SOURCE-SINK RATIO IN BARLEY (HORDEUM-VULGARE L) DURING GRAIN FILLING - EFFECTS ON SENESCENCE AND GRAIN PROTEIN-CONCENTRATION

The impact of source-sink manipulations on plant senescence and grain nitrogen (N) concentration was studied in barley plants (Hordeum vulga re L.) by ear halving or partial defoliation after anthesis. Plants we re sown in pots at two different dates (Experiment 1 and Experiment 2) under ample N supply and were periodically detillered. In Experiment 1, plants were grown at similar temperature regime (15/9 degrees C) un til 5 days after flowering, and then were subjected to either 15/9 deg rees C or 22/16 degrees C during grain filling. The temperature treatm ents were imposed to test if the effects of source-sink ratio on the c haracteristics under study were independent of temperature during grai n filling. The decrease in green leaf area and N concentration in stem and leaves after flowering were insensitive to alterations in source- sink ratio and similarly accelerated by temperature rise. The final we ight of individual grains decreased only moderately with temperature r ise and was not responsive to variation in the supply of assimilates. Instead, reduction in ear size increased grain N concentration under a ll environmental conditions, indicating that N accumulation in the gra in. was source-limited in control plants. It appeared that the grain N concentration necessary to achieve maximum grain weight was below tha t which maximizes grain N accumulation. Grain N limitation by the sour ce appeared after the initial phase of grain growth but was not eviden t at other times during grain filling. The temporal changes in grain N concentration in response to source-sink ratio during grain fill and the potential significance of the results for modelling of grain N con centration are discussed.