Importance of Sulphur in Plant Physiology

After reading this article you will learn about the importance of sulphur in plant physiology:- 1. Sulphate Assimilation and Reduction 2. Uptake and Translocation 3. Metabolic Functions 4. Critical Levels in Soils and Plants 5. Plant Growth and Nutrition.

Sulphate Assimilation and Reduction:

In higher plants and in green algae, the initial step of sulphur assimilation is believed to be the activation of the sulphate ion (SO) by ATP (adenosine triphosphate). In this reaction process, ATP sulphurylase enzyme catalyses the replacement of two phosphate groups of the ATP by sulphuryl group leading to the formation of adenosine phospho sulphate (APS) and pyrophosphate (Fig. 21.18)

The activated sulphate, adenosine phospho sulphate (APS), serves as substrate for the synthesis of esters of sulphate or reduction of sulphate. For the synthesis of esters of sulphate like sulpholipids the enzyme APS kinase catalyses the formation of phospho adenine phospho sulphate (PAPS) in an ATP-dependent reaction.

From PAPS the activated sulphate can be transformed to an hydroxyl group forming a ester of sulphate. The pathway, II of the above figure for the reduction of sulphate sulphur, the activated sulphate of APS and PAPS is transferred by APS or PAPS sulpho transferases to a thiol (R—SH) group of a suitable carrier. However, such transfer of sulphate to thiol groups, mediated by sulpho-transferases, is associated with the reduction process of sulphate to sulphite The subsequent reduction of carrier-bound sulphite to sulphide

For both types of reductases in chloroplasts reduced ferredoxin is the electron donor.

Such newly formed—SH group is transferred to acetyl serine, which is splitted into acetate and amino acid (cysteine). Cysteine, the first stable product of the assimilatory sulphate reduction, acts as a precursor for the synthesis of all other organic compounds containing reduced sulphur, as well as for other biosynthetic pathways like formation of ethylene.

Assimilatory sulphate reduction is believed to control by the following factors:

(i) Modulation of the activity of ATP sulphurylase

(ii) The availability of sulphate in situ at the site of ATP sulphurylase

(iii) Change in the level of APS sulpho-transferase

(iv) The availability of acetyl serine for cysteine synthase.

In higher plants the enzymes of the assimilatory sulphate reduction are localised in the chloroplasts but can also be found in the plastids of roots at a much lower level. In general, the reduction of sulphate has been found several times higher in green leaves than in roots, and in leaves the reaction is strongly stimulated by light.

Sulphate reduction in the leaves leads to export in the phloem of reduced sulphur compounds as glutathione to sites of demand for protein synthesis and possibly also for regulation the uptake of sulphate by roots.

Uptake and Translocation:

It is well known that plants mainly absorb sulphur in the form of sulphate sulphur (SO₄^2-). In the pH range to which roots are normally exposed, uptake is not very pH dependent. Selenate, however, which is closely related to SO₄^2-, reduces the uptake of sulphate by plants to a greater extent indicating selenate and sulphate possibly compete for the same absorption site.

It has been found that the absorption and subsequent translocation of sulphate within the plant takes place against an electro chemical gradient suggesting sulphate uptake is an active process. Besides, sulphate may be taken up by plants via a H⁺/SO₄^2- cotransport or an OH⁺/SO₄^2- anti-port. Carrier proteins located in the plasma lemma are responsible for the uptake of sulphate by plants.

Translocation:

Sulphate sulphur (SO₄^2-—S) is usually trans located in an upward (acropetal) direction and the ability of higher plants to move sulphur in a downward (basipetal) direction is relatively poor. The translocation of S from the roots and petioles takes place towards the younger leaves due to interruption of the SO₄^2- supply. The sulphur content in the old leaves.

Translocation does not contribute to the supply of S in young tissues indicating translocation does not occur against transpiration stream. It is evident that the partial supply of S to the plant very oftenly meets up from the atmospheric SO₂.

Once SO₂ is taken up by plants through the stomata it is believed to be distributed throughout the entire plant and it has been detected in different fractions of S namely protein, amino acid, sulphate sulphur.

Metabolic Functions of Sulphur:

Sulphate taken up by plants must be reduced because in the major S containing organic molecules, S is present in a reduced form. These organic compounds include cysteine, cystine and methionine as well as the proteins containing these amino acids. Usually, reduced sulphur is rapidly incorporated into an organic molecule, the first stable organic S compound as mentioned earlier being cysteine.

The SH (Sulphydryl or thiol) group from cysteine can be transferred to phospho homoserine to form cystathionine and subsequently breaks down to form homocysteine. This compound in turn, can be converted to methionine by a CH₃ group transfer. Again, it may be mentioned that plants are able to produce H₂S from SO₄^2- if the supply of SO₄^2- —S is very high.

