Abstract: 【Objective】The purpose of this experiment was to investigate the effects of exogenous cytokinin (6-BA) and nitrogen application with varied rates on photosynthetic characteristics of wheat post anthesis and provide a theoretical basis for enhancing photosynthetic productivity of wheat. 【Method】The experiment was conducted using a stay-green cultivar (Wennong 6) and a non-stay-green cultivar (Jimai 20) with three nitrogen rates, namely, N0(0), N1(240 kg·hm^-2) and N2(360 kg·hm^-2). Exogenous 6-benzyladenine (6-BA) (25 mg·L^-1) and lovastatin (300 mg·L^-1) were sprayed on the leaves at a rate of 100 mL·m^-2 for 3 days after anthesis (DAA). Then chlorophyll content, MDA content and antioxidant enzyme activity were determined using flag leaf samples. The content of four kinds of endogenous hormones was determined using high performance liquid chromatography method (HPLC). The chlorophyll fluorescence kinetics parameters of flag leaves induced by different treatments were determined using pulse modulation type fluorometer. 【Result】Spraying exogenous 6-BA significantly enhanced the max photochemical efficiency (Fv/Fm), actual photochemical efficiency (ΦPSII), photosynthetic electron transport rate (ETR) and photochemical quenching coefficient (qP) of flag leaves of two wheat cultivars at different growth stages post anthesis. The chlorophyll fluorescent parameters were significantly decreased by spraying lovastatin. It was found that under N0 treatment, spraying exogenous 6-BA increased ΦPSII of flag leaves of Jimai 20 by 12.08%, 6-BA combined with N1 treatment and N2 treatment increased ΦPSII by 14.21% and 9.43%, respectively. ΦPSII of flag leaves of Wennong 6 was increased by 12.44%, 14.84% and 11.58%, respectively, in above three treatments. In addition, in N0 treatment, spraying exogenous 6-BA enhanced photosynthetic electron transport rate (ETR) of flag leaves in Jimai 20 by 16.57%, 6-BA combined with N1 treatment and N2 treatment enhanced ETR by 25.81% and 18.83%, respectively. ETR of flag leaves of Wennong 6 was increased by 13.88%, 23.58% and 22.80%, respectively in above three treatments. Other indicators of flag leaves in both wheat cultivars showed the following rule that in 6-BA combined with N1 treatment, fluorescence parameters of flag leaves were increased more than 6-BA combined with N0 treatment or 6-BA combined with N2 treatment. Meanwhile, the cultivar, nitrogen fertilizer, single hormone effect, hormone combined with nitrogen fertilizer affected ΦPSII, ETR and qP significantly. In addition, Fv/Fm was affected significantly by cultivar, hormone but not affected by combined application of nitrogen fertilizer and hormone significantly. Besides, in N0 treatment, foliar application of the inhibitor of cytokinin lovastatin reduced ETR of flag leaves of Jimai 20 by 22.71%, lovastatin combined with N1 treatment and N2 treatment decreased ETR by 12.06% and 11.92%, respectively. Other fluorescence parameters including Fv/Fm, ΦPSII and qP of the two cultivars showed a downward trend. Increasing nitrogen application rate could slow down the decrease of fluorescence parameters of both wheat cultivars caused by the reduction of the synthesis of cytokinin. Exogenous 6-BA had significant effects on the content of endogenous hormones in two cultivars of wheat. Spraying exogenous 6-BA significantly increased the zeatin riboside (ZR) content and the auxin (IAA) content of flag leaves post anthesis in both cultivars. The gibberellins (GA3) content was significantly increased at 21-28 DAA. On the contrary, abscisic acid (ABA) was significantly decreased after spraying exogenous 6-BA. After spraying exogenous lovastatin, changes of endogenous hormones contents were on the contrary with the above results. With the increase of nitrogen application rate, the ZR and IAA content was enhanced at 21-28 DAA. The overall trend of the ABA content was decreased with the increase of nitrogen rate. In N1 treatment, the GA3 content was higher than that under the other two nitrogen rates at 7-14 DAA. Meanwhile, the ZR content was significantly affected by the cultivar, nitrogen fertilizer, single hormone effect, combined application of nitrogen fertilizer and hormone. In addition, in 6-BA combined with N1 treatment, chlorophyll content and antioxidant enzyme system activity of flag leaves were increased more than 6-BA combined with N0 treatment or 6-BA combined with N2 treatment. Combined application of nitrogen fertilizer and hormone had significant effects on chlorophyll content and antioxidant enzyme system activity. Lovastatin combined with N0 treatment reduced chlorophyll content and antioxidant enzymes activity more than that lovastatin combined with N1 and N2 treatments. This shows that increasing nitrogen fertilizer application could mitigate the senescence of the photosynthetic structure. Spraying 6-BA significantly affected the grain weight and yield (P<0.01), and had no significant effect on the spike number and grain number, and combined application of nitrogen fertilizer and exogenous 6-BA significantly affected the number of spike, grain number per spike, 1000-grain weight and yield. 【Conclusion】Obvious combined effects between nitrogen fertilizer and cytokinin exists, under 6-BA combined with appropriate nitrogen rate, the photosynthetic performance of wheat was improved higher than 6-BA combined with no N application or 6-BA combined with N2. The improvement of photosynthetic performance significantly improved grain yield.

Key words: stay-green wheat, hormone, nitrogen, photosynthetic characteristics, chlorophyll fluorescence