ImageOn Wednesday, November 18, 2009, Carl Leopold drew his last breath. Carl died in the home he shared with his loving wife and soul mate Lynn. Carl and Lynn lived in a warm and rustic home filled with music and constructed from the lumber milled from the mature white pine trees that Carl planted as saplings with his father, Aldo Leopold. On the day of his death, Carl was working on two papers—one on the phenomenon of memory in plants and another, with his sister Nina Bradley, on an update on the phenology of Sauk County, Wisconsin (see Phenological Changes Reflect Climate Change in Wisconsin, Proc. Natl. Acad. Sci. USA  96:9701-9704).

A memorial service will be held at the Unitarian Church of Ithaca, Sunday, December 13 at 2pm in the corner of Buffalo and Aurora Sts.
Aldo Carl Leopold was born on December 18, 1919 in Albuquerque, New Mexico where his father, Aldo Leopold, was working for the U.S. Forest Service and where his mother, Estella, had deep cultural roots. The family moved to Madison, Wisconsin in 1924. Carl was a teenager when his father acquired the famous “shack” on an abandoned farm in the banks of the Wisconsin River. There he worked with the family to convert a chicken coop into a cottage and to plant trees and native prairie plants in the famous experiment of land reclamation his father described in his “A Sand County Almanac.” Carl’s early fascination with photography provided many of the images we have of the transformation of the shack and surrounding farmland.

Carl received a bachelor’s degree in botany from the University of Wisconsin in 1941. He enlisted in the Marines during World War II and served in the Pacific. There he was unexpectedly asked to serve as defense council in courts martial for soldiers who were charged with being AWOL. Even more unexpectedly, he won these cases and became a hero to the enlisted men. After this brief legal career, Carl went to Harvard to study plant physiology with Kenneth Thimann. After completing his MS and PhD at Harvard, he worked briefly in industry, for the Hawaiian Pineapple Company, before he joined the faculty of Purdue University in 1949. In 1975, he was appointed Graduate Dean and Assistant Vice President for Research at the University of Nebraska. In 1977, Carl moved to the Boyce Thompson Institute for Plant Research in Ithaca, New York as William H. Crocker Scientist. He officially “retired” in 1990 but remained very active in science and environmental issues.

As a plant physiologist, Carl has worked in basic and applied research. Carl’s first book Auxins and Plant Growth (1955,1960) made him the unofficial ambassador of basic research to the scientists working in applied aspects of horticulture and agronomy. Carl also created the textbook, Plant Growth and Development (1964,1975) to “first develop the student’s reliance on experiments in forming generalizations about his science; second to depict science as a complex of imperfect approximations derived by the scientific method; and third, to reorganize the subject to make it more nearly representative of modern plant physiology in the laboratory and in the field.” While this is the way plant physiology courses are currently taught, a quick trip to the library to look at the previous plant physiology textbooks will show that this was a revolutionary idea in 1964. By initiating this exciting approach, Carl has directly and indirectly inspired three generations of plant physiologists.

Carl made significant contributions to a number of areas in plant physiology, including, flowering, apical dominance, phototropism, gravitropism, thigmomorphogenesis, fruit set, photoperiodism, phytochrome, dormancy, germination, growth, transpiration, translocation, stomatal regulation, mineral transport, membrane permeability, respiration, wounding, desiccation tolerance, chilling and water stress, and the physiology of  auxins, gibberellins, abscisic acid, ethylene and cytokinin. Through hard work accompanied by a sense of joy and purpose, Carl made many of the discoveries that form the foundation of our knowledge of how plants function—discoveries that are now considered to be self-evident facts. It would be possible to teach a comprehensive and intelligible plant physiological course based exclusively on the work of Carl and his students. We would like to elaborate on some of Carl’s outstanding contributions.

When reading Carl’s research papers, one can find a strong underlying feeling that while plant systems may have similarities with animal systems, one should not accept all the principles derived from animal research unchallenged. The importance of this outlook is demonstrated in a very interesting paper written by Carl that begins with a citation to Alex Comfort, the author of The Joy of Sex, and ends with a quote from St. Paul (Senescence in Plant Development, Science 134: 1727-1732). In this paper, Carl shows that while senescence in animals is not a selectable trait that can confer an advantage to an individual (an insight that is all too poignant today), senescence in plants serves many positive functions. For example, senescence allows the shedding of nonproductive, shaded leaves accompanied by the recovery of the nutrients that were in those leaves that can be used by the growing regions of the plant. The process of senescence is also useful to plants in  that it results in the breakdown of chlorophyll and provides fruits with a new coloration that makes them more attractive to the animals that disseminate them. In a series of papers, Carl, in collaboration with M. Kawase, E. Niedergang-Kamien, J. Janick and K. Muller, showed that senescence is a correlative phenomenon that is under hormonal control. One of Carl’s papers entitled, “Benzyladenine effects on bean leaf growth and senescence” (American Journal of Botany, 51: 294-298), was a particularly compelling and influential paper that demonstrated the contribution of cytokinin to regulating the competition between organs for essential nutrients. The loser, that is the organ with less cytokinin, became the senescent organ. Carl’s fundamental research on senescence with B. Poovaiah had practical utility in that it led to a method of storing apples so that we can eat crispy apples throughout the year.