About 2% of the organic reduced sulphur in plants is present in the water-soluble thiol (—SH) fraction and in general the tripeptide glutathione accounts for about 90% or more.

Glutathione serves many functions in plants and it is synthesised in two steps:

First Step:

Glutamate and cysteine → Glutamylcysteine.

Second Step:

Glycine is coupled to glutamylcysteine mediated by glutathione synthase (an enzyme with an absolute requirement of magnesium for activity).

Phytochelation:

In plants the glutathione content is usually higher in leaves than in roots, and in leaves more than 50% of it is localised in the chloroplasts. Glutathione is readily water soluble and a powerful antioxidant in plants, and is most probably of much importance than the cysteine-cystine redox system.

In the chloroplasts, the antioxidants glutathione and ascorbate both play a key role in detoxification of oxygen radicals and hydrogen peroxide.

Glutathione may also function as transient storage pool of reduced sulphur and thereby maintain a certain cellular cysteine concentration. Glutathione is also the precursor of phytochelatins which function is detoxifying certain heavy metals in higher plants.

Cells respond to high concentrations of heavy metals such as cadmium and zinc, by the synthesis of polypeptides high in cysteine content known as “metallo thioneins”.

In plants, polypeptides of low molecular weight termed as “phytochelatins”. Phytochelatins are capable of binding heavy metals via thiol co-ordination resulting detoxification of those metals like Cd, Zn etc. The synthesis of phytochelatins in roots is stimulated by cadmium and less stimulated by zinc and copper, and negligibly by nickel.

Thioredoxins, another family of thiols in higher plants, are low molecular-weight proteins of about 12 KDa (Kilo Dalton) with two well conserved cysteine residues which form a redox active intermolecular disulphide bridge.

Plant cells contain two different systems capable of reducing thioredoxins::

(i) In chloroplasts the ferredoxin or thioredoxin system, and

(ii) In the cytoplasm the NADP (Nicotinamide diphosphate) or thioredoxin system.

In chloroplasts, thioredoxins function act primarily as regulatory proteins in carbon metabolism. It is also evident that the reduced sulphur is a structural constituent of various, co-enzymes and prosthetic groups such as ferredoxin, biotin (vitamin H), and thiamine pyro phosphate (vitamin B₁).

In many enzymes and co-enzymes such as ureas, sulpho-transferases and co-enzyme A, the —SH groups act as functional groups in the enzyme reaction. Ferredoxin (non-haem iron sulphur protein) is low molecular weight protein containing a high proportion of cysteine units and an equal number of S and Fe atoms.

In the glycolytic pathway, decarboxylation of pyruvate and the formation of acetyl co-enzyme A are catalysed by a multi-enzyme complex involving three sulphur containing co-enzymes, thiamine pyro-phosphate (TPP), sulphydry-disulphide redox system of the lipoic acid and the sulphydryl group of co-enzyme A that are depicted below:

The acetyl group (—CO—CH₃) of the co-enzyme A is then transferred to the tricarboxylic acid (TCA) cycle or to the fatty acid synthesis pathway.

The most important sulphur containing compounds of the secondary metabolism are the allins and glucosinolates and the characteristic behaviour of those two compounds are as follows:

Allin is the common name for S-alkyl cysteine sulphoxides, a characteristic compound of the genus Allium.

More than 80% of the total sulphur in onion (Allium Cepa L) may be bound to such compound, as S-propyl cysteine sulphoxide. Enzymatic cleavage of alliins is carried out by the enzyme alliinase.

Loss in cellular compartmentation by mechanical damage of the tissue largely enhances the activity of enzyme through increased availability of the substrate and leads to the formation of allicins (Precursors of a large number of volatile substances such as mono- and di-sulphides with a characteristic odour or pungency).

Glucosinolates, a secondary metabolic product most commonly found in Brassicaceae. It contains sulphur both as a sulphydryl and a sulpho group, the side chain R varies between plant species.

Glucosinolates are stored in vacuoles and the hydrolysis is catalysed by the cytosolic enzymes myrosinase which is present in only a very small number of cells in a particular organ like leaf or seed. Hydrolysis results the formation of glucose, sulphate and volatile compounds like isothiocyanates in Brassica napus.

Glucosinolates serve important functions as sulphur storage in plants. During periods of low supply of sulphur to the plant, but due to higher requirement like rapid vegetative growth, seed formation etc., glucosinolates are degraded by myrosinase and both sulphur molecules are re-utilised through the normal sulphur assimilation pathway.

Besides, many plant species including garlic oil also contain volatile S-compounds as di or ply sulphides.

