Paper Reviewed
Pandey, R., Dubey, K.K., Ahmad, A., Nilofar, R., Verma, R., Jain, V., Zinta, G. and Kumar, V. 2015. Elevated CO2 improves growth and phosphorus utilization efficiency in cereal species under sub-optimal phosphorus supply. Journal of Plant Nutrition 38: 1196-1217.

Phosphorus (P) is an important macronutrient necessary for plant photosynthesis. When present in sufficient quantities, it has been shown to benefit plants by stimulating the formation of oils, sugars and starches, fostering rapid tissue growth and development, increasing stalk and stem strength, improving resistance to disease, enhancing crop quality, aiding flower and seed production, and benefiting a host of other growth- and development-related factors and processes. Out in the real world, however, P availability is often limited. Consequently, plants have developed multiple adaptive mechanisms (morphological, physiological and molecular) to help them cope with P insufficiency (Pi).

Despite such adaptive mechanisms, there are concerns that Pi will increase in the future as atmospheric CO2 concentrations rise. This hypothesis is based upon the recognition that elevated CO2 stimulates plant photosynthesis and growth. Such stimulation, however, is expected to require additional amounts of P in order for plants to sustain the projected CO2-induced growth enhancements. Otherwise, if P is limiting in the growth medium, or if plant adaptive mechanisms cannot compensate for the increased P demand, the growth benefits of CO2 enrichment may be reduced, and possibly wholly overcome, by Pi.

In a test of this hypothesis, Pandey et al. (2015) investigated the interactive effects of elevated CO2 and P nutrition on the growth response of three cereals: two wheat varieties (Triticum aestivum L. cv. PBW-396 and Triticum durum L. cv. PDW-233) and rye (Secale cereal L. cv. WSP 540-2). These three cereal species were grown in controlled environment chambers at the National Phytotron Facility, Indian Agricultural Research Institute, New Delhi, under conditions of 22°C/12°C day/night temperatures, a 10-hour photoperiod (450 µmol m^-2 s^-1 PAR) and relative humidity of 90 percent. Atmospheric CO2 concentrations were maintained at either ambient (380 ppm) or elevated (700 ppm) and supplied P levels were either low (2 µM) or sufficient (500 µM). And, after 15 days post germination various growth and biochemical analyses were performed. So what did the authors' analysis reveal?

First off, Pandey et al. note that elevated CO2 increased the total plant dry weight of all three cereal species by an average of approximately one-third, regardless of P treatment level. In addition, they write the "total plant dry matter accumulated in plants grown with low-P under elevated CO2 was equivalent to those grown with sufficient P under ambient CO2" (emphasis added), which observation, they say, "indicates efficient utilization of tissue P under higher CO2." Thus, elevated CO2 was able to completely compensate for the reduction in total plant dry weight that was induced by the low P treatment.

Elevated CO2 was also shown to benefit whole plant leaf area, which increased by 15 percent in low-P conditions, and by a much larger 43 percent under sufficient P. It also altered the partitioning of plant dry matter; under elevated CO2, both P treatments experienced a higher number of lateral roots per plant, greater root length and increased root surface area, which led to an increase in plant root/shoot ratio of approximately 20 percent in both P treatments.

Nutrient uptake per unit root mass and total P content per plant were also enhanced under elevated CO2. Under P sufficient conditions these parameters increased by 19 and 49 percent respectively, whereas under low P conditions they were stimulated by 55 and 26 percent. Lastly, Pandey et al. report that these several benefits of elevated CO2 combined to induce a whopping 59 percent average increase in plant phosphorus use efficiency (PUE, defined as a unit of dry matter produced per unit of P uptake) among the three studied cereals when grown under P-limiting conditions.

Given the above findings, it is clear that elevated CO2 was able to sufficiently compensate for an increased P demand by (1) stimulating the cereals' root systems, which allowed the plants to acquire greater amounts of P from the growth medium, and (2) by improving the ability of Pi-compensating mechanisms within the plants to utilize P, as evidenced by the large increase of PUE observed under low P conditions. And that is news worth celebrating.