Which factor does not affect the rate and amount of uptake of a herbicide by a weed?

The rate and amount of uptake of a herbicide by a weed is influenced by various physical and biological factors. Air temperature affects the metabolic processes of the plant, which in turn influences herbicide absorption and effectiveness. Similarly, relative humidity can impact how much moisture is available in the leaves, affecting the permeability and, consequently, the uptake of the herbicide.

The age and size of the weed matter significantly because younger, actively growing plants are generally more susceptible to herbicides. They typically have higher rates of metabolic activity and a larger surface area relative to their size, enhancing herbicide absorption.

On the other hand, the number of weeds being treated does not directly impact an individual weed's ability to absorb the herbicide. While treating a higher number of weeds can affect overall control or effectiveness due to competition for resources and possible variations in application, it doesn't change how each weed takes up the herbicide once applied. Therefore, this factor is correctly identified as not affecting the uptake rate or amount of herbicide by individual weeds.