Question 1: ‘All elements that are present in a plant need not be essential to its survival’. Comment.
Answer: An element should fulfill certain criteria to be categorized as essential element. Otherwise, it cannot be considered as essential elements. Out of the numerous elements present in plants, only 17 are considered as essential elements.
Question 2: Why is purification of water and nutrient salts so important in studies involving mineral nutrition using hydroponics?
Answer: Purification of water and nutrient salt is important to rule out other influencing factors. Use of pure nutrients and water will help in obtaining accurate scientific results.
Question 3: Explain with examples: macronutrients, micronutrients, beneficial nutrients, toxic elements and essential elements.
Answer: Macronutrients: Elements which are present in large amounts in plant tissues are called macronutrients. They are in excess of 10 mmole per kg of dry matter. Carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium and magnesium are the macronutrients.
Micronutrients: Elements which are present in small amounts, i.e. less than 10 mmole per kg of dry matter are called micronutrients. Iron, manganese, copper, molybdenum, zinc, copper, boron, chlorine and nickel are the micronutrients.
Beneficial Elements: Apart from the 17 essential elements, many other elements are required by plants. These are called beneficial elements. For example; sodium, silicon, cobalt and selenium are required by higher plants.
Toxic Elements: If a mineral ion concentration in tissues reaches to a level that it reduces the dry weight of tissues by about 10%, the mineral then becomes toxic. This shows that any element can become toxic if it crosses a certain threshold in plants.
Question 4: Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.
Answer: Following are the five deficiency symptoms and related minerals:
- Yellowing of lower leaves: Magnesium deficiency
- Pale green leaves: Nitrogen deficiency
- Purple leaf tints with bronze or brown leaf edges: Potassium deficiency
- Reddish purple undersides of leaves: Phosphorous deficiency
- White deposits on leaves: Carbon dioxide deficiency
Question 5: If a plant shows a symptom which could develop due to deficiency of more than one nutrient, how would you find out experimentally, the real deficient mineral element?
Answer: For this, we need to tabulate all the available symptoms in different parts of the plant. Then the symptoms are compared with the symptom table; to arrive at a conclusion about the deficiency of a specific element.
Question 6: Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?
Answer: Appearance of deficiency also depends on the mobility of the element in the plant. Some elements are actively mobilized in plants and are exported to young developing tissues. Deficiency of such elements first appears in the older tissues. For example; the deficiency symptoms of nitrogen, potassium and magnesium are first seen in the senescent leaves. This happens because these elements are mobilized to younger leaves.
Question 7: How are the minerals absorbed by the plants?
Answer: Absorption of minerals takes place in two main phases. In the first phase, passive absorption takes place through apoplast pathway. In the second phase, absorption takes place through symplast pathway. The first phase involves passive transport (facilitated diffusion), while the second phase involves active transport. After that, minerals are transported through xylem.
Question 8: What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium. What is their role in N2 fixation?
Answer: Rhizobium bacteria need symbiotic association with legume plants to carry out nitrogen fixation. Root nodules contain the necessary enzymes for nitrogen fixation and thus enable rhizobium to fix nitrogen. The enzyme nitrogenase facilitates the conversion of nitrogen into ammonia which is the first stable product of nitrogen fixation. Ammonia is then converted into glutamic acid. Glutamic acid is then utilised by plants to make amino acids; which are then utilised to make protein.
Question 9: What are the steps involved in formation of a root nodule?
Answer: Development of root nodules happens in following steps:
- Rhizobium bacteria contact a susceptible root hair and divides near it.
- Successful infection of the root hair results in curling of the root hair.
- The infected thread carries the bacteria to the inner cortex. The bacteria get modified into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation.
- A mature nodule is complete with vascular tissues. The vascular tissues of the nodule are continuous with those of the root.
Question 10: Which of the following statements are true? If false, correct them:
(a) Boron deficiency leads to stout axis.
(b) Every mineral element that is present in a cell is needed by the cell.
Answer: Out of all the mineral elements, only 17 are considered as essential elements.
(c) Nitrogen as a nutrient element, is highly immobile in the plants.
Answer: Nitrogen is highly mobile in plants.
(d) It is very easy to establish the essentiality of micronutrients because they are required only in trace quantities.