Land plants are constantly growing in environments not optimal for their growth and development, drought and saline stresses are often encountered by plants. These stresses will significantly affect the transpiration, ion absorption and photosynthesis of the plants. Because the photosynthesis of land plants is always accompanied by transpirational water loss, and because the xylem sap being transported to leaves during the transpiration process is not pure water, this in turn will result in the accumulation of ions in plant leaves. This is because the when the water in the xylem sap is transpired into the air, ions will be left in leaves. For this reason, the ratio between the amount of carbon assimilation and ion accumulation in plants may vary significantly under different environmental conditions even within the same plant. This is an important part of plant metabolism in terms of ionic nutrition and salt resistance. However, the relation between the simultaneous change and accumulation of ions and photosynthetic yield is rarely addressed and little is known about the relationship between the accumulation of ions and photosynthetic assimilatant in Vitex trifolia L, a salt and drought resistant plant, the effect of environmental factors on that relationship is even more scarce. In this paper, the photosynthesis and transpiration in relation to ion accumulation in V. trifolia under varied light intensity (on cloudy/overcast and sunny days, respectively). The diurnal accumulation of photosynthetic assimilatant was calculated through the integration according to the instantaneous photosynthetic rate of leaves against time, and the ion accumulation in leaves was yielded by integrating the instantaneous ion concentration of xylem sap against time. In this way, the ion concentrations, in relation to the transpiration rate and the water potential of V. trifolia, as well as the photosynthetic rate, were analysed on cloudy/overcast and sunny days, respectively. Under natural conditions, the photosynthetic rate and the transpiration rate of V. trifolia, both on cloudy/overcast days was highly light dependent. On cloudy/overcast days, the pattern of photosynthetic rate and the transpiration rate of V. trifolia agreed very well with the changes in light intensity. However, on sunny days, the pattern of photosynthetic rate and the transpiration rate of V. trifolia did not follow the pattern of changes in light intensity exactly, especially in the afternoon when the water potential of the plant was low. This was an indication that the water status of the plant, may also have played a role in regulating the transpiration and photosynthesis of the plant. In terms of the overall water use efficiency (WUE) of a day, the performance of V. trifolia on sunny days was significantly higher than that on cloudy/overcast days. The photosynthetic assimilatants accumulated on sunny days was four times of those accumulated on cloudy/overcast days, but with only just three times of the water consumed on cloudy/overcast days. In agreement with the report of some literatures, both the concentrations of sodium and potassium in xylem sap went down slightly with the decline of tissue water potential of the plant both on sunny and cloudy/overcast days, indicating that the ion concentration in xylem sap was regulated. The amount of sodium accumulated in V. trifolia was always higher than that of potassium either on sunny and cloudy/overcast days, suggesting that the sodium ions may have been positively absorbed in order for the plant cells to gain a lower osmotic potential to maintain a higher driving force for water absorption, which may be one of the reasons that V. trifolia had got a high resistance to salt and drought stresses.