I have also observed improved values in leaf sap testing during dry, sunny conditions compared to periods after rain. I believe there are two contributing factors: firstly, with more sunlight, the plant undergoes increased photosynthesis, resulting in better nutrient values. Secondly, as you rightly pointed out, the nutrients are less diluted in the sap due to higher transpiration, leading to higher values.

My consultant at NCC emphasized that this effect is notably reflected in the sugar content. In instances of high nitrogen, plant cells tend to absorb more water, causing a dilution effect and consequently reducing sugar content. In my interpretation, I consider sugar levels exceeding the optimal range as a stress factor for the plant.

there are companies like (https://www.soilbeat.com/) actively addressing this issue by incorporating weather data into the interpretation of leaf sap analyses. I am hopeful that a solution will be available soon.