温度和供氮形态对龙眼吸收氮素动力学的影响

Nitrogen Uptake Dynamics for Longan as Affected by Temperature and Nitrogen Form

研究龙眼幼苗在不同温度(10、15、20、25、30、35℃)和不同氮营养(硝态氮、3/4硝态氮+1/4铵态氮、1/2硝态氮+1/2铵态氮、1/4硝态氮+3/4铵态氮、铵态氮)供应条件下吸收氮素的动力学特征,比较龙眼吸氮能力变化,并探讨龙眼对氮形态的偏好性,为龙眼不同季节(物候期)选择适用氮肥形态提供依据。结果显示,温度和供氮形态对龙眼吸氮能力有显著影响(P<0.05)。在中低温度(10~25℃)等量供应硝态氮和铵态氮条件下,龙眼根系吸收氮素(硝态氮和铵态氮之和)的最大吸收速率(I_(max))最高或较高,而亲和力(Am)和离子吸收补偿点(C_(min))则随温度和氮形态变化不大,故此时龙眼吸氮能力强。在高温(30℃)且仅供应铵态氮时,根系具有最高I_(max)和A_(m),同时C_(min)最低,最有利于龙眼吸氮。在极端高温(35℃)时将铵态氮和硝态氮以3∶1的比例配合供应,则I_(max)最高,A_(m)较高且C_(min)较低,有利龙眼对氮素的吸收。另外,等量硝态氮和铵态氮共存条件下,龙眼在低温(10~15℃)时吸收铵态氮的I_(max)更高、A_(m)更强而且C_(min)更低,明显偏好铵态氮;在中高温(20~35℃)时吸收硝态氮的I_(max)更高,但A_(m)更弱且C_(min)更高,对氮形态的偏好性不明显。对照华南龙眼生长物候期,建议在龙眼秋梢生长期施用铵态氮为主、硝态氮为辅;在花前至小果期可以1∶1的比例施用硝态氮和铵态氮;在果实膨大期,可全部施用铵态氮或以3∶1的比例施入铵态氮和硝态氮。

Longan(Dimocarpus longan Lour.),a perennial fruit tree grown in tropical and subtropical areas,is subjected to great variation of temperature during a growth period.Temperature mediates the capability of plant root to absorb nutrients from the growth media.Generally,nitrogen(N)is the most-demanded element for most of the higher plants,and preference to N form is normally observed.Herein,N adsorption experiment with longan seedlings was conducted to investigate nitrogen(N)uptake dynamics affected by temperature(10,15,20,25,30 and 35℃)and N form(nitrate,3/4 nitrate+1/4 ammonium,1/2 nitrate+1/2 ammonium,1/4 nitrate+3/4 ammonium,and ammonium),with the objective to select suitable N source for longan growth with the seasons(phenophases).The N consumption formulas in nutrient solutions were fitted to calculate N uptake dynamics by longan roots,and N absorption capability was compared as temperature and N form varied.Further,the preference to N form for longan was examined as well.N uptake by longan was significantly influenced by both temperature and N form(P<0.05).When equal amount of nitrate and ammonium was supplied at low to medium temperature(10-25℃),longan roots took up N(the sum of nitrate and ammonium)with the highest or higher maximum adsorption rate(I_(max)),however,the affinity to N(A_(m))for longan roots and the ion adsorption compensation point(C_(min))did not vary considerably,leading to a powerful adsorptive capacity for N by longan roots.While the temperature rose to high level(30℃)and ammonium was solely applied,the peak values of both I_(max)and A_(m)and the bottom C_(min)were simultaneously observed,which facilitated N uptake by longan roots.If ammonium and nitrate were amended with the ratio of 3:1 at extremely high temperature(35℃),longan roots took up more N with the upmost I_(max),higher A_(m)and lower C_(min).In addition,while the same levels of nitrate and ammonium were fed,longan roots took up ammonium with higher I_(max),stronger A_(m)and lower C_(min),suggesting preference to ammonium at low temperature(10?15℃).However,the root adsorbed nitrate with higher I_(max),but weaker A_(m)and higher C_(min)at medium to high temperature(20?35℃),indicating no preference to N form.Based on the phenophase of longan grown in South China,it is suggested that N source with the majority of ammonium and minor nitrate facilitates N uptake by longan during the growth of the autumn shoot.Nitrate and ammonium are recommended to be used with the ratio of 1∶1 to enhance N adsorption during the period before anthesis to fruitlet development.Ammonium or the combination of ammonium and nitrate with the ratio of 3∶1 is proposed during fruit swelling stage in longan.