Magnesium stress affects how well cucumber plants can make food through photosynthesis. When there's too much magnesium, the plants have less of the green stuff (chlorophyll) they need for photosynthesis. But when there's not enough magnesium, they have even less chlorophyll. The plants also have trouble breathing in carbon dioxide and letting out oxygen when they don't have enough magnesium. Overall, not having enough magnesium hurts the plants' ability to make food, while having too much magnesium affects how they breathe.