Photosynthetic characteristics and antioxidative metabolism of flag leaves in responses to nitrogen application in wheat during grain filling

Chinese. 小麦灌浆过程中旗叶光合、抗氧化代谢与氮素营养密切相关，本研究通过田间试验，以进一步揭示其关系。试验选用多穗中粒（MM）型品种藁城8901（GC8901）和大穗大粒（LL）型品种山农1391（SN1391），设12 g/N•m2（LN）和24 g/N•m2（HN）2个氮肥处理，在田间条件下研究了小麦灌浆期旗叶光合色素含量、氮素含量、光合特性和抗衰老酶活性对氮素水平的响应。小麦开花后旗叶氮素含量与叶绿素含量呈下降趋势，净光合速率（Pn）、实际光化学效率（ΦPSⅡ）、开放的光系统Ⅱ反应中心的量子效率（Fv′/Fm′）和光化学猝灭系数（qp）在花后14 d左右开始下降，光系统Ⅱ的最大光化学效率直到衰老末期才有小幅下降，而非光化学猝灭系数（NPQ）呈上升趋势。旗叶光能的吸收和利用之间存在矛盾，这是促其衰老的重要原因。与MM型小麦品种GC8901相比，LL型品种SN1391群体较小，籽粒灌浆后期旗叶的光合色素含量高，光系统Ⅱ的活性下降慢，净光合速率高；同时，抗衰老酶活性下降慢，膜脂过氧化程度轻。氮素水平对2个品种籽粒灌浆过程中旗叶的光合及抗氧化代谢的影响不同。LL型品种SN1391表现为，增施氮肥后旗叶光系统Ⅱ的开放程度和实际光化学效率提高，热耗散降低，净光合速率提高；旗叶的SOD、POD、CAT和APX活性均提高，膜脂过氧化程度降低。增施氮肥后MM型品种GC8901光系统Ⅱ活性下降，用于热耗散的能量增加，光合速率降低；抗氧化酶系活性下降，膜脂过氧化程度加重。小麦生育后期旗叶光能吸收和利用间的矛盾是促其衰老的重要原因。不同类型小麦品种旗叶生育后期的光合及抗氧化代谢存在显著差异。施用氮肥对小麦旗叶光合、衰老特性的影响因品种类型而不同。

English. The purpose of this study was to reveal the relationship of photosynthetic characteristics and antioxidative metabolism to nitrogen nutrition in flag leaves of field-grown wheat during grain filling. A two factorial experiment was conducted with two wheat cultivars (SN1391, a large-spike and large grain cultivar and GC8901, a multiple-spike and medium grain cultivar) and two nitrogen application rates (12 and 24 g/N•m2). Four-treatment combinations was arranged in a completely randomized block design with three replicates. The results showed that the nitrogen (N) content and chlorophyll content of wheat flag leaves decreased after anthesis. The photosynthetic rate, effective quantum yield of PSⅡ (ΦPSⅡ), efficiency of excitation capture by open PSⅡ reaction centers (Fv′ / Fm′) and photochemical quenching coefficient (qp) began to decrease on 14 days after anthesis, however, the maximal efficiency of PSⅡ photochemistry (Fv/Fm) decreased slightly until the late period of senescence and the non-photochemical quencing coefficient (NPQ) increased during flag leaves senescence. So, a conflict existed between absorption and utilization of light energy in flag leaves during senescence, which might accelerate the senescence of flag leaves. The lower plant population during grain filling of SN1391, as compared with GC8901, was helpful to leaves during senescence to maintain the higher content of photosynthetic pigment, higher activity of PSⅡ and, in consequence, higher Pn. The delayed decrease in antioxidative enzymes activity and the lower degree of membrane lipid peroxidation in the senescing leaves of SN1391 were beneficial to protecting the photosynthetic apparatus, leading to the prolonged duration of CO2 assimilation. With the increase of N application, the chlorophyll content of SN1391 flag leaves and the efficiency of excitation captured by open PSII centers increased. At the same time, the thermal dissipation in SN1391 flag leaves of high N (HN) treatment decreased, but it was improved greatly, which was favorable to the increase of photosynthetic rate (Pn). The SOD, POD, CAT and APX activities in the flag leaves of SN1391 increased markedly in HN treatment, so that it could clean more active oxygen and decreased the degree of membrane lipid peroxidation. In this way, the ability of SN1391 to protect photosynthetic apparatus was improved in HN treatment. In the HN treatment, the decreased activity of PSⅡ and the increased thermal dissipation resulted in the decline of Pn in flag leaves of GC8901. At the same time, the decreased antioxidative enzymes activity and the increased degree of membrane lipid peroxidation had indirectly made unfavorable influence on CO2 assimilation. The results here suggested that the conflict between absorption and utilization of light energy in senescing leaves was an importment reason which induced and accelerated the senescence of wheat leaves, the photosynthetic characteristics and antioxidative metabolism of flag leaves during grain filling were markedly different among wheat cultivars, and the effects of nitrogen rate on the photosynthetic and senescent characteristics of flag leaves varied with wheat cultivars.