Stable isotope carbon study: long‐term partitioning during progressive drought stress in Brassica napus var. oleifera
Abstract. Long-term carbon partitioning was analysed by stable carbon isotope labelling of CO2 during the adaptive response of Brassica napus to progressive drought stress. This method allowed us to distinguish between the pre-existing carbon, which had accumulated before the change in CO2 isotope composition (on 20 d) and the recent photosynthetic input occurring during the following period. Three successive adaptive phases characterized the plant response to drought. In the first period of water shortage (20–30 d), growth was progressively slowed down: the recent C input (28.3 mg per plant) was mainly allocated to mature leaves (14 mg) and roots (8.9 mg); the pre-existing C chains were partly lost by respiration (12.1 mg) and partly translocated to the apex (2.3 mg). The increase in dry matter (13.0 mg) was mainly due to the recent C input (19.1 mg) in shoots but the roots appeared also as an important sink despite their small dry matter increase (3.2 mg). Root dry matter turnover corresponded to the elimination of pre-existing C chains (6.0 mg) and their renewal with the recent C input (9.2 mg). Root sink strength was related also to the development of drought-induced short tuberized roots (0.1 mg from pre-existing C and 0.3 mg from recent C). In the second period of water stress (from 30 to 40 d), the whole plant biomass remained constant in spite of efficient allocations from pre-existing and recent C sources to two main sinks: shoot apex (3.1 and 5.4 mg, respectively) and short tuberized roots (1.0 and 0.2 mg, respectively). The hypocotyl acted as a transient storage organ for the recent C input (2.0 mg). In the third period of recovery upon rehydration (from 46 to 51 d), the short tuberized roots gave rise to a new root system using C chains from pre-existing and recent sources (1.7 and 2.8 mg, respectively). Such concomitant sink-source behaviour of tuberized roots underlines the adaptive value of drought rhizogenesis.