Jour. N. Y. Bot. Gar. 34(400): 73-80. 1933.
The inheritance of fragrance in Gladiolus species crosses
Forman T. McLean
There is very little exact information about the inheritance of fragrance in flowers. Most of the breeding work with flowers has been to increase the size or to improve the form, color, or growth habits, and even in roses and violets, where fragrance is an important attraction, apparently very little attention has been given to improving or even retaining this desirable trait. In view of the dearth of data on the subject, the results of some experiments with gladiolus species crosses made with the purpose of introducing fragrance into the garden gladiolus should prove interesting.
The garden strains of hybrid gladioli are generally regarded as scentless, and even the Nanus gladioli used for greenhouse forcing are never sweet-scented, but a number of the wild species of South Africa are fragrant, notably Gladiolus tristis, which is strongly scented at night. Several crosses have been made during the past hundred years between G. tristis and large-flowered but scentless species and also garden hybrid gladioli. Herbert (1822-47), Colville (1823), Van Fleet in the United States about 1890, Van Tubergen in Holland, and Diener in California recently, all report on hybrids of this species. Colville's red Colvillei, and Van Tubergen's Charm, Prunella, and Glow are good examples of the results of these crosses. They are all of slender growth, early-flowering habit, and are intolerant of our hot summers, just as is the parent species, G. tristis. They are also scentless and sterile, so they offer little encouragement that a fragrant garden race will ever develop from this use of G. tristis. Several crosses made by the author between G. tristis and Primulinus hybrid gladiolus varieties confirm this view. The seedlings resulting, to the number of more than three hundred, were sterile, small and not sweet.
Hybrids raised by Herbert, and reported about 1822, between Gladiolus tristis and G. recurvus, were apparently sweet-scented, and were named by him G. fragrans. From this, G. recurvus would appear more promising as a parent, though it is a weak grower and poor seeder. But attempts to cross this species with garden varieties uniformly failed, after repeated trials. Then Herbert's old cross between Gladiolus tristis and G. recurvus was repeated, using a strong, vigorous form of G. tristis concolor, designated as Tristis II, and two forms received as G. recurvus, one with bronze-colored flowers and the other violet-colored. The violet-colored strain was raised from seed, all of the seedlings being essentially alike. Since the resulting hybrids or their derivatives might prove valuable as parents for crossing with the garden varieties, it seemed desirable to study the hereditary tendencies in them, even though the number of seedlings, all of which had to be reared in the greenhouse in winter, was necessarily limited. Accordingly, detailed records were kept of all of these hybrids.
Both Gladiolus tristis and G. recurvus are fragrant, but the fragrance of each is distinct. Tristis has a strong narcissus-like scent, but only at night or after darkening. G. recurvus has a more delicate scent, resembling the violet, which is strongest by day. The fragrance of both the first generation, Fl, and the second generation, F2, hybrids from these species varied in intensity. They were roughly grouped as scentless, faint, or medium-sweet, though in the medium group some were sweeter than others. Since Blakeslee and others have found that people vary greatly in their ability to detect different odors, it may be of interest to record that the same was found to be true here, and that the results given here are entirely based on observations of my own and of one assistant, Miss Helene Lunt, whose perception of the two scents here recorded was approximately like my own.
Some of the results may be found in TABLE I.
The F1 hybrids, both of Recurvus bronze, numbering 177, and of Recurvus violet, numbering 90, show strong dominance of the violet day-scent of Recurvus (called R. Day-scent in the table), and as strong recessiveness of the night-scent of Tristis. Among 165 F2 hybrids, about one third are scentless and two thirds have day-scent, while night-scent is again almost absent. In the backcross of the F1 to Tristis II, nearly one fifth of the progeny are day-scented, while more than one half are night-scented.
