(1) Figure A shows respiration, B shows transpiration and C shows photosynthesis. Plants can carry out photosynthesis, respiration and transpiration when there is light in the daytime, but at night, because there is no light, photosynthesis cannot be carried out normally, so they can only carry out respiration and transpiration.

(2) Photosynthesis can only be carried out under light, and curve I in Figure B can only be carried out during the daytime of 6- 18, so curve I represents photosynthesis; Curve ⅱ represents respiration, which corresponds to the physiological activity in Figure A, and the place where this physiological activity takes place is mitochondria.

(3) Only about 65,438+0% of the water absorbed by plants through roots is utilized by plants, and about 99% is discharged into the atmosphere by transpiration. Therefore, in arid desert, the water absorbed by plants is mainly used for transpiration of physiological activity B in Figure A. 。

(4) The main purpose of agricultural production is to obtain organic matter produced by photosynthesis. In agricultural production, it is necessary to ensure various conditions of effective photosynthesis of crops, such as reasonable close planting, intercropping and interplanting, increasing light intensity and increasing carbon dioxide concentration. At night, plants only breathe without photosynthesis. Proper temperature reduction can weaken the respiration of plants and reduce the consumption of organic matter.

So the answer is: (1)A, B, C;

(2) A; Mitochondria;

(3)B；

(4) Intercropping, interplanting and reasonable close planting; Proper cooling can weaken the respiration of plants and reduce the consumption of organic matter.