To determine the correct statements regarding mineral nutrition in plants, let's analyze each statement step by step:
### Step 1: Analyze Statement I
**Statement I:** "Solution culture/Hydroponics contains all essential minerals except one, the usefulness of which is to be determined."
- **Explanation:** Hydroponics is a method of growing plants without soil, using a nutrient solution that contains all essential minerals. However, it is common practice to exclude one mineral to study its effects on plant growth, which helps in determining its usefulness.
- **Conclusion:** This statement is **correct**.
### Step 2: Analyze Statement II
**Statement II:** "Na, Si, Co, and Selenium are beneficial elements required by higher plants."
- **Explanation:** Sodium (Na), Silicon (Si), Cobalt (Co), and Selenium (Se) are indeed considered beneficial elements for certain plants, but they are not classified as essential elements for all higher plants. Essential elements are those that are necessary for a plant's growth and reproduction.
- **Conclusion:** This statement is **correct**.
### Step 3: Analyze Statement III
**Statement III:** "Zn is the activator of nitrogenases while Mo is the activator of alcohol dehydrogenase."
- **Explanation:** This statement is incorrect. Zinc (Zn) is actually the activator of alcohol dehydrogenase, while Molybdenum (Mo) is the activator of nitrogenase, which is involved in nitrogen fixation.
- **Conclusion:** This statement is **incorrect**.
### Step 4: Analyze Statement IV
**Statement IV:** "Zn is needed for auxin synthesis."
- **Explanation:** Zinc is indeed required for the synthesis of auxins, which are important plant hormones involved in growth and development. Auxin synthesis is linked to the amino acid tryptophan, and zinc plays a crucial role in this process.
- **Conclusion:** This statement is **correct**.
### Final Conclusion
Based on the analysis:
- **Correct Statements:** I, II, and IV.
- **Incorrect Statement:** III.
### Answer
The correct statements are I, II, and IV.
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