Understanding Pecan Tree Dichogamy
If you have been following the Pecan South’s Pecan Newsletter this fall, you will have read several growers’ comments on pollination issues affecting their crop. Here, I’ll review the mechanics of pecan pollination and how different environmental factors impact pollination each spring. This review should help you design new plantings in the spring of 2020 and beyond.
Generally speaking, most horticultural plants only successfully pollinate when viable pollen comes in contact with a receptive stigmatic surface. This is true for both bee-pollinated plants and wind-pollinated plants, such as pecan. Adequate pollination of a pecan tree is critical for the nut’s yield and kernel quality.
Trees with separate male (staminate) and female (pistillate) flowers fall into two categories: monoecious or dioecious. For instance, American holly is dioecious because it has male trees with only staminate flowers and female trees with only pistillate flowers. Pecan trees are monoecious since their flowers are unisexual, but both male and female flowers exist on the same tree. Because the staminate flowers and pistillate flowers mature at different times, a pecan tree has dichogamous flowering. For a variety such as ‘Pawnee,’ which dehisces pollen from the catkins before the nuts become receptive to pollination, it would be classified as a protandrous or type I variety. But for varieties such as ‘Elliott,’ which have female flowers receptive for pollination before the catkins release pollen, these varieties would be classified as a protogynous or type II pecan variety. Because different pecan trees have different bloom patterns or dichogamies, pecan as a species is considered to be heterodichogamous.
The staminate inflorescence, or catkin groups, is composed of three catkins joined at the peduncle. The catkins—some growers refer to them as tassels—are easily spotted in the spring hanging down at each node of last year’s vegetative growth. Protogynous cultivars typically have long, thin catkins, while protandrous cultivars usually have catkins that are shorter and of greater diameter. Looking at the pecan cultivars in Figure 2, you can see that regardless of whether the variety is a protandrous (‘Pawnee,’ ‘Oconee,’ ‘Desirable’) or protogynous (‘Sumner,’ ‘Schley,’ ‘Stuart’) dichogamy, the central catkin of a catkin group is typically the longest strand.
Protandrous cultivars typically have fewer staminate flowers per catkin (~72 per catkin) than protogynous cultivars (~123 per catkin). Each staminate flower is composed of a central bract and two lateral bracteoles. Protogynous cultivars typically have male flowers with long, thin bracts, while protandrous varieties usually have male flowers with short, broad bracts (Woodroof, 1924).
Each male flower has three to seven anthers with four pollen sacs per anther, and according to Woodroof’s (1930) calculations, approximately 365 pollen grains per sac. Therefore, each male flower would shed 4,380 to 10,220 pollen grains for each flower on the catkin. This amount of pollen shed means that an average of three catkins, staminate inflorescence, would produce more than two million pollen grains. A medium-size tree would create thousands of catkins, so having an abundance of pollen available to fertilize all of the nutlets in an orchard shouldn’t be a problem.
Not only do protandrous and protogynous pecan catkins differ in appearance, but they also differ in their sequence of development. Let’s use the 2019-2020 growing season to illustrate the difference between the two. Protandrous, or type I, pecan cultivars initiated anthers on catkins in the buds of the staminate inflorescence in the summer of 2019, and these catkins will provide pollen for the 2020 crop next spring. In contrast, protogynous, or type II, pecan cultivars will initiate anthers on catkins in the spring of 2020, and these varieties also will provide pollen to fertilize the 2020 crop next spring. (Wetzstein and Sparks, 1984; Haulik and Holtzhausen, 1988).
Female flowers are differentiated during the early stages of bud growth in the spring. There were no apparent differences in time of differentiation of pistillate flowers by protogynous and protandrous cultivars. Pistillate flowers are borne in a spike at the end of the current season’s shoot. The basal flowers are the oldest, while the youngest flowers at the apex are often underdeveloped and abort in the first drop. The number of flowers produced on a single inflorescence varies with shoot length, cultivar, and season. Pistillate flowers consist of a bilobed stigma on a stigmatic disk surrounded by three bracteoles and a bract. The bracteoles and bract are fused at the base to form the involucre or shuck (Manning, 1940).
So, how do we know if a nutlet is receptive to be pollinated? Occasionally, you will read an article that states that the color of the stigma can determine receptivity, but that is not a reliable index. The developing stigma occurs in a range of colors, sizes, and shapes. Color varies between cultivars from deep red, as in ‘Pawnee,’ to vivid green, as in ‘Oconee.’ Therefore, receptivity is typically judged by the presence of a “viscous fluid” on the stigmatic surface (Adriance, 1931; Woodroof and Woodroof, 1926) and by adherence of applied pollen to the stigmatic surface (Smith and Romberg, 1941; Madden and Brown, 1973).
Earlier, we discussed the reason pecans are heterodichogamous, but we still have one more facet to address to get a full picture of pecan pollination. Cultivars such as ‘Creek,’ ‘Jackson,’ and ‘Pawnee’ have pollen release that partially overlaps nutlet receptivity and are considered to have incomplete dichogamy. Whereas, cultivars such as ‘Kanza,’ ‘Maramec,’ and ‘Nacono’ have pollen release and nutlet receptivity totally separated and are considered to have complete dichogamy.
Why does this separation of pollen release and nutlet receptivity matter? When a tree has complete separation of male and female bloom (complete dichogamy), another tree must cross-pollinate it. If another tree is not around, the nutlets do not pollinate and dehisce shortly after bloom, and so, we lack a crop. Trees that have some overlap (incomplete dichogamy) can have some self-pollination and may have a partial crop. However, self-pollinated nuts tend to have poor nut growth and development, and typically have a greater level of abortion than cross-pollinated nuts. Therefore, self-pollination is undesirable since it has been shown to reduce nut quality and significantly reduce crop yields (Romberg and Smith, 1946; Marquard, 1988).
