Chestnut fertilization

In the lower part of the shoot, buds develop in the leaf axils; these act as reserve buds and only break dormancy if the terminal buds die, usually remaining dormant and later withering. In the middle part of the shoot, male catkins 15–20 cm long and 1.5 cm thick develop in the leaf axils. At the apex of the shoot, smaller and narrower leaf axils produce smaller, mixed catkins, with female flowers on the lower part and male flowers in the middle and upper parts. In the leaf axils at the shoot tip, buds containing flower-bearing shoot primordia differentiate.

Under our domestic conditions, only varieties of the European chestnut (Castanea sativa) are cultivated. In other chestnut-growing countries, cultivars of Japanese chestnut (C. crenata), Chinese chestnut (C. mollissima), American chestnut (C. dentata), and selected hybrids from these and some other non-cultivated species—such as Seguin chestnut (C. seguini) and Allegany chestnut (C. pumila)—are grown. The flowering and fertilization of these chestnut species are similar to that of the European chestnut; they differ mainly in earlier flowering, which reduces their production reliability in our conditions. When flowering coincides, they can effectively pollinate European chestnut varieties. Considering that only European chestnut varieties are grown in Hungary, we will focus solely on their fertilization.

The quantity of male chestnut flowers primarily determines the conditions for pollination. Chestnut is mainly wind-pollinated, although insect pollination is also possible to a lesser extent. Its male flowers produce nectar, attracting insects both for the nectar and the pollen. Chestnut orchards and forests are especially valuable for beekeeping, as they provide forage for bees after the acacia bloom, and pure chestnut honey is highly prized for its culinary and sensory qualities.

The large amount of pollen produced also indicates wind pollination—each anther of the many male flowers on a large catkin contains more than 2,000 pollen grains. During dispersal, sufficient pollen reaches female flowers even 200–300 meters away from the pollen-producing trees to ensure successful pollination.

Some chestnut varieties may have inherited developmental defects in their male flowers, which inhibit pollen production. Several high-yielding and commercially valuable cultivars fall into this category and can only be fertilized when grown near a pollen-donor variety.

There are four types of developmental abnormalities in chestnut male flowers.

• Pollenless (astamineae): These develop normally sized catkins, but the male flowers do not open and produce neither anthers nor pollen. Several Italian “Maroni” chestnut cultivars belong to this group.
• Partially fertile (brachistamineae): Unlike the previous type, they develop a few short anthers with filaments 1–3 mm long, but these contain only a small amount of poorly germinating pollen (e.g., Perella, Nerüw varieties).
• Medium-length filamented (mesostamineae): They produce several short anthers with 3–5 mm filaments, which contain a sufficient amount of well-germinating pollen (e.g., Pistolese variety).
• Long filamented (longistamineae): They produce many normally developed long anthers with filaments 5–7 mm long, containing a large amount of well-germinating pollen.

In our domestic chestnut populations, the first two types (with reduced or non-functional male flowers) are rarely found; all cultivated varieties develop adequate male flowers.

Pollen production in Hungarian chestnut types is good, pollination conditions are favorable, and they provide excellent forage for bees.

An important factor for chestnut productivity is the proportion of female flowers that develop on the shoots. Like walnut, chestnuts produce female flowers only on the shoots originating from the terminal buds of the current year.

A higher number of fruiting shoots is advantageous because, as the number of catkins per shoot increases, the commercial value of the fruit decreases and the nuts remain small. In extreme cases, on clustered-type shoots, only female flowers may develop instead of mixed catkins. Such catkins are fertilized up to the tip, but the resulting fruits are so small that they have no market value.

The number of fertile flowers in the inflorescence is a trait that affects both yield and fruit size.

A chestnut catkin (burr) often contains more than one flower—frequently three, but sometimes ten or more. If more than one flower is fertilized and develops, the resulting nuts are often deformed, flattened, and small. This is typical of the large-fruited Italian “Maroni” cultivars, where usually only one flower is fertilized and develops. In cultivars with good yield and fruit size, 2, or at most 3 nuts develop per burr. All of our cultivated clonal varieties belong to this type.

Some varieties frequently exhibit twin germination. In this case, the nut contains not only multiple embryos but also multiple cotyledons, surrounded by the inner skin. The seemingly uniform large nut is so interlaced with this internal skin that peeling is very difficult.

Chestnut trees require cross-pollination. The male and female flowers of a tree do not bloom simultaneously, and it is common for female flowers to develop earlier than male flowers.

If cross-pollination is absent, male flowers from later-developing mixed catkins fertilize only 15–50% of the female flowers, which is sufficient only for moderate to poor fruit set.

The receptivity of the chestnut stigma, as with other fruit species, cannot be visually determined, because there is no secretion to aid pollen adhesion nor a color change associated with growth and ripening. Pollination studies show that the central and lateral flowers are capable of fertilization 8–21 days after the stigma develops. Pollinations occurring within 8 days after stigma appearance are unsuccessful.
For optimal productivity, it is recommended to plant multiple mutually pollinating varieties in chestnut orchards established from clonal cultivars. It is also advantageous to choose a large-fruited cultivar as a pollen donor. According to Solignat’s studies, pollination by large-fruited pollen donor varieties increases the size of the resulting nuts by 13–20% compared to pollination by smaller-fruited donors.