不同施氮水平对巨桉幼树耐旱生理特征的影响

采用盆栽方法,研究了巨桉幼树在N0(不施氮)、N1(1.4g尿素·盆-1)、N2(2.8g尿素·盆-1)3个氮处理水平下,连续干旱不同时间分别停水0(D0)、3、6、9、12、15、18d]时巨桉的生理响应。结果显示:(1)除D0外,试验期内N1和N2处理的巨桉叶片含水量(LWC)、叶片相对含水量(LRWC)和叶片保水力(LWHC)基本低于N0水平,尤其在干旱中期最为明显,表明在干旱胁迫前施氮可能对巨桉叶片水分生理产生负面作用。(2)干旱初期,氮处理间的可溶性蛋白(SP)和可溶性糖(SS)含量的差异不大,而干旱处理后期(9~18d),N0处理的SP和SS较初期明显增加,但N1和N2处理相对于N0变化较为平缓,表明施氮不利于SP和SS积累;N1和N2处理下脯氨酸(Pro)含量的增幅随着干旱胁迫时间的延长明显大于N0处理。(3)随干旱时间延长干旱程度加重,N1、N2处理巨桉叶片过氧化氢(H2O2)和丙二醛(MDA)含量明显高于N0处理,表明施氮使得巨桉在干旱条件下水分缺乏更为严重,产生更多的活性氧(ROS)。(4)整个干旱处理期内,施氮并未显著改变巨桉的超氧化物歧化酶(SOD)活性和抗坏血酸(AsA)含量,但N1和N2的过氧化物酶(POD)活性明显高于N0。(5)施氮增加了巨桉叶片的色素含量并在干旱初期和中期保持较高水平,在干旱初期(0~3d)增加了巨桉叶片的净光合速率(Pn),但随着干旱时间的延长而迅速下降;施氮(N1、N2)的蒸腾速率(Tr)和气孔导度(Gs)在干旱初期均显著小于N0,但在干旱中后期(6d以后)各处理间差异不显著且均处于极低水平。研究表明,水分充足时施氮有助于增强巨桉的光合同化能力,促进其生长,但遇到持续干旱时施氮更易面临水分亏缺,降低其抵抗干旱的能力,因此在巨桉人工林的经营管理过程中,不应在干旱或季节性干旱即将到来之前施氮,若干旱过程中需施氮则应采取灌溉等途径保证其充足的水分供应。

Drought Tolerance Comparison of Eucalyptus grandis Saplings under Different Nitrogen Application Levels

A pot experiment was performed to study the drought resistance of Eucalyptus grandis Hill ex Maiden saplings under different nitrogen(N) application levels.The experiment consisted of two steps,firstly three N treatments (N₀,supplied no extra urea,and N₁ and N₂,supplied urea 1.4 and 2.8 g·pot^-1,respectively) were carried out in advance for 20 days and afterwards the drought stresscontinuous drought for 0 d(D₀),3,6,9,12,15 and 18 d)] was conducted respectively to each N treatment.The results showed that:(1)Under the drought condition,the leaf water content (LWC),leaf relative water content (LRWC) and leaf water retention (LWHC) of saplings in N₁ and N₂ were lower than those in N₀ level,and the differences in the middle stage of drought were most obvious,which indicated that nitrogen application before drought probably led to negative impact to leaf water physiology of E.grandis saplings.(2)As for the soluble protein (SP) and soluble sugar (SS),the differences between nitrogen treatments were not significant in the early days of drought,but when drought for 9 ~ 18 d,the increase of SP and SS in N₀ were more obvious than those in N₁ and N₂,and the change trend of those in N₁ and N₂ were relatively gentle,indicating that nitrogen addition was adverse to the accumulation of SP and SS of E.grandis saplings.However the increase rate of proline (Pro) in N₁ and N₂ was higher than that in N₀ with the extension of drought.(3)With the aggravation of drought stress,the contents of H₂O₂ and MDA in E.grandis saplings in treatment N₁ and N₂ were obviously higher than those in treatment N₀,which indicated that nitrogen addition brought about more serious water deficiency of E.grandis saplings and produced more reactive oxygen species (ROS).(4)Nitrogen addition did not significantly change SOD activity and AsA content of E.grandis saplings in the process of drought stress,but POD activity in N₁ and N₂ was obviously higher than that in N₀.(5)Nitrogen addition increased the pigment content of E.grandis saplings and kept at a higher level and correspondingly the net photosynthesis rate(P_n) was at a high level at the beginning of drought stress (0~3 d),but with drought time prolonged,P_n declined more rapidly,and change trend of transpiration rate (T_r) and Stomatal conductance (G_s) were nearly same as P_n.The results suggested that nitrogen addition could enhance the photosynthesis and assimilation ability of E.grandis when soil moisture is sufficient,and promote its growth.However it is more likely to encounter water deficit in face of drought stress and reduce its ability to drought resistance.Thus in the process of management of E.grandis plantation,nitrogen addition should not be implemented before the coming of drought or seasonal drought.Otherwise irrigation or some watering measures should be taken.