To study the impact of nitrogen application on the photosynthetic structure and photosystem activity of tomato (Lycopersicon esculentum Mill.) leaves during the recovery stage after heat stress, the OJIP curve and JIP parameters were determined through a control experiment in an artificial climate chamber. The tomato variety was “Jinfen No. 1”. Four day/night temperature levels (25 °C/15 °C as control CKT; 30 °C/20 °C, lightly high-temperature LHT; 35 °C/25 °C, moderate high-temperature MHT; 40 °C/30 °C, severe high-temperature SHT) were set for a duration of 7 days. Five nitrogen supply levels (N1–N5: 0, 1.3, 1.95, 2.6 and 3.75 g/plant, respectively; 2.6 g/plant is the recommended nitrogen application rate, CKTN4) were applied. The results showed that the O, K, J, I and P phases on the chlorophyll a fluorescence curve were significantly affected by different nitrogen treatments in heat stress recovery. Compared with CKT, with the increase in nitrogen supply, the fluorescence intensity of SHTN2-SHTN5 treatment increased significantly at P, I and J phases, while that of MHTN1-MHTN4 treatment decreased. The fluorescence intensity of SHTN5 and SHTN3 increased by 13.27% and 10.10% in the P phase, 13.52% and 12.1% in the I phase and 20.16% and 26.18% in the J phase, respectively. There were highly significant differences (p < 0.01) in the impact of high temperatures and nitrogen levels on the fluorescence parameters. On the 1st day after short-term heat stress, N had no significant effect on ${\mathrm{F}}_{\mathrm{v}}/{\mathrm{F}}_{\mathrm{M}}$, ${\mathrm{F}}_{\mathrm{v}}$, ${\mathrm{F}}_{\mathrm{o}}$ and ${\mathrm{F}}_{\mathrm{M}}$; however, their interaction was significant (p < 0.05). On the 8th day, there were no significant interaction effects between HT and N for ${\mathrm{F}}_{\mathrm{v}}/{\mathrm{F}}_{\mathrm{o}}$, ABS/RC and ${\mathrm{D}\mathrm{I}}_{\mathrm{o}}/\mathrm{R}\mathrm{C}$. ${\mathrm{F}}_{\mathrm{v}}/{\mathrm{F}}_{\mathrm{o}}$ proved to be sensitive to the application of both high temperatures and nitrogen. Under all five nitrogen applications, temperature played a significant role in increasing ${\mathrm{D}\mathrm{I}}_{\mathrm{o}}/\mathrm{R}\mathrm{C}$, especially for N2 and N3. The results indicated that decreasing the nitrogen application under SHT resulted in a higher number of active $\mathrm{R}\mathrm{C}\mathrm{s}$ and an increased value of specific energy flux ($\mathrm{A}\mathrm{B}\mathrm{S}/\mathrm{R}\mathrm{C}$, ${\mathrm{T}\mathrm{R}}_{\mathrm{o}}/\mathrm{R}\mathrm{C}$ and ${\mathrm{D}\mathrm{I}}_{\mathrm{o}}/\mathrm{R}\mathrm{C}$), indicating the enhanced ability of $\mathrm{R}\mathrm{C}$ to reduce plastoquinone. The study provides a reference for the diagnosis of nitrogen nutrition under high-temperature stress using chlorophyll fluorescence methods.

In recent years, there has been increasing interest in floral volatiles due to their important function in reproduction, self-protection and so on. Although some progress was made on deciphering emissions of floral volatiles and on related mechanisms under a variety of environmental factors, the effects of Zn ²⁺ concentrations on mitochondrial function and floral volatile emissions are yet to be revealed. Dendrobium huoshanense petals were used as materials and were treated with a 0–8 mM ZnSO ₄ solution. Intracellular Zn ²⁺ concentrations were evaluated by the fluorescent dye method and atomic absorption method. Mitochondrial respiratory chain complex activities and the precursor and ATP contents were determined by the biochemical method. Floral volatile components were analyzed by the gas chromatograph-mass spectrometer (GC-MS). The results indicated that ZnSO ₄ application significantly increased intracellular Zn ²⁺ concentrations. Elevated intracellular Zn ²⁺ concentrations differently affected mitochondrial respiratory chain complex activities, precursor and adenosine triphosphate (ATP) productions and floral volatile emissions. Moreover, positive correlations exist among the activities of mitochondrial respiratory chain complexes, productions of precursors and ATP and emissions of floral volatiles. It is concluded that Zn ²⁺ concentrations induce the activity changes of mitochondrial respiratory chain complexes, especially complex II and V, which promote or inhibit the emissions of floral volatiles by affecting the precursor and ATP levels that are closely related to the production of terpenoids, benzoids and fatty acid derivatives. The research will contribute to understanding the relationship between Zn ²⁺ concentrations and floral volatile emissions from the perspective of mitochondrial function.