(1) In Experiment 1, the independent variable is EBR solution, the observation index is the photosynthetic oxygen release rate of leaves, and the dependent variable is the photosynthetic rate of cucumber seedlings, so the experimental purpose of this experiment is to explore the appropriate concentration of EBR to promote the photosynthetic rate of cucumber seedlings. In this experiment, the temperature is an irrelevant variable, and its setting should follow the principle of equivalence. The purpose of placing a glass tank filled with water in front of high-pressure sodium lamp is to avoid the temperature change caused by high-pressure sodium lamp irradiation from affecting the experimental results.

(2) According to the components of the mother liquor and the distilled water added in the preparation of EBR solutions with different concentrations, it can be known that the components of "a solution" sprayed in the control group of Experiment 2 are ethanol, polysorbate -80 and distilled water.

(3) From the figure 1, it can be seen that EBR treatment can obviously enhance the photosynthetic efficiency, and with the passage of time, the photosynthetic rate gradually decreases.

(4) CO _ 2 is transformed into photosynthetic products in chloroplasts through the fixation of carbon dioxide and the reduction of three carbon compounds.

(5) As can be seen from Figure 2, the stomatal conductance increased obviously 24 hours after EBR treatment, but as can be seen from Figure 3, the increase of intercellular CO2 concentration was not obvious at this time. Combined with the analysis of Figure 1, the main reason is that photosynthesis is accelerated and the utilization rate of CO2 is accelerated.

(6) According to the changes of chlorophyll content and the ratio of chlorophyll a to chlorophyll b before and after treatment with EBR, we can know whether EBR affects the synthesis of photosynthetic pigments, so we can know whether EBR affects the light reaction or dark reaction of photosynthesis.

According to Figure 3, the intercellular CO2 concentration in the experimental group is higher than that in the control group after 24 hours of EBR treatment, and the net photosynthetic rate is higher after 24 hours of EBR treatment combined with Figure 1EBR treatment, that is, the net photosynthetic rate is higher after 24 hours of EBR treatment, but the utilization of carbon dioxide is not much. It can be seen that it is not the carbon dioxide concentration that affects the photosynthetic rate at this time, and the relationship between photosynthesis and light intensity of leaves after 24 hours of EBR treatment can be inferred that EBR affects

The relationship between photosynthesis and intercellular CO2 concentration in leaves of 168h after EBR treatment can only tell the changes of photosynthesis before and after EBR treatment, but can't know whether EBR affects the light reaction or dark reaction of photosynthesis.

So the answer is:

(1) Explore the appropriate concentration of EBR to promote the photosynthetic rate of cucumber seedlings? Avoid the influence of temperature change caused by high-pressure sodium lamp irradiation on experimental results.

(2) ethanol, polysorbate -80 and distilled water

(3)EBR treatment can obviously enhance the photosynthetic efficiency, and with the passage of time, the photosynthetic rate gradually decreased.

(4) Fixation of carbon dioxide? Reduction of three-carbon compounds

(5) Photosynthesis is accelerated, and the utilization rate of CO2 is accelerated.

（6）④