Objectives Chilling stress severely inhibits the normal growth and blooming of tomato seedlings, and leads to fruit malformation. Chlorogenic acid (CGA), a phenolic acid, is the key metabolite in plant adaptation to abiotic stresses. We investigated the effect of exogenous application of CGA on the chilling tolerance of tomato seedlings of two varieties with different chilling tolerance, to explore the regulatory role of CGA and possible underlying mechanism.

Methods Tomato cultivars JF401 (high tolerant to chill) and JF101 (week tolerant to chill) were used to carry out a pot experiment under climate chamber conditions. Based on the previous result, six CGA root application concentrations were applied on tomato seedlings, including 0 (CK), 0.005, 0.025, 0.05, 0.1 and 0.25 g/L. The tomato seedlings had grown in normal temperature condition for 5 weeks before exposure to low temperature stress (day 15℃ and night 5℃) for 6 days. On the second day after the end of the chill stress, initial fluorescence yield (Fo), maximum fluorescence yield (Fm), photochemical burst coefficient (Qp), non-photochemical burst coefficient (NPQ) and actual photochemical efficiency of PSII (ФPSII) were determined in fully expanded leaf segments using a portable chlorophyll fluorometer. The fresh weight of shoot (SFW), root (RFW) and total seedlings (TFW) were weighed. The relative electrical conductivity (REC), chlorophyll pigment content (Chl.a, Chl.b and Car), free water content (LFW) and bound water content (LBW) of fully expanded leaves were determined. The leaf superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents were analyzed.

Results Within the tested CGA concentrations, the chilling tolerance of tomato seedlings showed a trend of first increase and then decrease, the peak D values, comprehensive evaluation index of plant cold tolerance, in JF401 and JF101 appeared at CGA 0.1 g/L and 0.05 g/L, respectively. Pearson correlation analysis showed that the D-value was positively correlated with Fv/Fm, Qp, ФPSII, Chl.a, Chl. (a+b), SFW, RFW, TFW, SOD, POD, CAT and LBW (r=0.72−0.98), negatively correlated with H₂O₂, NPQ, REC, MDA and LFW (r=0.63−0.98). Principal component analysis (PCA) revealed that the SFW, Chl.b, Chl. (a+b), Qp, ФPSII and LBW were the key indicators influencing the chilling tolerance of the two tomato cultivars at seedling stage. Under optimum CGA concentration, the ФPSII, Chl. (a+b), LBW and SFW in JF101 were 157.7%, 35.5%, 8.4% and 24% higher than CK, and in JF401 were 223.5%, 64.9%, 31.2% and 62% higher than the control.

Conclusions Under chill stress, application of CGA at an appropriate concentration could significantly improve the chilling tolerance of tomato seedlings, and the enhancement involves multi-pathway regulatory mechanisms such as improving light energy absorption and utilization, reducing water physiological metabolism, enhancing enzymatic antioxidant defenses and scavenging reactive oxygen species.