Effects of phosphorus on activities of enzymes related to nitrogen metabolism in flag leaves and protein contents in grains of wheat

Nitrogen metabolism in leaves has important function on protein accumulation in wheat grains,involving three key enzymes,i.e.nitrate reductase(NR),glutamine synthetase(GS)and endopeptidase(EP).It has been well documented the changes of related enzymes in nitrogen metabolism under different rates of nitrogen and potassium fertilizer.However,little research has been done about the effect of phosphorus(P)on the contents of protein components in wheat.The objectives of the present research were to investigate the effect of P on the activities of three enzymes concerned with nitrogen metabolism in flag leaves and protein accumulation as well as its component contents in wheat grains.The study was carried out from 2001 to 2002 with two wheat cultivars(Triticum aestivum L.cv.Lumai 22 and cv.Jinan 17)in field.Three P rates(P0,P1 and P2)were set in the experiment,and the amounts of P application were 0 kg,105 kg and 210 kg P_2O_5 per ha,respectively.The results showed that the activities of NR,GS and EP in flag leaves were increased with the increase of P application during the early and middle grain filling stages(Fig.13),and the effect of P application for Jinan 17 was greater than that for Lumai 22,but there was no significant difference between P1 and P2 treatment.The contents of soluble protein and free amino acid in flag leaves were increased by P application before 14 d and 7 d after anthesis respectively(Fig.4 and Fig.5).And P1 treatment was of benefit to the accumulation of free amino acid during early grain filling stage and the redistribution to grains during the whole grain filling stage,whereas P2 treatment was less effect on the redistribution.It indicated that the synthesis and accumulation of protein in grains were increased by P application during grain filling stage,especially in the early stage.The protein content in P1 and P2 was significantly higher than that in P0 during the early grain filling.When maturing,P1 treatment had the highest total protein content in both cultivars(Fig.6).In P1 treatment,there were little effect on the contents of albumin and globulin,and significant increase for the contents of gliadin and gluten in the two cultivars,which was greater for Lumai 22 than for Jinan 17(Table 1).These results suggested that P1 treatment had little effect on wheat nutritional quality,but could improve its processing quality.However,excessive P application could deteriorate the processing quality as for Jinan 17.Therefore,different cultivation management should be adopted according to the different quality demans of different wheat types.