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 vulgare L.) by ear halving or partial defoliation after anthesis. Plants were 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) until 5 days after flowering, and then were subjected to either 15/9 degrees C or 22/16 degrees C during grain filling. The temperature treatments were imposed to test if the effects of source-sink ratio on the characteristics under study were independent of temperature during grain 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 weight of individual grains decreased only moderately with temperature rise and was not responsive to variation in the supply of assimilates. Instead, reduction in ear size increased grain N concentration under all environmental conditions, indicating that N accumulation in the grain. was source-limited in control plants. It appeared that the grain N concentration necessary to achieve maximum grain weight was below that which maximizes grain N accumulation. Grain N limitation by the source appeared after the initial phase of grain growth but was not evident 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 concentration are discussed.