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‘All elements that are present in a plant need not be essential to its survival’ comment.

There are some elements such as Zn, Mo, Bo etc, are non essential and present in traces. They take part in some metabolic reactions but are not essential for survival.


Why is purification of water and nutrient salts so important in studies involving minerals nutrition using hydroponics?

It is important for the studies of physiological role and deficiency symptoms of specific element involving hydroponics.


Explain with examples: macro nutrients, micro nutrients, beneficial nutrients, toxic elements and essential elements.

Macronutrients. An element such as potassium, nitrogen or calcium, essential in large quantities for plant growth is termed macronutrient. They are generally present in plant tissues in concentration of at least 10 mol Kg-1 of dry matter. Examples are Carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, calcium, magnesium, sulphur, silicon and iron.

Micronutrients or trace elements, are needed in very small amounts (less than 10 mmol Kg-1 of dry matter). These include iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Beneficial nutrients. They are required by higher plants in large quantities. Examples are sodium, silicon, cobalt and selenium.

Toxic element. Any mineral ion that reduces the dry weight of tissues by about 10% is called a toxic element. Example – excess of manganese causes the development of brown spots in plants.

Essential elements. Such elements without which the physiological functions of plants are affected are called essential elements. These include calcium, hydrogen, oxygen etc.


Name at least five different deficiency symptoms in plants. Describe them and correlate them with the concerned mineral deficiency.

The names of five different deficiency symptoms are:

(a) Chlorosis: It is the loss of chlorophyll leading to the yellowing of leaves. It is caused by the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo.

(b) Necrosis: It is the death of the tissue particularly the leaf tissue and appears due to the deficiency of Ca, Mg, Cu, K.

(c) Inhibition of cell division:  Inhibition of cell division which affects the general growth of the plant and elongation of parts like roots is caused because of the deficiency of N, K, S, Mo.

(d) Delay in flowering: Some elements like N, S, Mo delay flowering if their concentrations in the plant are low.

(e) Stunted plant growth: Stunted growth occurs due to deficiency of N, P, Ca.


If a plant shows a symptoms which could develop due to deficiency of more than one nutrient, how would you find out experimentally the real deficient mineral element?

In such a case in order to find out the real cause of the symptom, one has to study all the symptoms developed in all the various parts of the plant and compare them with the available standard tables.


Why does deficiency symptoms appear first in younger parts of certain plant while in others they do so in mature organs?

The parts of the plant that show the deficiency symptoms depend on the mobility of the element in the plants. Where the elements are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are broken down, making these elements available for mobilising to younger leaves.

The deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs, for example, elements like sulphur and calcium are a part of the structural component of the cell and hence are not easily modified.


How are the minerals absorbed by the plants?

The process of intake of nutrients from the soil is called absorption. Plants absorb minerals from the soil through the roots by two ways. 

(a) Passive absorption and

(b) Active absorption.

(a) Passive absorption. It is simple process of diffusion of ions from higher concentration to lower concentration into root cells.

(b) Active absorption. The uptake of minerals ions against concentration gradient is called active absorption. Such a movement of ions is carried out with the expenditure of energy (ATP). Carriers are involved in active mineral absorption.

Carrier + ion Carrier ion complex Carrier ion enters inner space.


What are the conditions necessary for fixation of atmospheric nitrogen by Rhizobium? What is their role in N2- fixation?

The most important condition necessary for fixation of atmospheric nitrogen by Rhizobium is that it should occur in a symbiotic relationship with the roots of leguminous plants like alfalfa, sweet clover, sweet pea, lentils, peas, beans etc. Although Rhizobium also occurs as free living but can carry out fixation of atmospheric nitrogen as a symbiont.


What are the steps involved in formation of a root nodule?

Nodule Formation. Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant. Principal stages in the nodule formation are summaried as follows:

Rhizobia multiply and colonise the surroundings of roots and gets attached to epidermal and root hair cells. The root hairs curl and the bacteria invade the root hair. An infection thread is produced carrying the bacteria into the cells which leads to the differentiation of specialised nitrogen fixing cells. The nodule thus formed, establish a direct vascular connection with the host for exchange of nutrients.

Development of root nodules in soyabean:

(a) Rhizobium bacteria contact a susceptible root hair divide near it. 

(b) upon successful infection of the root hair cause it to curl. 

(c) Infected thread carries the bacteria to the inner cortex. The bacteria germ modified into rod-shaped bacteriods and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation. 

(d) A mature nodule complete with vascular tissues continuous with those of the root.

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