But the question remains: what is the advantage of pecans having dichogamous flowering? This type of flowering encourages genetic diversity by maximizing outcrossing (Thompson and Romberg, 1985). Dichogamy encourages cross-pollination and discourages self-pollination. More genetic variation leads to better pecan tree survival, continued evolution of the species, and better climate and disease adaptation. Thompson and Romberg (1985) reported that standard outcrossing patterns in native pecan populations result in about a 50 to 50 ratio of protandrous and protogynous trees.
Now that we have discussed the structures and the mechanics of pecan pollination, we can learn how to use this information to design a better orchard. The first step to ensure adequate pollination is to determine what varieties will best pollinate each other. Choosing a pollinator is usually accomplished through the use of a pollination chart. Pecan scientists recommend finding at least two to three varieties that shed pollen at a similar time as the main variety’s nutlet receptivity. Pollination charts are available from several sources, with one found at the LSU Pecan Station website.
I wish I could tell you it is merely a matter of planting a combination of type I and type II varieties to pollinate each other. But we must determine if the pollination of the different varieties is best suited for a specific variety. Let’s look at planting ‘Caddo’ as a protandrous (type I) variety and using a couple of protogynous (type II) varieties, ‘Kanza’ and ‘Fortkert,’ as pollinators.
‘Kanza’ and ‘Caddo’ are complementary to each other; their pollen sheds overlap a good portion of each varieties’ period of nutlet receptivity. So, we should expect a good crop set on both cultivars. However, ‘Forkert’ would be a horrible choice for ‘Caddo,’ as neither variety would supply pollen when the nutlets of each variety were receptive. This example should help illustrate why we recommend using three to four compatible varieties when designing an orchard.
Additionally, you should also take into account that the ratio of protandrous to protogynous varieties does not have to be 50 to 50. We have already discussed the copious amounts of pollen a single pecan tree can supply. It is not uncommon for the main variety in the orchard to comprise 80 to 90 percent of the trees with two or three pollinator varieties evenly distributed over the remaining 10 to 20 percent. As long as the pollinators are distributed uniformly throughout the planting, pollination shouldn’t be an issue. Small or narrow orchards may not need pollinators interspersed in the planting if native or seedling trees are growing along the farm’s outside edges. Still, generally, any large commercial orchard should have pollinators no further than 150 feet away. A common layout would be to have a row of pollinators followed by four rows of your main variety with this sequence repeated across the orchard.
You must remember that the degree of dichogamy varies within pecan trees and can be greatly affected by weather each year. Moist, warm springs favor male flowers, whereas cool, dry springs favor female flowers. A cultivar may show total separation of pollen shed and pistil receptivity in some years, while the same cultivar may have significant overlap of pollen shed and pistil receptivity the following year. This difference is especially true along the Gulf Coast, where you can have large fluctuations in the chill hours accumulated from year to year. Like other fruit tree crops, the chill hour requirement for pecan budbreak varies among cultivars. In 2017, a year when many locations received less than half of their normal chill hour accumulation, we saw significant shifts in pollination patterns between varieties that normally complimented each other well during pollination. Sustained high winds coupled with low humidity tend to shorten the period of effective pollination, both by speeding pollen dehiscence and by reducing the period of pistillate receptivity (Woodroof and Woofroof, 1929).
Conversely, high humidity delays pollen dehiscence and extends the period of pistil receptivity. Woodroof and Woodroof (1929) reported that pecan pollen continued maturation but did not dehisce if relative humidity exceeded 85 percent, with subsequent dry conditions resulting in periods of very heavy shed. Once the stigma receives pollen, the stigmatic cells collapse and dry after pollen hydration and germination, causing the stigma to appear brown and dried (Wetzstein and Sparks, 1989).
Let’s say you decided that this pollination stuff is too much of a headache, so now you are not going to plant grafted trees, but instead, you plan to establish a ten-acre seedling orchard by planting a seed of a known variety. You want big nuts, so you decide to plant ‘Mahan,’ a protogynous variety, and have a seedling orchard. So, you think you do not have to look at a pollination chart; you’re not even worried about future pollination. No problem, right?
Well, first, you have to consider that a single gene with simple dominance controls patterns of dichogamy. Protogyny (type II) is the dominant character with all protandrous (type I) cultivars being homozygous recessive. So, what does that mean? How does that affect my seedling orchard? Because ‘Mahan’ is a homozygous protogynous variety, all of your orchard’s seedlings will be protogynous too. Because of that, there is a good chance you will have pollination problems in your seedling orchard.
So, you change your mind again. A friend of yours has an orchard with ‘Oconee’ and ‘Cheyenne’ as the two main cultivars—both of which are type I varieties. You decide to collect ‘Oconee’ nuts to plant in your ten-acre seedling orchard. Well, it just so happens that ‘Cheyenne’ has a later pollen shed than ‘Oconee’ does, such that its pollen shed almost completely overlaps ‘Oconee’s’ nutlet receptivity. What does that then mean if all of your ‘Oconee’ nuts were pollinated by ‘Cheyenne’? That means you have a ten-acre orchard in which every tree is protandrous (type I), so you may still have pollination issues in the future.
Above all, if you are designing a new orchard to plant this spring, it doesn’t matter if you are planting grafted trees or nuts for a seedling orchard. You still need to consider what varieties are going to be planted and what impact those varieties will have on the future pollination and cropping success of your orchard. Do your research, map it out, and you can protect your future crops.