INHERITANCE OF FRAGRANCE IN GLADIOLUS TRISTIS—G. RECURVUS HYBRIDS
|Recurvus Day-scent||Tristis Night-scent|
|Recurvus, both bronze and violet||Medium||None|
|F1 Tristis X Recurvus, bronze||8||59||110||173||0||4|
|F1 Tristis X Recurvus, violet||3||17||70||88||1||1|
|F2 Tristis X Recurvus, bronze||62||39||64||157||3||5|
|F1 bronze X Tristis II||126||22||9||74||23||60|
If the presence of day-scent were a simple dominant, then the expected ratios of day-scent to its absence in the F2 would be 3:1, and in the back-cross 2:2. On this basis, the day-scent approached expectations in the F2, but fell far short in the backcross. On the other hand, if we regard the presence of the night-scent of G. tristis as a simple recessive, we should expect it to reappear in one fourth of the F2, and in one half of the backcrossed hybrids. The night-scent fails to come up to the expected proportion in the F2, in which the day-scent is well represented, but it fully meets the expected ratio in the back-cross, in which the day-scent has less than half of the expected representation. This relationship might be regarded as indicating a mutual antagonism of the two factors for scent, such that the presence of day-scent in many of the F2 hybrids might suppress development of night-scent in these individuals, and conversely the presence of night-scent in more than half of the back-cross hybrids might serve to suppress the expression of day-scent in them. The two factors for fragrance, even if they are thus mutually antagonistic, are otherwise independent, not allelomorphs, for a large number of both the F2 and of the back-cross individuals are entirely scentless, and a very few others have both the day-scent and the night-scent combined in the same individual.
|FIGURE I. A group of Gladiolus tristis x G. recurvus hybrids.|
This apparent antagonism between the two scents in these hybrids may have other causes than purely hereditary ones. The odors are in each case due to aromatic chemical compounds. Dr. Bogert, of Columbia University, very kindly examined the night-scented gladiolus flowers, found they contained a rather unstable aldehyde easily decomposed by light from an ultraviolet lamp, and expressed the opinion that the compound responsible for the odor is either cinnamic aldehyde or a closely related compound. Since aldehydes are easily oxidized, it is entirely possible that the substance responsible for the violet day-scent and the aldehyde causing the night-scent may under ordinary circumstances react on each other so as either entirely to suppress both odors, or for the more abundant compound to suppress the other. Since heredity must rest in the last analysis on chemical compounds which act as carriers of heredity, it seems reasonable that in some cases the inheritance may be purely quantitative, the unit of amount being the almost infinitely small molecule, the number of which carried in inheritance may be from one to several thousands, or any quantity in between, instead of a small and relatively finite number of genes.
This hypothesis of a quantitative basis of inheritance in certain cases seems best to fit the inheritance of fragrance in the descendants of these two sweet-scented gladiolus species. While the data are recorded as either medium or faintly sweet with each odor, actually the intensities intergrade imperceptibly, from the faintest trace, only noted by the most sensitive and trained nose, to strong odors, sufficient to scent a whole room or greenhouse to a degree noticeable to practically everybody. For instance, while nearly all of the direct crosses between Gladiolus tristis and the scentless species and hybrids are properly regarded as scentless, a very small proportion have a barely perceptible fragrance detectable by only a few people. In my experiments, including nearly three hundred such hybrids, about five per cent of them had such barely noticeable odor. So the night-scent of G. tristis can be regarded as only relatively recessive, though for practical purposes it is recessive.
Under these circumstances, dealing with magnitudes of fragrance which intergrade into each other, and for which no exact standard of measure is yet available, close analysis of the inheritance ratios would not be significant, even if possible. The general facts: that the day-fragrance is in general dominant, the night-fragrance relatively recessive; that the two may combine in exceptional individuals of the F1, the F2, and more frequently in the back-crosses to G. tristis; and that entire absence of either fragrance may result in the F1 generation from all-fragrant parents and grandparents—all of this stands out clearly. They further serve as a practical and reliable guide to future breeding work with these species and hybrids.
