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

This study compared the growth and photosynthetic responses of one C (Panicum laxum L.) and two C 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 (P) 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 (0.6-2.6 mmol P m) than in the two C grasses (0.8-4.1 mmol P m), and P constituted ~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 (50%) than the C 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 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 but not the C plants. Thus, P deficiency reduced growth in parallel with reductions of photosynthesis in the C grass, and independently of photosynthesis in the two C grasses. We propose that this may be related to a greater P requirement of C relative to C photosynthesis. Photosynthetic P use efficiency was greater and increased more with P deficiency in the C relative to the C species. The opposite was observed for whole-plant P-use efficiency. Hence, the greater P-use efficiency of C 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 grasses.