Dr. Howard Brett Frost’s groundbreaking citrus research, including the Frost Washington Navel Orange, transformed the industry and continues to impact the fruit we enjoy today.
Do you have a Washington Navel tree in your yard? If so, chances are that you can thank Riversider and geneticist par excellence Dr. Howard Brett Frost. His contributions to the global citrus industry were immense, and he was a legend in his own time, partially due to his unique personality. The vast majority of Washington Navel oranges should be properly called “Frost Washington Navel Orange.”
Dr. Frost came to Riverside to join the newly formed University of California Citrus Research Station in October 1913 when it was still west of downtown on the slope of Mt. Rubidoux. He had been hired to become its first citrus breeder. Genetics as a field was still largely in its infancy. Remember, Mendel’s ignored work had not been rediscovered until 1900.
Cornell University became the hotbed for merging the study of genetics with plant breeding. Frost, who grew up in New York state, enrolled at Cornell after a stint teaching at a rural one-room elementary school. He received a B.S. there in 1908, his M.S., and eventually a Ph.D. in Genetics in 1913. It must have been an adventure to move from snowy New York to sunny SoCal!
He settled into Riverside and got to work on citrus. Frost plunged into accumulating genetic information about citrus as well as details about its seed reproduction. He was the first to determine the normal “diploid” chromosome number for citrus (18 = 2n), an important contribution (Frost 1925a) at a time when the Chromosomal Theory of Inheritance was becoming widely accepted. That information made possible the identification of chromosomal variants, such as rare plants with a doubled number of chromosomes (36 = 4n = the “tetraploid” number) (Frost 1925b), that proved useful for creating seedless varieties.
Howard Frost was to become the premier expert on nucellar embryony in citrus. It takes a little science to understand nucellar embryony. In most plants, a seed contains a single embryo that is the product of sexual reproduction that has grown from a fertilized egg cell. In many species of citrus, as well as some citrus relatives, a seed often contains multiple embryos, one sexual embryo, and more that have been recruited from the nucellus, the tissue surrounding the egg. If you plant the seed, you get multiple seedlings.
Nucellar embryos are products of the mother’s tissues. Therefore, they aren’t the result of sex; genetically, they are as true to the maternal type as a photocopy is to the original. There’s a bonus to being a nucellar seedling. Nucellar tissues are virus-free (Frost and Soost 1968; Zhang et al. 2018). Thus, as Frost showed in comparative experiments, trees grown from nucellar seedlings are more vigorous and yield more fruit than those budded off the same tree. Thus, Frost made crosses involving established varieties to create vigorous, virus-free asexual lines from nucellar seedlings.
Studying genetics or evaluating new varieties of a long-lived tree takes substantial time. In the case of citrus, the time from making a cross to harvesting a mature seed is almost a year. The time from planted seed to fruit-bearing tree ranges from five to ten years. Multiple trees must be field-tested for years in evaluating new varieties under various conditions. It is to Frost’s credit and genius that so many of the commercialized varieties were the result of seeds that he obtained from crosses early in his career. Here is a sample of some of those varieties. Look how long it took to commercialize them (and note that he “formally” retired in 1948)!
VARIETY & YEAR OF ORIGINAL CROSS/SELECTION YEAR COMMERCIALIZED
Frost also studied annual species so that he could advance his knowledge in genetics while waiting for his citrus work to bear fruit, so to speak. Therefore, Frost continued doing inheritance studies on his dissertation plant, the garden plant known as “stock” (Matthiola incana = “grayish-white Matthiola”). His experiments led to understanding the genetics of “doubled” flowers and contributions to the ornamental industry. Frost was also one of the pioneers who examined the genetic basis of traits in wild plant populations. He noticed wild radishes growing on roadsides in Riverside, Corona, and Whittier sporting different flower colors – pink, purple, yellow, and bronze. He transplanted these plants into his research plots and grew them along with various radish cultivars to study their inheritance. The resulting paper, based on hundreds of crosses and thousands of measurements, revealed a passionate and careful scientist with an eye for detail (Frost 1923). But stock and radish were side projects.
But citrus was Frost’s true love. As Walton Sinclair wrote (in Reuther et al. 1968), “Frost’s genius lay in a total immersion in research, a gift for meticulous observation, and a rigorous objectivity sustained through decades of citrus experimentation. Few colleagues grasped the implications of Frost’s solitary and unheralded quest and foresaw the revolutionary impact it would have on today’s citrus industry.”
Given such focus, “Dr. Frost was characterized by outstanding integrity and complete devotion to his work. His attitude was unfailingly modest and unselfish; it was a rarity for him to speak critically about anyone. He was tireless, remarkably thorough, and gave great attention to detail. …His total involvement in research was well known; it caused many visitors and co-workers to be late for lunch, or even supper.” (Cameron et al. 1970).]
