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Source/Sink Relations -- Summary
Plants grown in elevated CO2 for extended durations often, but not always, exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by reduced rates of photosynthesis resulting from decreased activity and/or amount of the primary plant carboxylating enzyme rubisco.  When this phenomenon occurs, leaf nitrogen content often decreases, as nitrogen previously invested in rubisco is transferred to other parts of the plant.  Photosynthetic acclimation to elevated CO2 can be induced by insufficient plant sink strength, which often leads to carbohydrate accumulation in source leaves and reductions in net rates of photosynthesis.  Acclimation can also result from the physical constraints of growing plants in pots or by limiting their access to important nutrients such as nitrogen.  In this summary, we review the results of various studies of source:sink relationships on plant growth responses to atmospheric CO2 enrichment.

In a simple experiment designed to determine how sink size naturally influences plant growth response to elevated CO2, Reekie et al. (1998) grew various Brassica species of differing sink capacity in growth chambers receiving atmospheric CO2 concentrations of 350 and 1000 ppm.  After four weeks, elevated CO2 had significantly increased the total dry weights of all Brassica species.  However, at final harvests made after six or twelve weeks of CO2 fumigation, significant CO2-induced increases in total dry weight only persisted in species possessing well-developed carbon sinks.  Thus, the author's data suggest that species with inherently low sink strength are more likely to experience CO2-induced acclimation than are species with inherently large sink strength that can naturally utilize the additional carbohydrates produced under CO2-enriched conditions to progressively increase their biomass.

In a non-invasive manipulation of plant source:sink ratios, Gesch et al. (1998) grew rice plants for one month in growth chambers having atmospheric CO2 concentrations of 350 and 700 ppm before reciprocally switching half of the plants in each chamber to the other CO2 growth concentration.  Within 24 hours of switching, plants moved into the elevated CO2 environment displayed a mean 15% increase in photosynthetic rate and a mean 19% reduction in rbcS mRNA level, whereas previously-CO2-enriched plants switched to ambient CO2 exhibited photosynthetic rates that were 10% lower than those of plants grown continuously at ambient CO2 and an analogous 19% increase in the amount of rbcS transcript.  These observations suggest that rice plants grown continuously in elevated CO2 experienced sink limitations to growth.

In the FACE experiment of Rogers et al. (1998), perennial ryegrass growing at atmospheric CO2 concentrations of 360 and 600 ppm was frequently cut to determine how physical changes in source:sink relationships influence transient growth responses to elevated CO2.  Regardless of cutting events, plants grown in elevated CO2 with high soil nitrogen availability exhibited no signs of CO2-induced photosynthetic acclimation.  In contrast, one day prior to cutting, plants grown in elevated CO2 with low soil nitrogen availability exhibited 25% reductions in leaf rubisco content.  However, one day after cutting, this acclimation response was completely eliminated, probably in response to rapid carbohydrate utilization to repair cut leaves.  Similarly, Bryant et al. (1998) reported that simulated grazing events reversed CO2-induced decreases in rubisco activity in leaves of chalk grassland species growing on nutrient-poor soils.  Thus, the results of these studies suggest that photosynthetic acclimation likely results from an indirect effect of low soil nitrogen on sink development, rather than from a direct effect of elevated CO2 on leaf photosynthetic capacity.

In the experiments of Farage et al. (1998), the authors investigated the role of nitrogen supply in inducing photosynthetic acclimation in CO2-enriched wheat.  In one experiment, plants were grown in pots placed within growth chambers receiving 350 or 650 ppm CO2 and were irrigated with fixed amounts of low or high nitrogen solutions on a regular basis, which is standard protocol for experiments utilizing potted plants.  In the other experiment, plants were grown hydroponically at 350 or 650 ppm CO2 to eliminate any root restriction effects on growth.  The plants were placed in nutrient solutions containing low or high concentrations of nitrogen, which were continually increased to match the rising demand of the growing plants.  All plants were grown for approximately five weeks and then harvested.

Wheat plants grown in pots exhibited photosynthetic acclimation when supplied with low fixed amounts of nitrogen; and elevated CO2 exacerbated this effect.  In contrast, hydroponically-grown wheat that received gradually increasing nutrient supplies, which became ever larger with increasing plant size, exhibited no signs of photosynthetic acclimation when grown at elevated CO2 even at low nitrogen availability.  These observations led the authors to conclude that low nitrogen fertilization may not lead to photosynthetic acclimation in elevated CO2, as long as the nitrogen supply keeps pace with the relative growth rate of the plants.  Consequently, it is important for researchers who use potted plants to increase plant nutrient supply in proportion to plant growth as their experiments progress, in order to avoid inducing photosynthetic acclimation via the dilution of tissue nitrogen contents that typically results from enhanced carbohydrate and biomass production in elevated CO2.

In summary, it appears that plants with inherently large sink capacity have the ability to respond quite strongly and persistently to atmospheric CO2 enrichment.  In other plants, a short-term reduction in source strength often overcomes sink-induced limitations on growth responses to atmospheric CO2 enrichment.  Indeed, foliar reductions resulting from clipping events - or non-invasive reductions in source strength - nearly always eliminated CO2-induced photosynthetic acclimation.  Moreover, it was shown that acclimation to elevated CO2 may actually be an indirect effect induced by insufficient amounts of soil nitrogen; for if soil nitrogen content is too low, additional sink strength cannot be developed in some plants, which consequently allows carbohydrate accumulation in source leaves that can induce photosynthetic acclimation by feedback inhibition processes.  Therefore, as the CO2 content of the air rises and increases plant photosynthetic rates, many plants will respond by reducing rubisco contents, which consequently frees up large quantities of nitrogen that may be used to enhance sink strength to keep plants from exhibiting photosynthetic acclimation.

Bryant, J., Taylor, G. and Frehner, M.  1998.  Photosynthetic acclimation to elevated CO2 is modified by source:sink balance in three component species of chalk grassland swards grown in a free air carbon dioxide enrichment (FACE) experiment.  Plant, Cell and Environment 21: 159-168.

Farage, P.K., McKee, I.F. and Long, S.P.  1998.  Does a low nitrogen supply necessarily lead to acclimation of photosynthesis to elevated CO2Plant Physiology 118: 573-580.

Gesch, R.W., Boote, K.J., Vu, J.C.V., Allen, L.H., Jr. and Bowes, G.  1998.  Changes in growth CO2 result in rapid adjustments of ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit gene expression in expanding and mature leaves of rice.  Plant Physiology 118: 521-529.

Reekie, E.G., MacDougall, G., Wong, I. and Hicklenton, P.R.  1998.  Effect of sink size on growth response to elevated atmospheric CO2 within the genus BrassicaCanadian Journal of Botany 76: 829-835.

Rogers, A., Fischer, B.U., Bryant, J., Frehner, M., Blum, H., Raines, C.A. and Long, S.P.  1998.  Acclimation of photosynthesis to elevated CO2 under low-nitrogen nutrition is affected by the capacity for assimilate utilization.  Perennial ryegrass under free-air CO2 enrichment.  Plant Physiology 118: 683-689.