Cucurbit Genetics Cooperative Report 4:32-33 (article 24) 1981
O. Shifriss
Department of Horticulture and Forestry, Cook College, Rutgers University, New Brunswick, NJ 08903
Gene B conditions precocious fruit pigmentation in Cucurbita pepo and C. maxima. But this gene can also affect plant growth, sexuality, and fruit quality. Some of these effects are beneficial and others, detrimental. According to my present hypothesis, the beneficial effects can be enhanced and the detrimental effects can be suppressed by gene substitutions at other loci (1). From this point of view, the coadaptation of B depends on its harmonious interactions with other genes.
Gene B brings about precocious pigmentation by blocking chlorophyll synthesis or by causing chlorophyll depletion early in fruit development. The substitution of B for B+ in different cultivars of maxima and pepo revealed the fact that B can block chlorophyll synthesis in other potentially green organs or tissues such as leaf blade, petiole, stem, tendril, and the calyx of staminate flowers. It appears that the genetic background can determine (i) the expression or suppression of B in any one of these organs or in all of them, (ii) the timing of B expression, and (iii) the sensitivity of B yo environmental stimuli.
One of my objectives is to identify some of the genetic elements which regulate the expression of B in different vegetative tissues, particularly in leaves and stems. Some data on the inheritance of resistance (inactive B) and susceptibility (active B) to leaf yellowing in a cross between two BB inbreds are presented in Table 1. From these data I cannot identify the number of segregated genes which affect the expression of B in leaves. Furthermore, I find it difficult to duplicate these data for some unknown reasons. Nevertheless, two facts are consistent in repeated inheritance studies of this cross. First, the frequency of the parental phenotypes in the F2 is relatively high. Second, selection in subsequent filial generations can lead to the development of a line that is more resistant than the resistant parent under field conditions. The evidence suggests that the expression of B in leaves is conditioned by relatively few genes and is greatly affected by non-genetic variations. Many observations support the conclusion that the gene pools of C. maxima and C. pepo consist of elements which can suppress or inactivate B in all organs other than the fruit and that these hypothetical elements will play a major role in the coadaptation of B in the cultivated Cucurbita.
Table 1. Inheritance of resistance and susceptibility to leaf yellowing in the cross between ‘Precocious Fordhook Zucchini’ (PEZ), B/B, resistant, and ‘Jersey Golden Acorn’ (JGA), B/B, susceptible for plants grown together in a controlled environment in 1978.z
Grading scale for classification of variations in extent of yellowing in the first and second true leaves |
||||||||||||||||||||||
Generation |
1-1y |
1-2 |
1-3 |
1-4 1-5 |
2-1 |
2-2 |
2-3 2-4 2-5 |
3-1 |
3-2 |
3-3 |
3-4 |
3-5 |
4-1 |
4-2 |
4-3 |
4-4 |
4-5 5-1 |
5-2 |
5-3 |
5-4 |
5-5 |
Total plants |
| P1, PFZ |
9 |
1 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
10 |
| P2, JGA |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
4 |
5 |
10 |
| F1 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
4 |
2 |
1 |
0 |
0 |
2 |
1 |
0 |
0 |
10 |
| F2 |
11 |
0 |
1 |
0 |
5 |
1 |
0 |
10 |
5 |
9 |
7 |
1 |
5 |
2 |
6 |
10 |
0 |
0 |
0 |
8 |
9 |
90 |
z 12 hr photoperiod, light from G.E. F96T12CW 1500 fluorescent tubes which generate 1.1 to 1.5 millieinsteins at the shoot apices, 22°C during day and 20°C at night, plants grown in mix in 10 cm clay pots. Although PFZ is resistant in this environment it is only moderately so under field conditions. Susceptibility in JGA is confined to the first 5-6 leaves as plants gradually turn green thereafter. Low temperatures and low light intensity are conductive to leaf yellowing in BB plants.
y The grades ranged from 1 to 5: grade 1 for a green leaf with less than 5 yellow spots and grade 5 for uniformly yellow leaf. Grade 1-1 means that the first and second true leaves were of grade 1. Grade 4-2 means that the first true leaf was of grade 4 and the second of grade 2.
Literature Cited
- Shifriss, O. 1981. Origin, expression and significance of gene B in Cucurbita pepo L. J. Amer. Soc. Hort. Sci. 106:220-232.