Proc. Am. Soc. Hort. Sci. 29:160-163 (1932)
Dichogamy—An Important Factor Affecting Production in the Persian Walnut
MILO N. WOOD
U. S. Department of Agriculture, Washington, D. C.

ALTHOUGH Delpino (1), Kerner and Oliver (2), Jessup (3), and Delong (4) mentioned dichogamy in Juglans, there was great diversity in opinion as to its extent and type. Many growers in the West have believed that light crops were due in some cases to sterility or lack of compatibility of pollen and pistil, or that pollination under natural conditions was inadequate in some way. Others believed that light crops could not be ascribed to any pollination factor. Although a limited amount of observational evidence was advanced to substantiate the various opinions, there had been no experimental studies upon which conclusions could he established regarding the relation of pollination to crop production.

In 1920 experiments were started and collection of data begun to determine whether walnut varieties were self-fertile, whether intersterility or incompatibility of pollen and pistil occurred, and whether dichogamy was responsible for light crops or complete crop failure in any of the commercial varieties. Details of the methods used are given in a forthcoming U. S. Department of Agriculture bulletin. Some phases of the investigations have been discussed in a previous article (5).

Briefly the findings to date may be summarized as follows: (1) All varieties are self-fertile. (2) All varieties are inter-fertile. (3) Inter-fertility exists between all varieties of Juglans regia and other species tested, including J. hindsi, J. californica, and J. sieboldiana. (4) Dichogamy is common.

In general, it may be said that light crops cannot be attributed to self-sterility, inter-sterility, or lack of compatibility between pistil and pollen. On the other hand, dichogamy is often the cause of crop failure in orchards planted to one variety, or to more than one variety when the blooming period of each variety differs from that of the others in the orchard.

Great variation was found in the extent and types of dichogamy occurring in the genus. Some varieties tend to be protandrous and others to be protogynous. Protandrous varieties include the Concord, Eureka, Franquette, Golden Nugget, Grove, Grenoble (French Mayette), San Jose Mayette, XXX Mayette, Payne and Praeparturiens. Among protogynous varieties are the. Erhardt, Lucretia, Meylan, Pride of Ventura, and most of the Santa Barbara Soft Shell seedling types. Protandry was most pronounced in the Franquette, San Jose Mayette, Payne, and Eureka varieties. The El Monte, Willson, and Persian were difficult to classify.

The fact that some varieties are protandrous and others protogynous in tendency accounts no doubt for the differences in opinion held by previous investigators regarding the type of dichogamy present in Juglans regia. If there were no variation in dichogamy in any variety it would be easy to plan an interplanting of varieties to insure proper pollination. Other factors, however, enter into the matter.

Dichogamy varies with the age of the tree. In the varieties studied it is more complete in young than in old trees. This apparently accounts for the fact that some varieties (for example the Franquette) do not bear well until the trees are fairly old. Variation in dichogamy with age of tree is especially pronounced in some of the protandrous varieties.

1The term "catkin" is applied in this
paper to the staminate flowers only.

Climate has a very definite effect upon dichogamy. Experiments in California show that the catkins1 respond more quickly to sudden spells of warm weather than do the terminal growths bearing the pistils, particularly after a cold winter. When warm weather continues for a number of days, the catkins in protandrous varieties may develop so much faster than the pistils, that no pollen is left by the time the pistils become receptive. On the other hand, in protogynous varieties such a season hastens the development of catkins so that the difference in time of bloom between catkins and pistils is lessened. When, after a warm winter, the blooming season is cool, protogynous varieties may develop pistils so far ahead of the catkins that the receptive period may be over before pollen is produced, while in protandrous varieties the coincidence of blooming of catkins and pistils is increased. So great is the effect of weather upon the blooming habit that a protandrous variety may in some seasons actually become protogynous, or, the season may be such that protogynous varieties become protandrous. In the varied climates found in California all degrees of protandry and protogyny occur. More specifically, dichogamy is affected by temperature, amount and intensity of sunlight, winds, air drainage, humidity of the atmosphere, soil moisture, and perhaps such factors as type of soil and rootstock.