The main component in garlic oil is diallyl-disulphide as depicted below:

CH₂ = CH—CH₂—S—S—CH₂—CH = CH₂

Sulphur in its non-reduced form like as sulphate ester, is a component of sulpholipids and hence a structural constituent of all biological membranes. In sulpholipids the sulpho group is attached to C6 sugar by an ester bond as follows:

Sulpholipids are particularly abundant in the thylakoid membranes of chloroplasts, about 5% of the chloroplast lipids are sulpholipids involved in the regulation of ion transport across bio membranes. The plant species in which glucosinolate compounds are mainly found that have been shown above.

The high S contents usually found in the Cruciferae which are contributed mostly by glucosinolate compounds. The total S content in plant tissues is in the order of 0.1 to 0.5 per cent in the dry matter.

Structural formula of some of the important S-containing compounds is depicted as follows:

As soon as the demand for S by the plant was satisfied, the content of SO₄—S was found increased while that of amount of organic sulphur content remained almost constant. It is evident that the oil content in mustard has been largely dependent on the supply of sulphur to the plant.

Increasing sulphur application enhances oil content in mustard even after attaining maximum growth. During senescence, proteins are hydrolysed (proteolysis) and amino acid sulphur released can frequently be oxidised to SO₄—S form.

In tltis respect organic sulphur differs fundamentally from organic N, which cannot be oxidised to NO₃—N in plant tissues. The bulk of organic sulphur consists of protein sulphur in the form of cysteinyl-and methionyl-residues excepting the plants containing S-glycosides.

The N/S ratio of proteins does not vary much as the protein has a definite composition. Proteins in chloroplast and that associated with nucleic acids, however, have lower N/S ratios because of greater amount of S in these proteins.

Critical Levels in Soils and Plants:

Chemical analysis of plants is considered as an effective and quick method to diagnose visual and hidden S deficiencies. Critical level of S in plants grown on diversified types of Indian soils vary with type of crops, crop variety, crop species, age, plant parts etc.

Plant-absorbed S is almost equally divided between “grain and straw. The seeds of cruciferous plants namely mustard, raya and gobi sarson have the highest S content ranging from 1.10 to 2.00 per cent, legume crops ranging from 0.24 to 0.50 per cent and cereal crops ranging from 0.15 to 0.20 per cent.

From several experiments in India, it has been found that concentrations of 0.26, 0.24, 0.15 and 0.14 per cent sulphur in the youngest fully matured leaves (about 70 day old crops) of mustard, gram, black gram and pea respectively serve as the threshold concentration. In most of the cases, however, various crops exhibit a deficiency symptoms of S when plant leaves contain S less than 0.10 per cent.

It is known that S deficiency first appears in the early stages of crop and so soil analysis may have some advantages over that of plant tissue tests and visual deficiency symptoms. The critical level of S in soils may vary with soils and nature of extractants used.

There are various extractants which may be used for the available S status in soils namely 0.15% CaCl₂ extractable, Morgan’s reagent, calcium monophosphate, 1% NaCl, 0.5 M NaHCO₃, heat soluble 0.005 M DTPA and 0.01 M HCl.

Out of all above extractants, 0.15% CaCl₂ is superior to other extractants with the exception of soils containing higher amount of organic matter. The critical level of S in soils extractable with 0.15% CaCl₂ is 10 mg kg^-1. Soils testing < 10 mg kg^-1 S in soil may be considered as low category, between 10 to 15 mg kg^-1 in soil as medium and > 20 mg kg^-1 S in soil as high where no responses to S application.

Plant Growth and Nutrition:

The amount of S content in most crops is found in the same order of P content in crops. The uptake of S is not the same with that of phosphorus by plants, only because of the fact that the SO₄—S is not as strongly bound to soil colloids as phosphate.

Besides, a substantial amount of S taken up by crops from the atmospheric sources or from the applied S-fertilizers. Some of the atmospheric SO₂ is dissolved in precipitation and enters into the soil where it becomes oxidised to SO₄—S by the following reaction,

SO₂ + O₂ → SO₄.

This process may contribute acidity to the soil. It is evident that the same amount of S in the form of SO₂ is absorbed directly by crops where high concentrations of SO₂ are present in the atmosphere. On the other hand, a large quantity of SO₄—S can be leached when rainfall is high.

The amount of such leached S frequently far exceeds the amounts taken up by crops. It was also found that the amounts of S supplied by rainfall and fertilizer were less than that of S absorbed by crops and leached due to rainfall. Such higher amounts of absorption may be directly from the atmosphere.

