镁缺乏和过量胁迫对纽荷尔脐橙叶绿素荧光特性的影响

以2龄枳砧纽荷尔脐橙为材料,研究了镁缺乏和过量胁迫对叶片叶绿素含量和叶绿素荧光特性的影响。结果表明,镁缺乏胁迫导致老叶叶绿素含量显著降低,而新叶叶绿素含量无显著下降;镁过量胁迫显著抑制老叶叶绿素含量的下降,而促进了新叶叶绿素含量的降低。镁缺乏和过量均导致不同叶龄叶片的光化学效率(Fv/Fm)和相对电子传递速率(rETR)降低,但镁缺乏胁迫的影响显著大于镁过量胁迫。缺镁第4个月时,2龄秋梢、1龄春梢、1龄夏梢和晚夏梢叶的Fv/Fm分别比对照低了13.9%、12.6%、2.9%和2.0%,rETRmax值分别比对照低35.7%、56.2%、32.6%和15.2%;而镁过量胁迫叶片的Fv/Fm分别比对照低了0.5%、2.2%、3.4%和1.5%,rETRmax分别为对照的110.1%、68.8%、87.2%和84.5%。缺镁2龄秋梢、1龄春梢和1龄夏梢叶片的非光化学淬灭系数(NPQ)先升后降,热耗散能力显著下降,且显著低于镁过量胁迫。因此,在夏季高光照条件下缺镁胁迫对纽荷尔脐橙光合作用的影响显著,且大于镁过量胁迫,缺镁纽荷尔脐橙叶片易发生光抑制,产生光伤害。

Influence of magnesium deficiency and excess on chlorophyll fluorescence characteristics of Newhall navel orange leaves

To investigate the effect of magnesium (Mg) deficiency and excess on chlorophyll content and chlorophyll fluorescence characteristics of leaves, 2-year-old trees of Newhall navel orange (Citrus sinensis Osbeck) grafted on trifoliate orange (Poncirus trifoliata (L.) Raf.) rootstocks were transferred to pots containing a mixture of quartz sand-perlite (1:1, v/v) and irrigated with modified Hoagland-Aron nutrient solution containing three Mg concentrations: 0.1 mmol/L (Mg-deficiency), 1 mmol/L (control) or 10 mmol/L MgSO₄(Mg-excess). The relative chlorophyll content and chlorophyll fluorescence characteristics of leaves on different shoots were measured regularly in the three treatments. A portable chlorophyll meter (SPAD-502) was used to test the relative chlorophyll content of leaves. Simultaneously, the chlorophyll fluorescence characteristics of leaves were determined in situ by a pulse-amplitude modulated (PAM-2500) fluorometer. Results showed that Mg deficiency led to a more significant decline in chlorophyll content of 2-year-old autumn leaves than did Mg excess, whereas the latter caused a greater decline of 1-year-old leaves than did the former. In the 4^th month after Mg stress treatment, SPAD readings of leaves on 2-year-old autumn, 1-year-old spring, 1-year-old summer and 1-year-old late summer shoots in the Mg-deficient group decreased by 12.3% (P < 0.05), 10.5% (P < 0.05), 4.0% (P > 0.05) and 4.0% (P > 0.05), respectively, compared with the control. Those in the Mg-excess group were 107.3% (P < 0.05), 95.5% (P > 0.05), 88.9% (P < 0.05) and 92.5% (P < 0.05) of the control group, respectively. Moreover, both Mg-deficiency and excess reduced maximum quantum efficiency (F_v/F_m) and relative electron transport rate (rETR) of leaves on different shoots, with the impact of the former greater than that of the latter. In the 4^th month, compared with the control, F_v/F_m of 2-year-old autumn, 1-year-old spring, 1-year-old summer and 1year-old late summer leaves in the Mg-deficient group decreased by 13.9% (P < 0.05), 12.6% (P < 0.05), 2.9% (P > 0.05) and 2.0% (P > 0.05), respectively. Correspondingly, there was a decline of 0.5% (P > 0.05), 2.2% (P > 0.05), 3.4% (P > 0.05) and 1.5% (P > 0.05), respectively, in the Mg-excess group. Further investigation showed that the rETR_max of these leaves decreased by 35.7% (P < 0.05), 56.2% (P < 0.05), 32.6% (P < 0.05) and 15.2% (P < 0.05) in the Mg-deficient group, respectively, which corresponded to 110.1% (P < 0.05), 68.8% (P < 0.05), 87.2% (P < 0.05) and 84.5% (P < 0.05) of the control in the Mg-excess group. In addition, non-photochemical quenching (NPQ) of all leaves in the Mg-deficient group increased in the first 3 months and then declined dramatically in the 4^th month, although this was not as marked as in the Mg-excess group. In the 4^th month, the maximum NPQ of 2-year-old autumn, 1-year-old spring, 1-year-old summer and 1-year-old late summer leaves in the Mg-deficient group was lower than those in control, with a drop of 52.8% (P < 0.05), 26.6% (P < 0.05), 19.8% (P < 0.05) and 0.6% (P > 0.05), respectively. In the Mg-excess group, the peak value of NPQ of those leaves was 94.9% (P > 0.05), 110.3% (P > 0.05), 101.3% (P > 0.05) and 104.5% (P > 0.05) of the control group, respectively. Therefore, Mg-deficient plants exposed to high light intensity in summer would be subject to a reduction in photosynthetic capacity more easily than would Mg-excess plants. This indicated that Mg-deficient leaves would be susceptible to photoinhibition and photooxidation.