Plants poorly watered and suffering the full strength of the sun are said to wilt beneath its heat, describing the drooping of leaves as they lose the water that gives them structure. Further exposure can cause a plant to wither, shrivelling and losing color, which occurs when a plant lacks the nutrients to combat the effects of solar radiation. This lack of nutrients is due to insufficient flow of water to carry them up to the leaves from the roots, and the lack of the temperature-regulating power of water itself. It's a fitting analogy to the example in your question: a person toiling in the heat of the sun, presumably with little rest or refreshment.
If you regard water as a nutrient, then the sun does indeed cause its rapid loss through evaporation. In this particular act of removing water from a person, the sun could be described as dessicating that person. Often dessication unchecked leads to mummification, so it's a particularly impactful (and perhaps extreme) word for what might not be such a serious situation.
You can describe the conditions of hard, prolonged work under intense heat as deleterious, because they cause a person to deplete their body's stores of nutrients more quickly and, because they aren't being replenished, to a greater extent than the norm. This word suits a wide range of conditions, all of which tax the body and mind to expend more of its resources than it would under comfortable circumstances.
