Leaf N And P Resorption Characteristics In Lucerne Of Different Ages And Cultivars In The Rain-fed Region Of Eastern Gansu,China

Leaf nutrient resorption is a process that some nutrients can be removed from the senescing leaves and transported to other parts,which has been well developed as one of the most important strategies to enhance nutrient use efficiency and environmental adaptability of a plant in adverse conditions,especially playing a significant role in responding to drought stress and soil infertility etc.Lucerne(Medicago sativa)is one of the most important crops in the rainfed region.It has strong adaptability to drought and soil infertility,and strong ability of biological nitrogen fixation.However,the imbalanced rainfall and soil infertility impose heavy impacts on the efficient and sustainable use of lucerne in the rainfed region of eastern Gansu,China.Further elucidation of lucerne adaptation in this region plays a vital role in promoting the use and extension of lucerne.Leaf nutrient resorption is one of the key mechanisms that a plant adapts to adverse environments.However,there was rare knowledge on changes in leaf nutrient resorption under different stand ages and cultivars.Lucerne of different ages(2,4,7,11 and 14 y since establishment)and cultivars(M.sativa cv.Longdong,M.sativa cv.Algonquin and M.sativa cv.Xingjiangdaye)were used to analyze the characteristics of nitrogen and phosphorus resorptions and their regulation mechanisms.1)The age of lucerne stands imposed significant impacts on leaf nitrogen and phosphorus resorptions(P<0.05).With the increase in stand age,nitrogen resorption efficiency(NRE)first increased and then decreased,while nutrient resorption proficiency(NRP)tended to first decrease and then increse,and finally decrease.Phosphorus resorption efficiency(PRE)varied in the same way as NRE,while phosphorus resorption proficiency(PRP)decreased gradually with sand age.Generally,the resorption efficiencies and proficiencies of nitrogen and phosphorus in differently aged lucerne stands were significantly different between cuts and they were higher at the first cut than the second cut(P<0.05).2)There were few significant differences in nitrogen and phosphorus resorptions observed among cultivars(P<0.05).The NRE of Xinjiangdaye in the first cut at age 1 was lower than those of Algonquin and Longdong,and the NRE of Longdong was lower than those of the other two in the second cut of age 1 and in the first cut of age 2(P<0.05).The NRP of Xinjiangdaye in the first cut at age 1 was lower than that of Longdong,while it was opposite in the first cut of age 2(P<0.05).The PRE of Longdong in the first cut at age 2 was lower than those of Algonquin and Xinjiangdaye,and the PRE of Algonquin was higer than that of Xinjiangdaye(P<0.05).While the PRP of Longdong was lower than that of Algonquin in the second cut of age 2(P<0.05).Leaf nitrogen and phosphorus resorptions of lucerne at age 1 were lower than those at age 2,and it was higher in the first cut than the second cut(P<0.05).3)Leaf nitrogen and phosphorus resorptions were significantly influenced by nitrogen and phosphorus concentrations in leaves(P<0.05).Generally,The NRE and PRE were positively correlated with nitrogen and phosphorus concentrations in green mature leaves,while negatively correlated to the concentrations in senesced leaves(P<0.05).The NRP and PRP were negatively regulated by the concentrations of nitrogen and phosphorus in green mature leaves.4)Soil nutrient availability had a effect on leaf nitrogen and phosphorus resorptions.Only in the first cut of Longdong,nitrate nitrogen imposed negative regulation on NRE and NRP,while ammonium nitrogen showed positive effects.Available phosphorus negatively affected PRE and PRP.5)The BNF also showed effects on leaf nitrogen and phosphorus resorptions.In this study,the rate of BNF of differently aged lucerne stands was avergely 51% in the first cut.The NRE and PRE were negatively correlated with BNF in this cut,but the NRP and PRP was rarely affected by BNF(P<0.05).The rate of BNF was 34% in the second cut,which showed weak correlations with the resorptions.In addition,there was no solid correlation of nitrogen and phosphorus resorptions with BNF observed with different cultivars.In summary,the intraspecific genetic differences among cultivars have little effects on leaf nitrogen and phosphorus resorptions,but there were significant differences among cuts and ages of lucerne stands(P<0.05).