Always absorbed with the subject at hand, that fixed attention to detail made for outstanding science as well as legendary anecdotes. Here’s a sample:
“During an extended field trip one November, Frost is said to have asked, ‘What day is it? I promised my wife I’d be home for Thanksgiving!’” (Reuther et al. 1968).
“Frost was taking his turn at driving as a group of scientists returned from the San Joaquin Valley to Riverside. Suddenly, with no cars in sight, he slammed on the brakes. The response from the passengers was something like, ‘What the heck are you doing, Howard?!” He answered, “It just occurred to me that I never tried the brakes to see if they were working!’” (from Robert Soost).
One day while driving from his home in the Wood Streets to the Experiment Station, Frost noticed that the fuel indicator was on “Empty”. He promptly parked the car and proceeded on foot to the campus. There he called his garage. His serviceman drove to the car, started it, and proceeded to drive it to the service station where he filled it and then delivered it to Dr. Frost. (from Williard Bitters).
Another concern to detail involves disease transmission. “The original Citrus Experiment Station building had a large men’s room in the basement. One faculty member reportedly pulled open the exterior door and nearly pulled Frost off his feet because Frost was grasping the inside doorknob with his hand in his pants pocket and then around the knob to avoid direct flesh contact between hand and door knob”. (from Robert Soost).
His interests went beyond science. Frost was a proponent of the synthetic international language Esperanto, naming three of the cultivars that he created in that language: the Trovita (“found”) orange, the Frua (“early”) mandarin, and the Sukega (“very juicy”) grapefruit hybrid. He also had a keen interest in politics. For example, he was an ardent supporter of the Riverside ballot measure that enabled the city to create its own public electric utility rather than being served by Southern California Edison (from Robert Soost).
Frost was largely underappreciated during his time in the Experiment Station. Genetics was a new field, and none of his colleagues were geneticists. The long generation time from seedling to fruit delayed the results of certain experiments for a decade or more. Frost’s idiosyncratic attention to collecting data made him a bit of a recluse. In the end, he retired at the “Associate” level instead of the “Full” level typical for scientists of recognized accomplishment. Retiring in 1948, he even continued to attend to occasional data taking and experiments. It was not until 1966 that he received the Wilder Medal from the American Pomologist Society, the oldest and one of the most highly prized awards in American horticulture. He passed away in 1969.
Frost’s contributions are still with us. The nucellar selections remain very important. Likewise, His “Kinnow” mandarin is a major variety in Pakistan and India (Karp 2012). A new low-seeded “Kinnow LS” variety is an irradiated line derived from “Kinnow.” Frost’s work on Citrus genetics and reproduction provides the scaffold for current and future research.
Howard B. Frost is not the only Riverside scholar who has left his mark on things local to global. For a city its size, Riverside is the home of a notable number of research hubs and institutions of higher learning. Some examples are California Air Resources Board, California Baptist University, La Sierra University, Riverside City College, and the University of California Riverside. Riverside is indeed a city of scholars, and I look forward to profiling them now and then in future editions of Naturally Riverside.
This article benefited from the contributions of Drs. Willard Bitters, Charles Coggins, Robert Soost, and Tracy Kahn.
Cameron, J. W., and H.B. Frost. 1968. Genetics, breeding, and nucellar embryony. In: Reuther W. et al. The Citrus Industry. Volume two. Anatomy, Physiology, Genetics, and Reproduction. Revised edition. University of California. Division of Agriculture. Pp. 325-372.
Frost, H. B. 1923. Heterosis and dominance of size factors in Raphanus. Genetics 8:116-153. 116.pdf (nih.gov).
Frost, H. B. 1925a The chromosomes of Citrus. Journal of the Washington Academy of Sciences 15:1-13.
Frost, H.B. 1925b. Tetraploidy in Citrus. Proceedings of the National Academy of Sciences 11:535-537.
Frost, H. B., and R. K. Soost. 1968. Seed reproduction: Development of Gametes and Embryos. In: Reuther W. et al. The Citrus Industry. Volume two. Anatomy, Physiology, Genetics, and Reproduction. Revised edition. University of California. Division of Agriculture. Pp. 290-324.
Reuther W. et al. 1967. The Citrus Industry. Volume one. History, World Distribution, Botany and Varieties. Revised edition. University of California. Division of Agriculture. http://websites.lib.ucr.edu/agnic/webber/Vol1/Vol1TP.html
Reuther W. et al. 1968. The Citrus Industry. Volume two. Anatomy, Physiology, Genetics, and Reproduction. Revised edition. University of California. Division of Agriculture. http://websites.lib.ucr.edu/agnic/webber/Vol2/Vol2TP.html
Norm Ellstrand is an emeritus Distinguished Professor of Genetics and former Director of the California Agriculture and Food Enterprise at the University of California Riverside.
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