What was done
Thirty-day-old nodulated alfalfa (Medicago sativa L.) plants were grown in two temperature-gradient greenhouses (one maintained at an atmospheric CO2 concentration of 350 ppm and the other at a concentration of 700 ppm) in pots recessed into the ground in an alfalfa field under conditions of ambient temperature (TA) and elevated temperature (TE = TA + 4°C) and well-watered (to field capacity) and water-stressed (50% field capacity) conditions for one month, after which a first cutting took place, plus an additional month, after which a second cutting took place. After each cutting, plant dry matter production was determined, while taproots were analyzed for vegetative storage protein (VSP) contents.

What was learned
At the time of first cutting it was determined that the alfalfa plants had had their dry matter production boosted by an average of about 30% in the well-watered treatment (averaged across both temperature treatments) over the first growth period, but by only about 10% in the water-stressed treatment. At the time of the second cutting, however, it was found that the well-watered plants had experienced an average dry matter production increase on the order of 20% over the second growth period, while the plants in the water-stressed treatment had experienced a mean increase of fully 40%. In addition, Erice et al. report that over the first growth period "taproot VSP content increased in response to drought and elevated CO2."

What it means
The researchers state that "it has been demonstrated that nitrogen pools in alfalfa taproot, especially vegetative storage proteins, condition new regrowing shoots," and that appears to be what happened in their study. At the end of the first growth period, for example, the enhanced taproot VSP content in the water-stressed and CO2-enriched treatment may have been the reason why the elevated CO2 was so effective in stimulating biomass production in the water-stressed treatment over the second growth period.

This finding is somewhat analogous to the observation of Idso et al. (2001), who found that nitrogen reabsorbed from second-year leaves of sour orange trees (which hold most of their leaves for a period of two years) during the process of senescence in the fall was stored over winter in much greater amounts in putative vegetative storage proteins in first-year leaves of CO2-enriched trees than in first-year leaves of trees growing in ambient air, so that when the stored nitrogen was released in the spring to produce a flush of new leaves on the trees, leaf production on the CO2-enriched trees vastly outpaced the production of new leaves on trees growing in ambient air.

Reference
Idso, K.E., Hoober, J.K., Idso, S.B., Wall, G.W. and Kimball, B.A. 2001. Atmospheric CO2 enrichment influences the synthesis and mobilization of putative vacuolar storage proteins in sour orange tree leaves. Environmental and Experimental Botany 48: 199-211.