Carbon translocation and root respiration in potted apple trees during conditions of moderate drought

The experiment was conducted to investigate the partitioning of carbon 13 (C-13) and the relationship between root C allocation and root respiration rate in 'Redcort'/M.7 and rooted cuttings of B.9. Canopies were pulsed with (CO2)-C-13 and isotopic emissions from the soil were monitored after pulse labelling. Mildly water stressed potted trees were compared with a well-watered treatment. In 'Redcort'/M.7, water stress, over a 7-day period, reduced predawn leaf water potential and root efflux rate of (CO2)-C-13 (up to 55% reduction measured 60 hours after pulse labelling). However, C-13 partitioning to the roots was not reduced by low soil moisture. Evolution rate of C-13 from the roots was not related to the amount of C-13 allocated into the roots themselves. In B.9, (CO2)-C-13 emission from the soil was monitored for 16 days. A decrease in root respiration (up to 53% on the first day after the pulse) was the first response to soil water deficit treatment even when drought stress was not effective in reducing leaf gas exchange. These results suggest that during water deficit conditions roots appear to use a lower proportion of their total carbon content for respiration. C-13 partitioning within the tree was not affected by water supply over the duration of the experiment.