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Contribution of current photosynthates to root respiration of non-nodulated Medicago sativa: effects of light and nitrogen supply.

The effects of light (PFD) and nitrogen (N) supply on root respiration of new C (currently assimilated carbon, R(new)) and old C ( R(old)) were analysed in non-nodulated Medicago sativa. Plants were pre-treated with high/low PFD and high/low N supply with a regular 16/8 h light/dark cycle. Five to eight weeks after planting current photosynthates were labelled with (13)C and their contribution to root respiration was continuously measured during a 24 h day/night cycle. PFD conditions during labelling were either those of the pre-treatments (control, 25 or 6 mol m(-2) d(-1)) or, for high PFD plants, 6 mol m(-2) d(-1) by shortening the photoperiod or reducing irradiance. The fraction of new C in the respiratory CO2 increased during the light period, but remained constant in the dark period. In control plants, R(new) contributed 40 % to the daily root respiration in high PFD/high N conditions. Continuously low PFD increased (50 %) and low N decreased (26 %) the contribution of R(new). Exposing plants from high PFD pre-treatments to a short photoperiod or to low PFD stimulated R(old), indicating mobilisation of reserve C. This stimulation was more pronounced in plants with high N supply than in those with low N supply. Comparison with other legumes suggested that R(new) in root respiration was mainly defined by the ratio between the assimilatory capacity of the shoots and the maintenance costs of roots with a short-term capacity of buffering respiratory demand by mobilisation of reserves in situations of fluctuating PFD.