不同供镁水平对水稻叶片叶绿素荧光特性和能量耗散的影响

以高产杂交稻汕优63为材料,研究了不同供镁(Mg)水平对水稻叶片叶绿素荧光特性和抗氧化酶类活性的影响。结果表明,缺Mg使水稻叶片叶绿素含量降低,光合电子传递速率下降、CO2同化受抑制而导致Fv／Fm、PSⅡ等下降、净光合速率降低,加重了叶片受到光抑制的程度。尽管缺Mg叶片所捕获和传递到PSⅡ反应中心的光能减少,但是在强光下仍然积累了大量过剩激发能。在13:00时,对照叶片的qP下降,qN升高,过剩的激发能通过非光化学猝灭途径耗散掉以避免光合机构受到伤害,而缺Mg叶片qP下降的同时qN也显著下降,这可能是由于缺Mg使叶黄素循环受阻而阻碍了过剩激发能的耗散。缺Mg叶片中没有能够通过能量耗散系统及时耗散掉的过剩激发能导致了活性氧自由基的生成比例增加,诱导了抗氧化酶类活性不同程度的增加。大量活性氧自由基不能被抗氧化酶类及时清除掉,积累到一定程度引起了膜脂过氧化加剧而使叶绿体结构受到损伤,最终使得叶片失绿坏死。

Effects of different magnesium nutrition levels on chlorophyll fluorescence characteristics and excitation energy dissipation in rice leaves

As an essential element, magnesium (Mg) plays a key role in plant development. Some reports have showed that Mg-deficiency leads to the decline of the rate of photosynthesis (Pn) followed by the decrease in the yield of crop. In order to detect physiological mechanism for lowering Pn by Mg-deficiency, the diurnal changes in gas exchange and chlorophyll fluorescence parameters in the fully expanded 2nd top leaves in a hybrid rice Shanyou 63, which grow in three nutrient solutions containing essential elements with 0 mmol/L Mg²⁺ (deficient) ,1.65 mmol/L Mg²⁺ (control) and 8.25 mmol/L Mg²⁺ (high) respectively, were characterized in this experiment with a portable photosynthesis system (LICOR-6400, USA) during a sunny day with significantly variable photon flux density (PFD) from about 100 μmol/(m²·s) at 6:00 a.m. and 18:00 p.m. to 1400 μmol/(m²·s) at noon. Antioxidant enzymes activities and contents of photosynthet ic pigments, soluble proteins and malonyldialdehyde (MDA) from the same leaves were also determined at same time. The results showed that, compared with the control plants, markedly lowered Pn indicated that an aggravated photoinhibi tion occurred in Mg-deficient plants. Chlorophyll content, electron transport rate (ETR) and CO₂ assimilation were suppressed , the three major processes of photosynthetic reaction, which are primary reaction, proton electron transport and CO₂ assimilation respectively, are all inhibited under Mg-deficiency conditions. At 13:00, qP decreased and qN increased in control plants, indicating that non-photochemical quenching dissipates excessive excitation energy to protect photosynthetic apparatus from high light damage. On the contrary, both qP and qN decreased in magnesium deficientplants, showing that Mg deficiency impairs excessive excitation energy dissipation. Since xanthin cycle which is one of the important channels of excess excitation dissipation dependents on pH gradient across thylakoid membrane and Mg makes great contribution to the formation of thylakoid-across pH gradient, it suggests that Mg-deficiency suppresses xanthin cycle to block the increase of qN. Judging from the lowering of F₀, F_m, F₀' , F_m' , F_s and F_v ' /F_m' of Mg deficient plants and the rising in SOD, POD and CAT activities as well MDA contents, we suggest that, though the light energy harvesting and transferring to PSⅡ reaction center has decreased for rice plant subjected to magnesium deficiency, excessive excitation energy is still accumulated under high light conditions, which triggers the overproduction of reaction oxygen species and induces the increase of activities of antioxidant enzymes. Large quantities of free radicals not to be scavenged by antioxidant enzymes lead to the aggravation of membrane lipid peroxidation to the extent that the leaves loss green even died. The results also showed no significant difference in all parameters tested from five-fold controlled Mg²⁺ content treatment.