Abstract: Chrysanthemum is a typical short−day plant, which blossoms only when the sunshine length is shorter than the critical day length, and the critical day length is 12 h·d−1. In order to meet the market demand and promote the chrysanthemum to bloom in the long sunshine season, black shading materials are often used to shorten the day. There were many studies on short−day treatment to control the flowering period of chrysanthemum at home and abroad, but they failed to solve the problem of high temperature obstacle of willow buds in chrysanthemum during shading in summer. The appearance of willow buds indicates that chrysanthemum is still in the vegetative growth stage, and the process of flower bud differentiation is hindered. Continuous high temperature environment is an important factor leading to the emergence of willow buds in chrysanthemum. In this experiment, chrysanthemum variety "Hongmian" was used as the test material. The photoperiod experiment was carried out at high temperature of (32±2)℃/(22±2)℃(day/night), and the photoperiod duration was set as 7h/17h(Ph7), 8h/16h(Ph8) , 9h/15h(Ph9), 10h/14h(Ph10) and 11h/13h(Ph11), respectively with 13h/11h(CK) as control. The experiment began on July 20, 2019, and ended on August 25, 2019 when willow buds appeared in chrysanthemum seedlings. The photosynthetic structure of chrysanthemum leaves is very sensitive to adversity, which is the primary site of adversity damage. The light response curve, photosynthetic pigment content (including chlorophyll a, chlorophyll b, carotenoid and chlorophyll total) and rapid fluorescence induction kinetics curves of leaves were measured and analyzed before the formation of chrysanthemum willow buds. The curves of chlorophyll fluorescence kinetics OJ, OI, OK and IP phases were standardized as relatively variable fluorescence W, WOJ=(Ft−F0)/(FJ−F0), WOI=(Ft−F0)/(FI−F0), WOK=(Ft−F0)/ (FK−F0), WIP= (Ft−FI)/(FP−FI), and the fluorescence differential kinetics ΔW was calculated, ΔW=W−Wref, where Wref is the relatively variable fluorescence at the corresponding time of CK. That is, ΔWOJ=WOJ−Wref, ΔWOI=WOI−Wref, ΔWOK=WOK−Wref, ΔWIP=WIP−Wref, in order to understand the absorption and utilization of light energy by different photoperiod systems of chrysanthemum at high temperature in the process of photoreaction. By analyzing photosynthetic rate and the operation of photosynthetic electron transfer chain in photosynthetic structure (PSⅡ and PSⅠ), it is expected to provide scientific reference for the diagnosis and analysis of leaf photosynthesis and the study of photosynthetic performance when chrysanthemum is unable to differentiate normally. The results showed that: (1) the content of photosynthetic pigment was the lowest at ph7 and Ph8, and the reduction of NADP+ in the reaction center of photosystem II(PS II), the oxygen-releasing complex and the terminal electron acceptor bank of photosystem I(PS I) was slightly eased in the middle of the 26−day experiment, but it was inhibited at other times, and the photosynthetic capacity was the worst correspondingly. (2)The abnormal differentiation of willow bud inflorescence occurred at Ph10. The photosynthetic potential of Ph10 is great, but the oxygen-releasing complex of PSII is always inactive, and the photosynthesis changes with the strength and decline of the energy connection between PSII photosynthetic units. (3)The photosynthetic pigment of Ph11 leaves is the maximum after CK, and its photosynthetic performance is relatively stable. The continuously enhanced PSI and PSII activities make photoelectrons transfer normally under the premise of inactivation of oxygen-releasing complex. The photosynthetic system of chrysanthemum leaves treated with Ph7 and Ph8 was the most seriously damaged at high temperature. The photosynthetic system of chrysanthemum leaves treated with Ph10 was more sensitive, and the photosynthetic system of chrysanthemum leaves treated with Ph11 had stronger stress resistance.

Key words: Chrysanthemum, Photoperiod, Photosynthetic system, Photosynthetic electron transfer, Photosynthesis