Wardlaw I.F., 1988: Effect of phosphorus deficiency on source sink interactions between the flag leaf and developing grain in barley. Journal Of Experimental Botany9: 165-178
Photosynthetic rate and stomatal conductance of P-deficient barley plants were increased by manipulations that increased plant demand for carbohydrates (shading of the ear or removal of other leaves). This was associated with retarded senescence of the flag leaf. Similarly, small flag leaves (i.e., those having large sinks relative to their own size) had high rates of photosynthesis. These relationships were most pronounced under conditions of P deficiency. Distribution of 32p and 14c from the flag leaf was determined by the carbohydrate demand of a plant part and was unaffected by altering the sink strength for P. Thus, ear shading increased accumulation by developing grains of 32p and 14c applied to the flag leaf, grain removal reduced accumulation by grain of both radioisotopes, and removal of lower leaves increased movement of both radioisotopes to vegetative plant parts; in contrast, P addition to the ear had no effect upon 32p or 14c distribution. P deficiency reduced the amount of tillering and, therefore, the movement of 32p and 14c to developing tillers and increased retention of 14c and 32p in the sheath of the flag leaf but otherwise had no major effect on patterns of carbon and phosphorus distribution. We conclude that the major effects of P deficiency upon source-sink interactions are (1) to reduce the number or activity of sinks in the shoot (grains and daughter tillers) and (2) to increase the sensitivity with which photosynthesis responds to demand for carbohydrate. The strengthened source-sink interaction under conditions of P deficiency provides a regulatory mechanism for reducing carbohydrate accumulation under conditions where carbohydrates do not strongly limit growth.