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Clouding the issue. Climatologists trying to understand the interplay among the myriad factors associated with global warming, as well as meteorologists trying to predict erratic weather patterns, depend upon highly complex computational models to interpret the wealth of data they collect from earth-observing satellite-based instruments. The picture is clouded, however, by recent studies that indicate that clouds in the atmosphere absorb more solar radiation than any of the current models predict. Although the amount of excess absorption is small, the impact on how the data is understood may be significant, and until now researchers have been puzzled about how to account for the discrepancy.

Recent research by two faculty member in the CAS department of geography, Juri Knjazihhin, a research associate professor, and Ranga Myneni, an associate professor, may hold the key. Their results indicate that the difference between the projected level of absorbed radiation and the amount actually absorbed may be a product of how researchers mathematically handle the wide range in the size of cloud particles.

Standard modeling protocols average out the sizes of the particles to come up with a single average particle size. This is based on an “ensemble assumption” -- an assumption that all particle sizes are well represented in any cloud. But a realistic cloud contains a huge number of small drops and a tiny number of large ones. The assumption of an “average drop” simplifies the modeling protocol, but since drops are discrete, it poorly approximates reality. By replacing the assumption of an average-sized particle with a model that uses a cumulative distribution, one that has jumps, or steps, corresponding to the rarer particle sizes, Knjazihhin and Myneni were able to create a more accurate model.

The new approach designed by the researchers adds together two components that represent contributions from the ensemble of particles and from the large single drops within the cloud. The first component integrates the amount of energy absorbed as photons from a solar beam travel through the cloud -- the longer the photon path, the larger the amount of energy absorbed. The second adds the jumps that correspond to the rarer particle sizes to provide more accurate results.

This research was recently published in the “Papers of Note” section of the Bulletin of the American Meteorological Society.

Growing things. As even the most casual of gardeners will notice, plants respond to changes of environment. Leaves and stems grow toward light, roots toward darkness. Changes of temperature and the availability of nutrients likewise trigger developmental changes. It has long been known that auxin, a plant hormone, is an important mediator, regulating almost all qualities of plant growth -- stature, shoot and root architecture, shoot strength, seed and fruit size, ripening, and aging. Although the chemical structure of auxin has also been known for nearly 70 years, the genetic pathway by which it is synthesized has remained elusive.

New research by Anna Hull (GRS’02), a doctoral student in the laboratory of John Celenza, a CAS assistant biology professor, and collaborators from the Salk Institute and the University of Massachusetts, Amherst, has shed new light on this process.

Hull and her colleagues, working with the model plant species Arabidopsis thaliana, identified two cytochrome P450 enzymes, encoded by two genes they had previously identified in the species, and demonstrated the critical role they play in auxin synthesis. To do this they engineered Arabidopsis mutants, some of which produced an abundance of the enzymes and others that were unable to produce the enzymes. The mutant plants produced more auxin in the first group and reduced amounts of auxin in the second. Significantly, the mutant plants showed developmental changes consistent with the changes in auxin, indicating that by modulating auxin production, plant development can be modulated.

Since all higher plants share a close evolutionary link, this new piece in the auxin puzzle has relevance to other, more agriculturally important, plants.

This research was published in the December 1, 2002, issue of the journal Genes & Development. Other BU authors include Neeru R. Gupta (CAS’00, MED’04), who did her honors thesis in Celenza’s lab funded in part by grants from the Howard Hughes Medical Institute and BU’s Undergraduate Research Opportunities Program, and Kendrick A. Goss (GRS’03). Hull received the biology department’s 2002 Belamarich Award for outstanding thesis based on this work. Celenza and his colleague Jennifer Normanly of UMass-Amherst recently received USDA funding to continue the investigation.

"Research Briefs" is written by Joan Schwartz in the Office of the Provost. To read more about BU research, visit


15 May 2003
Boston University
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