 alternate l: 21 opposite +: 19 opposite l plants (contingency X2 = 16.6, p > .001). It is suggested that the gene for opposite leaves that is linked with m and l be given the symbol opp.
Literature Cited
- Tkachenko, N.N. 1935. Preliminary results of a genetic investigation of the cucumber, Cucumis sativus L. Bull. Appl. Pl. Breed. Ser. 2, 9:311-356.
- Youngner, V.B. 1952. A study of the inheritance of several characters in the cucumber. PhD. Thesis. Univ. Minnesota, St. Paul.