Why do tomato plant leaves turn dark green and stems purple?

Tomato plant leaves turning dark green and stems turning purple are signs of phosphorus deficiency.

Here's why:

* Dark Green Leaves: While a healthy green color is good, excessively dark green leaves can indicate that the plant is trying to compensate for a lack of phosphorus. Phosphorus is essential for photosynthesis, and when it's lacking, the plant produces more chlorophyll to try and make up for it.

* Purple Stems: Phosphorus is also important for the plant's overall growth and development. When phosphorus is deficient, the plant may produce anthocyanin pigments, which are responsible for the purple color in stems. This pigment is a natural defense mechanism that helps the plant to protect itself from stress.

Other symptoms of phosphorus deficiency:

* Stunted growth: The plant may grow slowly or even stop growing altogether.

* Weak stems: The stems may become weak and brittle.

* Poor fruit production: The plant may produce fewer tomatoes, and the tomatoes may be smaller and less flavorful.

* Older leaves may turn yellow or brown: This is because the plant re-absorbs phosphorus from older leaves to feed newer growth.

How to fix phosphorus deficiency:

* Soil test: Get your soil tested to determine the exact nutrient levels.

* Apply phosphorus fertilizer: Use a fertilizer that is high in phosphorus, such as superphosphate.

* Use compost: Compost is a great source of phosphorus and other nutrients.

* Improve drainage: Phosphorus is less available in wet soils, so make sure your soil is well-drained.

Important Note: While phosphorus deficiency is a common cause of dark green leaves and purple stems, other factors, such as cold temperatures or excess nitrogen, could also be involved. If you're unsure, it's best to consult a gardening expert or your local extension office.