It is evident that dichogamy varies not only according to the season, but according to geographic location. In general, coastal regions emphasize protogyny, and decrease protandry, while in the interior valleys protandry is pronounced and protogyny diminished. During the same season a variety having a protogynous tendency (such as Placentia) may be protogynous at Ventura on the coast and protandrous at Chico in the northern part of the Sacramento Valley. On the other hand, a variety having a protandrous tendency, such as Franquette or Payne, may be completely protandrous at Chico and nearly or wholly protogynous at Ventura. Actually, then, a protandrous variety would be much more likely to pollinate itself in coastal districts than in the interior, and a protogynous variety would be more likely to pollinate itself in the interior than on the coast; therefore one would at first thought be inclined, to conclude that protogynous varieties should be grown in the interior and protandrous varieties in coastal regions. Other considerations, however, make this impracticable. It happens that among the superior commercial walnut varieties most of the early bloomers are protogynous in tendency while many of the late bloomers are protandrous. Many of the earliest bloomers are unsuited to the interior because of the damage likely from late spring frosts. In some of the coastal sections (especially in southern California), the growing season for late blooming varieties is so short that the crop does not have time to mature properly.

Because of the variations in the completeness of dichogamy due to climate, season, and locality as well as to variety, it is not possible to foresee exactly what will be the effect of planting any combination of varieties unless blooming data have been kept in the locality for several years. However, varieties are fairly consistent in their time of bloom with relation to one another. Although in a given locality the dates of bloom of all varieties are closer in some seasons than in others, the sequence of blooming remains about the same; therefore a rough classification into early, intermediate, and late bloomers has value in planning a planting to favor pollination. Though the amount of inter-pollination secured will vary from year to year, and will not be the same in different districts for the same group of varieties, the proper grouping of varieties is likely to prove helpful in most years if not in all, a fact borne out by data collected from commercial orchards for the purpose of comparing walnut orchards consisting of several inter-planted varieties with those consisting of a single variety only.

Because no variety or locality is always free from dichogamy, it would appear that any grouping is better than none at all, but, to secure the best results, careful grouping based on data collected for the district is necessary. Inter-planting early and late varieties is not advisable because in some seasons their blooming periods do not overlap. Probably the best combinations are those which bring together early and intermediate bloomers, or intermediate and late bloomers, or all three. The fact must be considered, however, that the grower has to limit the number of varieties in his orchard to a few most suited to his purpose. The matter is further complicated by the consideration that when late varieties are grouped they are likely to suffer from the same pollination trouble at the same time. This is true to a somewhat less extent of intermediate, and of early bloomers. Furthermore, when protandrous varieties are planted together for pollination purposes, many of the pistils of the latest blooming variety must go unpollinated, and when protogynous varieties are grouped many of the pistils of the earliest blooming variety will be unpollinated The ideal arrangement would be to plant together a protogynous variety and a protandrous variety blooming at the same time, but in practice this is difficult because protogynous and protandrous varieties of commercial merit blooming simultaneously are few, and the two types are not suited to the same locality. For a number of years a careful search has been made to secure better pollinators for the leading commercial varieties. The protandrous variety Payne, and the somewhat protogynous Lucretia, seem to go well together. Frostfighter appears to be an excellent pollinator for Franquette in some localities, and Meylan in others. Certain strains of the California Black, "Bolivian Black," Japanese walnut, and some Persian seedlings may also be desirable pollinators for some of the commercial varieties, but records must be kept for a number of years before their value can be stated positively.

Grafting-in of pollinating varieties has already given remarkable results in orchards in which unproductiveness was due to dichogamy. Artificial pollination has greatly increased crop production in some dichogamous orchards. Incomplete or partial dichogamy generally results in considerable loss of crop. In such cases provision should be made for proper pollination.

LITERATURE CITED

  1. DELPINO, F. Ulteriori osservazioni e considerazioni sulla Dicogamia nel regno vegetale. Atti della Societa Italiana de Scienze Naturali. 1868-1874.
  2. KERNER and OLIVER. Natural History of Plants. Trans. 2: 313. 1895.
  3. JESSUP. W. H. Paper read before the Calif. State Hort. Soc. April 27, 1883.
  4. DELONG, B. M. Rept, Calif. State Board Hurt, 1895-1896.
  5. WOOD, M. N. Walnut pollination studies and blooming habits of the common varieties. Diamond Walnut News, XII: No. 2, April, 1930.