By Gayle Kida
Although there are currently thousands of small native American chestnut (Castanea dentata) sprouts in the understory of Connecticut's forests, only a rare few escape or survive blight long enough to grow to flowering size. Fewer still are located in or next to a clearing or canopy opening to receive sufficient sunshine to initiate flowers. Most often, a flowering tree has been invaded by blight and is heavily cankered; it will die back to the roots within one to a few years after producing its first flowers and burs. Without another flowering American tree nearby to pollinate it, the attempt to produce nuts will result in either infertile flattened husks or fertiles fathered by pollen from Chinese or other hybrid chestnuts. Japanese, Chinese and European chestnut cultivars were sold by many mail order sources to Connecticut residents going back more than a century, and hybrid replacements for dying American trees were tested and promoted to farmers and homeowners by the CT Agricultural Experiment Station beginning in the early 1930s.1
Above – A flowering twig tip with catkins a few days before pre-bagging the female flowers. One female (see arrow) is visible at upper right at the base of the topmost catkin, a bisexual catkin. Trees that are in the shade usually do not flower.
Below – Flowers at pre-bagging time. The four females (one is a double) vary in size and development ? how close they are to being fertile. Ideally the flowers are pre-bagged to protect them from undesired pollen just before they become fertile.
Click on any photo to view larger version. Photos by Gayle Kida
The American chestnut sprouts you come across in the woods may look like seedlings, but they are probably quite old. It has been suggested that more than 95% of stump sprouts in Connecticut are actually “old seedlings”, small trees established just before blight appeared in the state around 1910 ?1925.2 The sprouts you see are the latest attempts from surviving roots to form new trees, over many decades sprouts have re-emerged when above-ground stems became blighted and died back. Because these trees rarely sexually reproduce with another American, there can be no new pure species genetic recombination that may help our native chestnuts adapt to a changing environment.
Above – Before pre-bagging, all of the tip's male catkins and the male part of the bisexual catkins are trimmed off, removing or trimming back leaves that would take up room in the bag or get in the way of securing it. A finished pollination bag is fastened with a twist tie in the background of the photo.
Below – Females 10-14 days after pre-bagging are usually ready for pollination. The styles, structures connecting the receptive stigmas to ovules, have turned very pale yellow. The tree's remaining male catkins are bushy. Bisexual catkins are still in bud but ready to open.
Click on any photo to view larger version. Photos by Gayle Kida
Therefore each tree that successfully flowers is a temporary event, a window of opportunity to capture some of Connecticut's genetic diversity. A chestnut native to Litchfield County is likely to have different alleles (alternative forms of a gene) than a tree in Essex County.3 Through controlled pollinations, CT-TACF seeks to create twenty or more lines using our state “mother trees”. The goal is to intercross CT lines of backcross trees that are on average 15/16 American but containing a small amount of Chinese material including blight resistance genes4. It is worth considering that the Chinese “cousin” species whose genes bring disease resistance already shares genes with the American species because they both evolved from a common ancestor many millions of years ago.5
Above – One container that can be used to perform a controlled pollination. Only the very tips ? ends ? of the styles have the receptive stigmas. A small quantity of pollen is tapped into the depressed area of the lid, and the female flower is gently dunked, tilted and swirled so the stigmas get coated with pollen. But there are other more time-tested methods. The most established method involves dragging the stigmas through a film of pollen coating a glass slide, but some prefer to use a prescription lid, dunking the style tips into a pile of pollen. After controlled pollination the flowers are re-bagged to protect them from undesired pollen.
Below – This large cluster of burs actually came out of a pollination bag. Many plump fertile nuts are visible, showing the characteristic white pubescence (fine hairs) covering a large portion of the nut.
Click on any photo to view larger version. Photo by Gayle Kida
Performing controlled pollinations requires dedicated observation of the tree's flowers because weather conditions and temperature can speed up or slow development.6 The female flowers are predominantly on the highest branches of the tree, where they are most exposed to windblown pollen from another tree. Binoculars are often needed to view the flowers to gauge their readiness. Timing is crucial; a couple days too early or too late may greatly change the degree of unwanted pollen contamination or the final success of the harvest. Volunteers who help pollinate work with bucket truck operators or use ladders to access the flowers. The photos appearing with this article give a close-up view of controlled pollination, and show how the flowers may look at each stage.
- An “Historical Reference for Chestnut Introductions into North America” by Dr. Sandra L. Anagnostakis, document is online at: www.ct.gov/CAES/cwp/view.asp?a=2815&q=376740
- Chestnut: history and ecology of a transformed species by Frederick L. Paillet, Journal of Biogeography 29, pgs. 1524-1525. A pdf document is online at: harvardforest.fas.harvard.edu/publications/pdfs/Paillet_JBiogeography_2002.pdf
- An Update of Chestnut DNA Projects: Part II by Paul H. Sisco, Journal of the American Chestnut Foundation Vol. XX, no. 2 (Fall 2006), pgs. 40-42. A PDF of this issue of the Journal is available online at: www.tacf.org/pdfs/resources/journal/JrnlFall2006.pdf
- The Backcross Method, web page from the American Chestnut Foundation website, www.tacf.org/r_r.php
- Intercontinental genetic divergence of Castanea species in eastern Asia and eastern North America by F. Dane, P Lang, H Huang and Y Fu, Heredity (2003) 91, pgs. 317-318. A pdf is online at: www.nature.com/hdy/journal/v91/n3/full/6800300a.html
- Per Dr. Fred Hebard, personal e-mail communication, 2005
