3. Results

3.1Effects of E-2-hexenal on tomoto colour development

Postharvest colour developments in response to E -2-hexenal (E2H) with different concentrations (0.5-25μL L-1) were observed at 72 h. Tomatoes treated with E2H showed restrained coloration in concentration-dependent patterns (Figure 1A). The pericarps colour development was totally inhibited from the outermost to the innermost by E2H, but the columellas were not affected by the E2H treatments (Figure 1B). In the skins, patchy green areas were separated by red areas (Figure 1C).

3.2 Effects of combined E-2-hexenal and ethylene on tomato colour development

E2H, ET, combined E2H and ET, and different priorities of E2H and ethylene (ET) treatments on the tomatoes showed different patterns at 72 h. Tomato treated with ET (12 h) and then E2H (12 h) showed red colour like the ET or the control group. Tomato treated with E2H (12 h) and then ET (12 h), as well as the combined E2H and ET group showed inhibited colour development like the E2H group (Figure 2).

3.3 Effects of E-2-hexenal on the transform of chloroplast to chromosome

During the transformation of chloroplast to chromosome, the IMS (inner membrane structure) including inner envelope membrane (IEM) and the thylakoid membranes proliferated disappeared, and produced new structures such as tubular elements (te), plastoglobules (pg) and crystal remnants (cr) via membrane fusion or vesicles budding (Figure 3B-D). Based on the dynamics of im, te, pg and cr during the transform processes of chloroplast to chromosome (Figure 3I), four stages (S1: chloroplast; S2: transition states 1; S3: transition states 2 and S4: chromosome) of the plastids were suggested in this study, for example, The im vanishes at S3; te and pg emerge at S2; cr emerges at S4. (Fig. 3I).
In the control group, all the plastids observed by TEM were statistically analyzed (Figure 3A-D). The proportions of plastids in S1, S2, S3 and S4 were 35%, 17%, 17% and 31% separately. No plastids in the S1 and S2 states were detected in the E2H group, and the proportions of S3 and S4 were 44% and 56%, respectively (Figure 3E and F).
The transform from S1 to S4 in the control group were programmed, im disappeared, te appeared, pg gradually accumulated, and further cr accumulated (Figure 3A-D). However, most of the plastids in the E2H group completed transformation without sufficient accumulation of pg and no cr observed (Figure 3E and F). Therefore, the pg count per plastid in the E2H group was significantly lower than the control (Figure 3J).

3.4 Effects of E-2-hexenal on pigment contents

The a* values indicate red-green difference, lower value indicates the fruit colour close to green, higher value indicates the fruit colour close to red (Gao, Liu, Kan, Chen & Chen, 2019). The a* values of the control increased during 18-72h. Both E2H (2.5) and E2H (12.5) treatment decelerated the increase speed of the a* values. More obviously, no a* value increase was observed in the E2H (12.5) group (Figure 4A).
The chlorophyll is the main pigment of green tomatoes and the concentrations decreased during postharvest ripening in the control group in 18-72 h. Both l E2H (2.5) and E2H (12.5) decelerated the decrease speed of chlorophyll contents. In 36-72 h, E2H (12.5) group showed more obvious effects then the E2H (2.5) group (Figure 4B).
The β-carotene and lycopene are two pigments of red tomatoes and the concentrations increased during postharvest ripening in the control group. Both E2H (2.5) and E2H (12.5) inhibited theβ-carotene and lycopene increase, especially E2H (12.5) stopped the accumulation of β-carotene and lycopene totally in 18-72 h. Interestingly, the concentration of β-carotene showed more complex patterns than the lycopene in response to E2H treatments. The increase and decrease of β-carotene in response to E2H treatments formed “peaks” at 36 h in E2H (2.5) group and 6 h in the E2H (12.5) group separately (Figure 4C and D).

3.5 Effects of E-2-hexenal on LOX and HPL gene expressions

The responses of both LOXC and HPL to E2H were fast (6 h), but they showed obviously different patterns (Figure 5).
The expression of LOX upregulated during postharvest colour transformation in the control group. The E2H (2.5) group showed upregulation of LOX than the control at 6 h, and downregulation than the control at 36-72 h. The E2H (12.5) group showed upregulation ofLOX than the control at 6-18 h, and downregulation than the control at 36-72 h. Both E2H (2.5) and E2H (12.5) groups showed upregulation and then downregulation trends to form expression “peaks” at 6 h (2.5 μL L-1) and 6-36 h (12.5 μL L-1) separately (Figure 5A).
The expression of HPL downregulated during postharvest colour transformation in the control group. Both E2H (2.5) and E2H (12.5) groups showed sharply decreases since 6h and kept at lower expression levels than the control at all the time points (Figure 5B).

3.6 Effects of E-2-hexenal on the titratable acids, ascorbic acids, soluble solids and malondialdehyde contents

The E2H at both concentrations increased the contents of TA (6 h) and SSC (6 h and 36 h) rapidly, and the effects of E2H (12.5) were more significant (Table 2). The E2H (2.5) (36-72h) and the E2H (12.5) (36h) both increased the contents of ascorbic acids in tomatoes, and the effects of E2H (2.5) were more significant (Table 2). The E2H affected the MDA contents. The E2H (2.5) increased the MDA contents at 18-36 h while the E2H (12.5) increased them at 6 and 36 h, whereas at 72 h, the contents of the MDA in the control group was higher than that of the two groups treated with E2H (Table 2).