Drought affects the fluxes of carbon to roots and soil in ¹³C pulse-labelled plants of wheat

The allocation of C between roots and shoots, and the fluxes of C to roots and soil were measured in wheat grown under two soil moisture regimes during the vegetative stages of growth. Wheat plants were grown in columns of soil in a glasshouse, and the fate of pulse-labelled ¹³C was followed as root and shoot C. root and microbial respiration, rhizodeposition and shoot respiration. Relatively more assimilates were allocated to the roots under limited water conditions as indicated by higher percentages of the excess ¹³C recovered in the roots. Changes during the vegetative growth in the excess ¹³C allocated to the roots occurred at the onset of tillering and booting stage, and were associated with changes in the activity of the sinks. The excess ¹³C below ground under limited soil water increased by 21% at leaf formation and by 43% at booting relative to that under adequate soil water. The increase resulted mainly from an increase in the excess ¹³CO₂ respired from roots and microorganisms but not from the recovery of ¹³C in the soil. We conclude that the increase in the C below ground under limited soil water arose from more C made available to the roots by the reduction in shoot C. This was due to a smaller reduction in root growth rates than in shoot growth rates under the water-limited conditions.