LAUSR

Plant species vary in their phosphorous (P) use efficiency under suboptimal P supplies using different strategies, but the mechanisms are not clearly documented for some alternative plant species. Safflower was considered as low input oil crop, but its P uptake efficiency mechanism was not fully investigated. Therefore P uptake efficiency of safflower was studied as compared to sunflower under semi-controlled conditions in sandy and loamy soils. Both species responded strongly to increasing P supplies in both soils and performed better in loamy soil. Both species had similar agronomic P efficiency in both soils, indicated by similar external P requirement under P-deficient conditions. Under P deficiency, safflower had less relative shoot and root production when they were grown in sandy soils and the opposite was found in terms of loamy soil. Safflower had the disadvantage of less root length and root shoot ratio in both soils under low and high P supplies but had the advantage of higher specific root density, less root radius, and slower shoot growth rate. Under P deficiency in both soil types, both species responded similarly in terms of P influx, depleting P from soil solution and P concentration in shoots. Under high P supply, P influx and P concentration in shoots was less in safflower in both soil types. Safflower was characterized by higher shoot demand on roots for P under low and high P supplies in both soil types. Therefore the cause of high root demand on P in safflower roots at low and high P supplies stems from the low root shoot ratio of safflower at both P supplies, low P concentration and low P influx of safflower at high P supplies not because of higher shoot growth rate of safflower at low and high P supplies. Therefore using different measures of utilization efficiency parameters to differentiate plant species and genotypes to superior and inferior could be in some cases misleading.