Manganese efficiency of bread (Triticum aestivum) and durum (Triticum durum) wheat cultivars grown in a low Mn soil

Bread wheat cultivars are usually more manganese (Mn) efficient than durum wheat cultivars when grown in low Mn soils. In order to test if differences in Mn efficiency are due to differences in uptake efficiency and whether this would be due to differences in the size of the root system, the Mn net influx (In), uptake kinetics, and/or Mn insolubilization/solubilization in the rhizosphere. Two bread (Triticum aestivum L. cvs. PBW 343 and PBW 502) and two durum (Triticum durum L. cvs. PDW 274 and PDW 233) wheat cultivars were grown in a Mn-deficient soil (DTPA-extractable Mn 3.5 mg (kg soil)-1 and a Mn soil solution concentration of 0.037 nmol ml-1) at two Mn levels (unfertilized (Mn-0) and fertilized with 1 mmol Mn (kg soil)-1 (Mn-1) as MnSO4<middle dot>H2O). To determine the Mn In two harvests were made, the first harvest 14 days and the second 22 days after sowing. At the second harvest shoot Mn concentration was 7 mg per kilogram dry matter (kg DM) in durum and 11 mg (kg DM)-1 in bread wheat cultivars indicating a clear suboptimal soil Mn supply. Shoot dry weight (SDW) of durum wheat was only 30 to 40% of that of bread wheat, showing that durum wheat cultivars were less Mn efficient than bread wheat cultivars and this was because of a lower uptake efficiency. The lower Mn uptake efficiency of durum wheat was because of a smaller Mn influx, which was only 20 to 25% of that of bread wheat while all cultivars had a similar root surface area per unit of shoot. Manganese concentration in soil solution was 0.037 nmol ml-1 in the unplanted unfertilized soil and plant growth reduced it to about 0.030 nmol ml-1 not because of Mn uptake but because of Mn insolubilization. To study Mn uptake kinetics the same four wheat cultivars were grown in nutrient solution with a Mn concentration varying from 0.037 to 2.7 nmol ml-1. At the lowest Mn concentration maximum or close to maximum yield with shoot Mn concentration of 33 mg (kg DM)-1 for durum 22 mg (kg DM)-1 for bread wheat were obtained. The uptake isotherm, following a Michaelis-Menten function, was similar for both wheat species. Therefore, the higher Mn In of bread wheat in soil was not because of a more efficient uptake physiology. Measurement in the bulk soil indicated a Mn insolubilization by plant growth which was confirmed by model calculations using uptake kinetics and ion transport in soil. The calculations showed that both wheat species apparently insolubilized Mn in the rhizosphere. The Mn insolubilization was stronger in durum than in bread wheat and probably caused its lower Mn In.