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Classification and characteristics of heat tolerance in Ageratina adenophora populations using fast chlorophyll a fluorescence rise O-J-I-P

Classification and characteristics of heat tolerance in Ageratina adenophora populations using fast chlorophyll a fluorescence rise O-J-I-P
Shiguo Chena,*, Juan Yanga, Mansong Zhanga, Reto J?rg Strassera,b, Sheng Qianga,*
aWeed Research Laboratory, Nanjing Agricultural University, Nanjing 210095, China
b Bioenergetics Laboratory, University of Geneva, CH-1254 Jussy/Geneva, Switzerland

Abstract
Croftonweed originated fromMexico is a worldwide notorious invasive weed. The objectives of this study
were to screen heat tolerance in different croftonweed populations, determine the effect of heat stress on
two photosystems and probe the mechanism of acquired heat tolerance. According to conventional tests
of plant injury and fast chlorophyll fluorescence rise kinetics, four different croftonweed populations
collected from South Chinawere successfully classified into three categories by exposing whole plants to
heat treatment at 40 C: sensitive, intermediate, tolerant. Evidence from the JIP-test indicated that
inhibition of the oxygen evolution complexes (OEC) and inactivation of PSII reaction centers (RCs) were
the primary cause of heat damage. In mild heat stress (<40 C), slightly damaged the OEC without
creating a visible K-step in the fluorescence rise OJIP curve. In moderate heat stress or stronger (40 C), a
pronounced K-step due to irreversible severe damage on the OEC occurred. Additionally, inactivation of
PSII RCs, down-regulation of energetic connectivity of PSII units, destruction of PSII antenna architecture,
losing overall photosynthetic activity of PSII, increase of PSI activity also took place. Furthermore, the
tolerant population had lesser damage degree on photosynthetic capacity relative to intermediate and
sensitive populations. Finally, a reliable model, based on the most sensitive parameter PIABS and VK as a
characteristic parameter for heat stress, is presented for ranking and identifying heat tolerance in
different croftonweed populations. The heat sensitivity index (HSI) is also introduced as an indicator of
plant heat sensitivity. The smaller the HSI value is, the higher the level of tolerance to heat stress is. We
also found that the tolerance degree of four croftonweed populations to heat stress is significantly
correlated to the extreme high temperature. This indicates that acquired heat tolerance in croftonweed
populations results from plant adaptation to ambient high temperatures. Acquirement of heat tolerance
confers a possible risk for croftonweed to spread further to currently hotter areas.

Keywords: JIP-test Heat tolerance Adaptation