What does foliage turning purple on a greenhouse plant indicate?

Foliage turning purple on a greenhouse plant is most commonly associated with a phosphorus deficiency. Phosphorus is an essential macronutrient that plays a critical role in plant energy transfer, photosynthesis, and nutrient uptake. When plants do not receive enough phosphorus, they can exhibit stunted growth and develop purplish hues on their leaves due to the accumulation of anthocyanins, which are pigments produced in response to stress. This color change is like a visual signal that something is amiss in the plant's nutrient uptake.

While excess nitrogen can also cause various issues in plants, it typically leads to lush, green growth and can even mask deficiencies, not induce purple foliage. Pest infestation can cause discoloration and other damage, but purple leaves are not a direct indicator of pests specifically. Water stress may affect plant health and lead to changes in leaf color, but it commonly causes yellowing or wilting rather than specifically turning leaves purple. Hence, the presence of purple hues is a strong indicator of insufficient phosphorus levels in the plant's environment.