(1) In the curve in Figure B, only respiration occurs before 6: 00 a.m., and soybean seedlings begin to photosynthesize at 6: 00 a.m., so the indoor oxygen amount gradually decreases in 6-8 h, and at point B, the photosynthetic rate reaches the highest value in a day. If the soybean seedlings under B-spot illumination were moved to the dark, the light reaction was immediately blocked, and the ATP and [H] produced were reduced, thus inhibiting C3 in the dark reaction. It leads to the increase of C3 content in chloroplasts. (2) Because magnesium is the main component of chlorophyll synthesis, if soybean seedlings are cultivated with magnesium-deficient total nutrient solution, the chlorophyll content of soybean seedlings will decrease, the photosynthetic rate will slow down, and the respiration will not be affected, so the point A in the curve of Figure B will move to the right. (3) Due to the high light intensity in Xinjiang during the day, there are many organic compounds synthesized during the day, but the temperature is low at night. Breathing at night consumes less organic matter, so it accumulates more organic matter in one day, so the grapes in this area are big and sweet. (4) Because inorganic salts are absorbed by root cells by active transportation, which requires energy, loosening soil can promote aerobic respiration and energy supply of root cells. So the answers are: (1) soybean seedlings start photosynthesis, (2) suitable plants lack chlorophyll, and (3) light time.