Another advance in plant physiology was made by Carl in collaboration with N. Niedergang-Kamien, R. Hertel, A. Christie, O. Hall and R. dela Fuente in the 1950s, 1960s and 1970s when they studied the fundamentally important process of polar auxin transport. They showed that polar auxin transport was separable into two distinct transport processes: auxin entry into a cell and auxin exit from a cell. They observed that auxin exit required oxygen and was inhibited by metabolic inhibitors and low temperature, indicating that it was an active process. Moreover, they found that agents that bind to sulfhydryl groups of proteins also inhibit auxin transport. After characterizing the mechanism of inhibition by TIBA kinetically, they suggested that a transport protein on the plasma membrane was responsible for the transport of auxin out of the cell.

In a particularly significant pair of papers (Mathematical Model of Polar Auxin Transport, Plant Physiology 41:1476-1480 and Kinetics of Polar Auxin Transport, Plant Physiology 41:1481-1484), Carl showed that the marked polarity of auxin transport in tissues (basipetal transport/acropetal transport = 100), so necessary for normal development, is determined by the small polar ratio of individual cells (1.01-1.1). The linear arrangement of hundreds of cells in a file in a tissue provides the amplification mechanism to convert a minute cellular polarity into a conspicuous tissue polarity.

Carl made other valuable contributions to the concept of hormone receptors. While studying abscission with R. Biggs and B. Rubinstein in the 1950s and 1960s, Carl showed that abscission not only depends on the concentration of auxin but also on the sensitivity of the tissue to auxin. Moreover, he then showed that both the concentration of auxin and the tissues’ sensitivity to it is under developmental control. To our knowledge, this was the first work on receptor physiology in plants, a field that is currently blossoming.

By combining vision with experience, theory with skill, and enthusiasm with caution, Carl became a leader in the field of gravity sensing in plants from the 1960s through the 1990s, mentoring R. Hertel, R. dela Fuente, F. Sack, T. Blörkman, D. Perdue, A. LaFavre, M. Suyemoto, S. Wettlaufer and us. Carl came up with a model of gravisensing that provided the first and only serious challenge to the sedimenting statolith model of gravisensing in more than 100 years. Carl has also pioneered work in elucidating the series of biophysical steps that occur after the perception of gravity. In this regard, Carl demonstrated the importance of Ca2+ and electrical currents in transducing and amplifying the energy of the gravistimulus into the graviresponse.

Recognizing the importance of seed preservation in ensuring an adequate food supply for the world’s population, Carl, along with S. Yentur and D. Parrish, began to study the physiology of seeds.  As a natural outgrowth of Carl’s interest in senescence, Carl concentrated on the mechanism of aging in seeds. In this research direction, Carl collaborated with B. Webster, W. Bramlage, D. Priestley. L. Baird, M. McBride, M. Musgrave, R. Tully, V. Seewaldt, G. Feigenson, F. Goodsaid-Zalduondo, W. Galinat, R. Alscher-Herman, K. Niklas, B. Tiffney, J. Chabot, M. Hanna, S. O’Neill, R. Willing, M. Miller, R. Obendorf and C. Vertucci.
Through the development and utilization of a vast array of biophysical and biochemical techniques, Carl discovered the physico-chemical nature of the dry, anhydrous seed and the reasons why the viability of seeds decreases during storage. Carl found that seeds that could be stored successfully had a high capacity to bind water tightly. In collaboration with R. Williams, I. Bernal-Lugo and W. Sun, Carl showed that in recalcitrant seeds the small amount of water retained in the dried seed combined with non-reducing sugars to form a glassy state that would protect the membranes and proteins in the embryo. Using the knowledge Carl gleaned from studying the preservation of proteins in stored seeds, Carl developed a method to preserve peptide hormones like insulin in the glassy state so that they can be pulverized into a powder and inhaled by diabetics as an alterative to self-injection. In order to understand further, what happens in the glassy state in seeds, Carl, along with M. Cohn, F. Bruni and G. Careri applied percolation theory to describe how the glassy state can act as a solid-state gate that inhibits respiratory activity in seeds by preventing proton movement and metabolic electron transport, which would result in a slow decline in the dormant seed.  In seeds that are less capable of forming a glassy state, the Amadori and Maillard reactions, which cause reducing sugars to nonenzymatically bind to proteins and inactivate them, takes place. These indiscriminant glycation events also result in the loss of seed viability. In collaboration with J. Lozano and S. Blackman, Carl investigated the biochemical basis of desiccation tolerance by screening for the genetic markers that coded for small hydrophilic stress-induced proteins that were correlated with good seed storage. Carl shared “what he learned from seeds” in a warm personal account (Learning from Seeds, Seed Science Research 9:111-116).