Several other traits of the species and hybrids were observed and recorded, besides their fragrance. Fourteen characters other than odor, in which the two species differ, were quite consistently observed and recorded in the hybrids. The flowers differ in size, Gladiolus tristis being two or more inches across, G. recurvus about one inch. One and one-half inches is put as the dividing line between large- and small-flowered hybrids. The height of flower stalk also exceeds the length of the longest leaf in Gladiolus tristis, but is much shorter than the leaf in G. recurvus. The flower segments of G. tristis are clear-colored, except for central lines or blotches, while recurvus has segments dusted or veined with darker color. The body color of the flower is cream or pale yellow in tristis, white in recurvus. Similarly, we find an open, flaring shape of flower contrasted with a narrow, bell-shaped one; segments with pointed tips versus rounded ones; narrow segments, less than one-half as broad as long, versus broad segments; plain-margined segments versus wavy-margined ones; upper segments with dark central blotches versus non-blotched ones; clear-colored segment-tips versus lines on the tips; pale throat of lower segments versus dotted throat; greenish lines along centers of throat segments versus no lines; cream-colored anthers versus anthers with dark sides; basal leaf-sheath brown versus sheath dotted green and brown. To find whether there is any apparent linkage between the dominant violet day-scented and any of these characters or the night odor, a count was made of the number of times each character was associated with violet odor, or both characters were absent, and how many times the two characters were independent of each other. Expressing the degree of correlation as a fraction, with the number of agreements as the numerator and the number of disagreements as the denominator, the following are the results:
LINKAGE OF VIOLET ODOR IN SECOND GENERATION GLADIOLUS TRISTIS—G. RECURVUS HYBRIDS
|F2||F1 x T.||F2||F1 x T.|
|Flower size||68/75||Segments blotched||81/58||46/107|
|Cream color||79/70||61/93||Lines on tips||81/72|
|Segments dusted||74/65||126/69||Throat dusted||92/75||113/44|
|Open shape||70/69||Throat lines||87/79|
|Tips pointed||84/66||Anthers lined||78/88|
|Segments broad||99/64||93/64||Odor at night||61/106||69/88|
|Segments wavy||73/65||89/65||Height/leaf length||63/102||65/92|
|Leaf sheath dotted||89/45||92/62|
If there is linkage, it should obviously appear in both the F2 and in the F1 x Tristis. If there is an apparent positive correlation in one set of progeny, and an apparent negative correlation or none in the other set, then the figures cannot indicate any interdependence. The only evident correlations in both sets are violet day-odor with broad flower segments, F2 99/64 = 62 per cent correlation and F1 x T 93/64=59 per cent correlation. Also, violet day-odor with height of plant, F2 63/102 = 62 per cent negative correlation and F1 x T 65/92 =159 per cent negative correlation. Both of these show very weak linkage, if any, of the breadth of segments and shortness of flower stalk with the violet scent. The fact that the degree of correlation is about the same in both cases may be purely accidental, as an attempt to establish a similar correlation between broad flower-segments and short stalk failed.
Theoretically, if the inheritance is carried by the chromosomes, then the number of entirely independent characters of any plant should be limited by the haploid number of chromosomes, which is found to be 14 in G. tristis and related Gladiolus species. Among the 16 pairs of characters here considered, these two sets are the only ones that suggest any correlation to violet odor, and in them the linkage is so weak, if it exists at all, that it need not be seriously considered in further breeding work. Thus, none of the contrasted characters of the two species need necessarily accompany the dominant fragrance into hybrids with the garden gladiolus varieties. Traits that the species have in common, such as slender growth, requirement of cool weather for growth, etc., may be linked with the character for violet scent. Studies of such hybrids are now being undertaken. From the breeding records, it is already evident to what extent the characters for fragrance are transmitted, and that the two types of fragrance are not linked, either with each other or with the other recorded characters to any great degree. Thus genetic records have proved distinctly valuable in guiding practical breeding work, even with so evanescent a character as flower fragrance.