i) Temperature, concentration of CO2, availability of water, availability of minerals, intensity of sunlight, duration of sunlight, color of sunlight reaching the leaves are all factors that can limit photosynthesis. If any one of these factors is limiting, that factor will determine the rate of photosynthesis.
ii) Use of fertilizers and irrigation will improve yields of field crops. Removing plants that shade the field will improve yields. In a greenhouse setting, increasing CO2 concentrations will help as will the use of artificial light of high intensity and selected colors during winter months.
Any factor other than the one under consideration that affects a reaction becomes a limiting factor when we change it, it will cause a further increase in the rate of the reaction.
So for example. If you are investigating the effect of CO2 concentration on the rate of Photosynthesis then as you keep on increasing the concentration of Co2 there will come a point where you'll see that increasing the concentration of Co2 causes no further increase in the rate of the reaction. Some other reaction such as temperature or the availability of water may need to be changed to increase the rate further.
Temperature causes an increase in enzyme-activity for example due to more frequent collisions between enzyme and substrate to form an enzyme substrate complex. Furthermore, it causes an increase in water uptake due to an increase in the rate of transpiration.
Water is a limiting factor because it is required in the light dependent reaction of photosynthesis.
Similarly, light intensity is also a limiting factor as the light dependent reaction depends on it.
On the whole, increasing each of these will actually increase the rate of a reaction.
The independent variable is "Light Intensity" So any other factor will only become limiting when we are using the higher amount of our independent variable. At lower light intensities it will it self limit it. So, at high light intensity Co2 becomes the limiting factor which needs to be increased to increase the rate of Photosynthesis further.