Phosphorus deficiency inhibits growth in parallel with photosynthesis in a C-3 (Panicum laxum) but not two C-4 (P. coloratum and Cenchrus ciliaris) grasses

This study compared the growth and photosynthetic responses of one C-3 ( Panicum laxum L.) and two C-4 grasses ( Panicum coloratum L. and Cenchrus ciliaris L.) to changes in soil phosphorus ( P) nutrition. Plants were grown in potted soil amended with six different concentrations of P. One week before harvest, leaf elongation and photosynthetic rates and the contents of carbohydrate, P and inorganic phosphate ( Pi) were measured. Five weeks after germination, plants were harvested to estimate biomass accumulation. At each soil P supply, leaf P contents were lower in the C-3 ( 0.6-2.6 mmol P m(-2)) than in the two C-4 grasses ( 0.8-4.1 mmol P m(-2)), and Pi constituted similar to 40-65% of total leaf P. The P deficiency reduced leaf growth, tillering and plant dry mass to a similar extent in all three grasses. In contrast, P deficiency suppressed photosynthetic rates to a greater extent in the C-3 ( 50%) than the C-4 grasses ( 25%). The foliar contents of non-structural carbohydrates were affected only slightly by soil P supply in all three species. Leaf mass per area decreased at low P in the two C-4 grasses only, and biomass partitioning changed little with soil P supply. The percentage changes in assimilation rates and plant dry mass were linearly related in the C-3 but not the C-4 plants. Thus, P deficiency reduced growth in parallel with reductions of photosynthesis in the C-3 grass, and independently of photosynthesis in the two C-4 grasses. We propose that this may be related to a greater Pi requirement of C-4 relative to C-3 photosynthesis. Photosynthetic P use efficiency was greater and increased more with P deficiency in the C-4 relative to the C-3 species. The opposite was observed for whole-plant P-use efficiency. Hence, the greater P-use efficiency of C-4 photosynthesis was not transferred to the whole-plant level, mainly as a result of the larger and constant leaf P fraction in the two C-4 grasses.