Background
In introducing their intriguing new study, the authors write that "brassinosteroids (BRs) are a family of over 40 naturally occurring plant steroid hormones that are ubiquitously distributed in the plant kingdom," citing Clouse and Sasse (1998), Bishop and Koncz (2002), Krishna (2003) and Montoya et al. (2005). And they go on to report that "BRs play prominent roles in various physiological processes including the induction of a broad spectrum of cellular responses, such as stem elongation, pollen tube growth, xylem differentiation, leaf epinasty, root inhibition, induction of ethylene biosynthesis, proton pump activation, regulation of gene expression and photosynthesis, and adaptive responses to environmental stress," citing Clouse and Sasse (1998), Dhaubhadel et al. (1999), Khripach et al. (2000), Krishna (2003) and Yu et al. (2004), while noting that "as potent plant growth regulators, BRs are now widely used to enhance plant growth and yield of important agricultural crops," citing Khripach et al. (2000) and Divi and Krishna (2009).

What was done
Working with well-watered and fertilized plants that had reached the three-leaf growth stage in pots containing a 6:3:1 (by volume) mixture of peat, vermiculite and perlite that were located within controlled environment growth chambers maintained at either ambient (380 ppm) or enriched (760 ppm) atmospheric CO2 concentrations, and with or without being sprayed with a solution of brassinosteroids (0.1 µM 24-epibrassinolide), Jiang et al. measured - among several other things -- rates of net photosynthesis, leaf area development and shoot biomass production over a period of one additional week.

What was learned
The six scientists determined that their doubling of the air's CO2 concentration resulted in a 44.1% increase in CO2 assimilation rate; and they write that the BR treatment "also significantly increased CO2 assimilation under ambient atmospheric CO2 conditions, and the increase was close to that by CO2 enrichment." Most interesting of all, in this regard, they report that the combined treatment of "plants with BR application under CO2-enriched conditions showed the highest CO2 assimilation rate, which was increased by 77.2% relative to the control." Likewise, they found that "an elevation in the atmospheric CO2 level from 380 to 760 ppm resulted in a 20.5% and 16.0% increase in leaf area and shoot biomass accumulation, respectively," while the plants that received the BR application "exhibited 22.6% and 20.6% increases in leaf area and shoot biomass accumulation, respectively." Most importantly of all, however, they report that, once again, the combined treatment of "CO2 enrichment and BR application further improved the plant growth, resulting in 49.0% and 40.2% increases in leaf area and shoot biomass, relative to that of the control, respectively."

What it means
Both atmospheric CO2 enrichment and BR application work wonders in promoting the early growth and development of cucumber plants, thereby providing hope that the two phenomena - one due to the direct effects of man (BR application) and one due to humanity's indirect effects (the mining and burning of fossil fuels) - working together and in concert with still other crop improvements that may possibly be devised, will yet enable earth's growing human population to successfully feed itself near the midpoint of this century, when it is estimated there will be another two billion people inhabiting the planet.

References
Bishop, G.J. and Koncz, C. 2002. Brassinosteroids and plant steroid hormone signaling. The Plant Cell 14: S97-S110.

Clouse, S.D. and Sasse, J.M. 1998. Brassinosteroids: essential regulators of plant growth and development. Annual Review of Plant Physiology and Plant Molecular Biology 49: 427-451.

Dhaubhadel, S., Chaudhary, S., Dobinson, K.F. and Krishna, P. 1999. Treatment with 24-epibrassinolide, a brassinosteroid, increases the basic thermotolerance of Brassica napus and tomato seedlings. Plant Molecular Biology 40: 333-342.

Divi, U.K. and Krishna, P. 2009. Brassinosteroid: a biotechnological target for enhancing corn yield and stress tolerance. New Biotechnology 26: 131-136.

Khripach, V., Zhabinskii, V. and De Groot, A. 2000. Twenty years of Brassinosteroids: steroidal plant hormones warrant better crops for the XXI century. Annals of Botany 86: 441-447.

Krishna, P. 2003. Brassinosteroid-mediated stress responses. Journal of Plant Growth Regulators 22: 289-297.

Montoya, T., Normura, T., Yokota, T., Farrar, K., Harrison, K., Jones, J.G.D., Kaneta, T., Kamiya, W., Szekeres, M. and Bishop, G.R. 2005. Patterns of dwarf expression and brassinosteroid accumulation in tomato reveal the importance of brassinosteroid synthesis during fruit development. Plant Journal 42: 262-269.