Carl continued to work as a plant physiologist in the new millennium. Along with his dear colleagues and fellow retirees, Mark Jaffe and Dick Staples, Carl worked on the physiology of contractile roots (Light Activation of Contractile Roots of Easter Lily, American Society for Horticultural Science, 132: 575-582) and the mechanism of thigmomorphogenesis (Thigmo Responses in Plants and Fungi, American Journal of Botany 89: 375-382).

Carl has not restricted his creative scientific energies to doing experiments and writing them up. Carl has also written extensively about the scientific process (The Act of Creation: Creative Processes in Science, BioScience 28:436-440), the structure of the scientific community (The Man in the White Lab Coat, BioScience 17:233-235; Games Scientists Play, BioScience 23:590-594; The Burden of Competitive Grants, Science 203:607; Heroic or Bureaucratic Science?, BioScience 31:707; Effective Funding, Science 216:244; Ivory Tower: A Peer into the Review System, ASPP Newsletter 11:4-9; Weapons Research Extracts a Toll on Academic Science, The Scientist October 16, 1989, pp. 13-15) and the role science has played in determining the ethics of our society (The Science Community is Starved for Ethical Standards, The Scientist, 6:11; Ethics in Science and Science in Ethics, Ursus 1: 8-10).  At a time when science was increasingly being guided by financial interests, Carl remained dedicated to ethical thinking followed by intentional action and to science as a means of discovery and enlightenment.

Carl joined the ASPP in 1948 and became an Emeritus Member in 1990. Carl was the Vice President of ASPP in 1959 and the President in 1966. Carl served on the editorial board of Plant Physiology for 13-14 years. Carl was a regular contributor to Plant Physiology and to this newsletter.  Carl championed the cause of plant physiology as the representative of ASPP to AAAS, PGSA, NASA and the BSA. As a plant physiologist, Carl served the Nation by being a Senior Policy Analyst in the staff of the Science Adviser to the President during the Ford Administration (1974-1975), provided testimony before the United States House Committee on Science and Technology (December 1980 and February 1982) and served as a consultant to the National Science Foundation, Division of Policy Research and Analysis (1975-1977). In 2000, when Carl received the Golden Medal of the Royal Galician Academy of Science, in Santiago de Compostela, Spain, he gave his acceptance speech in Spanish.  In 1994, Carl was awarded the Charles Reid Barnes Life Membership Award for his wide-ranging and deep contributions to the society and to plant physiology itself. This past summer, the American Society of Plant Biologists again honored Carl as a physiologist and a citizen when it made Carl an ASPB Fellow.

As a conservationist, Carl was active in the Aldo Leopold Foundation, founding president of the Finger Lakes Land Trust and of a board member of the Black Locust Initiative. In 1992, Carl co-founded the Tropical Forestry Initiative, a non-profit organization that is notable as a demonstration project for the reforestation of tropical land with native forest tree species. Much of the 350 acres has been reforested with over 40,000 seedlings, which came predominantly from a native tree nursery Carl began. Carl measured and documented the growth of these huge tropical trees just as he measured and recorded the growth of the pines he planted with his father sixty years earlier. Carl’s daughter Susan has continued the Leopold legacy through a reforestation project in her home state of Washington.

On the day he died, Carl was actively trying to preserve the integrity, stability and natural beauty of Ithaca and its surrounds by preventing the possible use of horizontal drilling, hydrofracting and storing toxic wastes in an attempt to extract natural gas from the Marcellus Shale.  Carl was buried in Greensprings Natural Cemetery, of which he was a co-founder. In this last initiative, he completed the circle begun by his father’s quest to understand how to live on the land without spoiling it. Carl’s molecules and minerals will be recycled into the fruits of the native bushes that will feed the native birds and provide energy for their songs.  Carl’s spirit will live on within all who knew him.

Carl was not only a phenomenally talented and broad thinker with an enormous number of influential and lasting scientific accomplishments under his belt, but he was also a very kind and gentle person. He had an enormous curiosity and a genuine interest in other people and their ideas. Carl was a really good man. Carl loved the study of life; he believed in life, and he made the lives of those around him better. We will miss Carl’s heartwarming smile and his infectious enthusiasm for life.

Carl grew up with strong personal roots in the natural world and in the Land Ethic of his father and after growing his own youthful sapwood, colorful heartwood, and thin latewood, Carl became a mighty and graceful trunk that nurtured and enthusiastically supported many plant physiologists and conservationists who consider themselves to be “Leopold leaves” on the tree of Carl’s life.