Leaf pigment and photosynthetic gas exchange parameters
Table 1 shows that the chlorophyll and carotenoid contents and Pn decreased significantly under drought conditions, with decreases of 19.1%, 21.1%, and 64.5% under high light, and 20.7%, 18.7%, and 55.1, respectively, under 20% shading. Compared with natural light, shading increased the leaf chlorophyll and carotenoid contents, which increased by 4.9% and 9.4% under drought conditions and by 7.0% and 6.2%, respectively, under normal irrigation conditions. As the processing time progressed, the effects of high light and drought on plants increased. The chlorophyll content, carotenoid content and net photosynthetic rate of SW were significantly higher than those of the other three treatments at 20 d. The chlorophyll content of SW was 49.2%, 34.3%, and 45.0% higher than that of ND, NW and SD, respectively. The carotenoid content of SW was 37.6. %, 32.3%, and 27.3% higher, and the Pn was 6.3, 0.7, 2.3 times higher than the corresponding values of the other treatments. With prolonged drought stress time, the Pn decreased significantly. At 10 d, compared with SW, the net photosynthetic rate of ND, NW and SD decreased significantly, while the Gs and Pn showed the same trend, and the Ci decreased slightly, but this difference was not significant. We hypothesized that the decrease in Pn was due to the closure of the pores, resulting in a reduction in the carbon dioxide supply. When treated for 20 d, the decrease in Pn was intensified, and the Gs trend was basically the same, while the Ci of ND and SD was higher than that of SW, which may be the reason for the decrease in Pn caused by nonstomatal limiting factors; NW showed a decrease in Gs and an increase in Ci, which may have been caused by stomatal factors.
The trends in the diurnal variation of Pn were similar under the different treatments, and all showed a “double peak” curve (Fig. 2). The highest peak appeared at approximately 11:00, and the Pn was the highest for SW. Between 11:00 and 13:00, as the PAR and temperature rose, the Pn dropped rapidly and peaked at approximately 15:00.
The quantum efficiencies of PSII
Hendrickson et al. (2004) noted that the excitation energy of PSII absorption in plant leaves was mainly used in the following three ways: Y(II), Y(NPQ), and Y(NO). Under natural light, the photosynthetic mechanism operates quickly, and the light energy absorbed by the Welsh onion leaves was mainly used for photochemical processes and heat dissipation. SW had the highest Y(II), which was 22.1%, 20.0%, and 14.1% higher than that of ND, NW, and SD, respectively. The Y(NPQ) was highest with NW and lowest with SW. The Y(NO) of ND was significantly higher than that of NW, SD, and SW by 15.0%, 13.7%, 12.9%, respectively.
OJIP curve and JIP-test
All OJIP curves showed multiphase changes as the OJIP phase increased. At the O-step, the fluorescence yield was minimal because Q A was almost completely oxidized after dark adaptation. When exposed to saturated pulsed light, with the accumulation of QA-, the fluorescence yield gradually increased. Once QA completely entered the reduced state, the PSII reaction center was completely closed, and the quantum yield was no longer accepted. At this time, the maximum PF intensity (Fm) was reached. The occurrence of points J and I was mainly related to the exchange of a reduced QA with an oxidated PQ molecule at the QB site and the reoxidation of PQ. After 10 d of treatment, there was a significant difference in the change in the OJIP transient. Compared with that of SW, the Fm value of ND, NW, and SD decreased while the minimum PF intensity (Fo) increased (Fig. S1-A). This trend was more pronounced at 20 d. Fig.4 -A, D shows an induction curve normalized by the fluorescence value at point P. The normalized curve did not change significantly at 10 d but did at 20 d; the fluorescence signal between the O-P step of ND and SD increased significantly, especially the J-step. In the SW treatment, the OJIP curve showed similar trends over all measured days. Fig.4 -B, E shows a subtraction process with NW as a control. Four treatments were significantly different in the J-phase. At 10 d, ΔJ under ND and SD increased to 0.02 and 0.01 compared to that of NW, while that of SW decreased to 0.02. At 20 d, ΔJ increased to 0.08 under ND and SD and decreased to approximately 0.03 under SW. First, the O-J phase normalization was performed for each treatment and then subtracted from each treatment with NW as a control (Fig.4 –C, F). At 10 d, SD had a more obvious K peak than did NW. The difference between the treatments at 20 d was more noticeable than that at 10 d. The K peak of the ND treatment was 11.6% higher than that of the NW treatment, while the difference between the SD and NW was not significant, and the K peak of SW was 15.3% smaller than that of NW.
A large amount of raw data was obtained from the OJIP curve. To better reflect the relationship between the kinetic curves and the material, the methods of Strasser based on the biofilm flow were used to calculate the energy flow and energy ratio to measure the given physical parameters[12, 16]. The variation in the interior of the sample material in a given state was used to establish a highly simplified energy flow model diagram. The data from the energy model diagram was used for the JIP-test. Compared with SW, the FO of ND, NW, and SD increased significantly at 10 d, by 18.6%, 12.9%, and 19.2%, respectively. At 20 d, the FO value was highest in ND and lowest in SW. The difference in FO between ND and SD was not significant. The Fm value of SW was significantly higher than that of ND, NW, SD, by 29.6%, 16.6%, and 17.1%, respectively. The VJ reflects the degree of closure of the active reaction center at 2 ms of illumination. The VJ of ND and SD increased notably, while the difference between SW and NW was not distinct. When the donor side of the PSII (oxygen-releasing complex, OEC complex) was damaged, the chlorophyll fluorescence yield increased before the J point (approximately 300 μs), so the relative fluorescence value WK of this point increased as the donor of PSII. The degree of increase in WK represents the level of damage on the donor side of PSII. ND had the highest WK, and SW had the lowest. Sm is typically used to characterize the size of the PSII reaction center PQ pool. At 20 d, there was a significant difference in Sm between different treatments, with SW being the highest at 27.6%, 19.8%, and 26.6% higher than those of ND, NW, and SD, respectively. There were significant differences in the PIABS of the Welsh onion leaves under each treatment, in which drought stress notably decreased the PIABS, while the PIABS was increased by 20% shading.
P700 can absorb light at 820 nm when the light is in the oxidation state P700+, so the change in light absorption or reflection at 820 nm can represent the redox state of P700. The 820 nm light reflection kinetic curve in this test was represented by MR/MRO. Under different treatment conditions, the redox state of PSI changed in the leaves of Welsh onion. The lowest point of the MR/MRO rapid decline phase represents the turning point of the PSI oxidation state. The fast decline phase of the synchronously measured MR/MRO curve changed at 10 d, while the lowest point of the reduction was increased in ND, and the MR/MRO drop rate VPSI was significantly reduced (Fig. 5). After 20 d of treatment, the rapid drop in the MR/MRO of ND was significantly shifted backward and increased. In addition, the VPSI of ND was notably reduced compared with that of NW, SD, and SW by 27.5%, 20.5%, and 48.9%, respectively. For the rate of the reduction of P700+ by electrons from PSII and the reduced PQ pool, the rate of increase of VPSII-PSI in ND was significantly reduced after 10 d of treatment. After 20 d of treatment, the rate of increase of MR/MRO was highest in SW, followed by NW and SD, and was the lowest in ND. Among them, the rate of increase of MR/MRO in SW was 34.6%, 35.5%, and 80.9% higher than that in NW, SD and ND, respectively.