A requirement of S for optimum crop growth varies from 0.1 to 0.5 per cent by weight of dry matter. However, the requirements of S vary with the nature and type of crops. The requirements decrease in the order of:

Cruciferae > Leguminosae > Gramineae.

The deficiencies in sulphur content in seeds of Gramineae, Leguminosae and Cruciferae families are 0.18-0.19; 0.25-0.30 and 1.1-1.7 per cent (dry weight basis) respectively.

The sulphur content of protein also varies greatly between the protein fractions of individual cells and plant species. On an average, the protein of leguminous crops contains less sulphur than that of proteins in cereal crops, being N: S ratio as 40: 1 and 30: 1 respectively.

In sulphur deficient crops, the shoot growth is more inhibited as compared to root growth, leading to a decrease in shoot : root dry weight ratio in tomato from 4.4 to 2.0, Interruption of sulphur supply decreases root hydraulic conductivity, stomatal aftertime and net photosynthesis. Leaf areas of plants are reduced due to S deficiency resulting both smaller size and number of leaf cells.

It has been mentioned earlier that the synthesis of proteins in plants are also inhibited due to S deficiency which also affect the crop quantity. Sulphur is involved in the formation of chlorophyll, activation of enzymes, in the formation of glucosides or glucosinolates, and the SH-sulphydryl linkages which provide pungency to onion, oils etc.

Sulphur is a part of co-enzyme A, Pyrophosphate, vitamins viz. biotin and thiamine (B₁). Sulphur enhances crop yield, oil yield, protein content, quality of cereal crops in relation to milling and baking, and quality of various other crops.

Sulphur uptakes by various crops vary from 5 to 80 kg ha^-1 under actual held conditions. However, its uptake depends on various factors like type of crop and variety, intensity of crops, yield level, sulphur content in soil, amount of external sulphur input and nature of other interacting ions present etc. However, the uptake of S by different crops usually varies from 9 to 15 per cent of the N-uptake.

In oil seeds like mustard, the uptake of S is found highest, being one third of N- uptake. As a thumb rule, crops absorb same amount of S as with that of phosphorus uptake. Let P uptake is 100, then the uptake of S by mustard can be taken as 180, 78 for leguminous oil seeds (groundnut, soybean) and 102 for other oil seeds.

It is well known that iron chlorosis, one of the most wide spread nutritional disorders affecting field crops and fruit trees in particular, is easily recognised by the characteristic yellow leaves that result from a lack of chlorophyll due to insufficient supply of Fe. Iron deficiencies can be corrected using inorganic and organic compounds of Fe either through soil or foliar application.

The application of inorganic salts of Fe viz. iron sulphate to the soil has been found to be very less effective in correcting Fe deficiency because of its unavailability to plants resulting from various harmful reactions with different soil components. Again, iron chelates although, are very effective, but these are most expensive.

Therefore, the use of elemental sulphur to the soil is believed to be the most effective means of correcting Fe-chlorosis in plants. On its application to the soil elemental sulphur is subjected to microbial oxidation forming strong mineral sulphuric acid (H₂SO₄) which increases solubility of Fe.

Since the log K is a constant, if pH decreases one unit, the term log [Fe³⁺] has to increase three units. Three log units equal a factor of 1000 and Fe-containing compounds in soils and thus reduce the severity of Fe-chlorosis in plants.

It has been found that the application of 1 kg wettable S (80% S) per fruit free (banded on both sides of a tree now), allowed for excellent Fe-chlorosis control as compared to 50 g FeEDDHA (Chelate) per fruit tree.

It is usually considered that the nitrogen: sulphur ratio (N/S) in the protein fraction is a constant for a particular plant species. This ratio can change depending on the amount of sulphur available. Generally, as the supply of sulphur decreases, the N/Sp ratio widens, but in an acute sulphur deficiency, the ratio can actually decrease due to a change in the composition of the protein.

An accumulation of non-protein nitrogen sulphur will result in corresponding change in the total nitrogen/sulphur (N/S) in the plant. Therefore, a high N/S ratio indicates sulphur deficiency, while low ratio of the same indicates no sulphur deficiency. This N/S ratio is used to measure the sulphur status of the plant.

A ratio of 15 has been suggested as being applicable to most crops, with the exception of plants belongs to the Brassica family. Besides, total phosphorus/total sulphur ratio in many cases used as a useful guide to sulphur nutrition and a critical ratio (total P/total S) of about 1.0 has been found out for lucerne.

Unless other nutrients are present in sufficient quantities in the soil, responses to sulphur may not be evident, and conversely, unless sulphur is provided in adequate amount, responses to nitrogen, phosphorus and potassium may be very little.

Because of the similarity of charge and size, the SO₄^2- ion exerts a decreasing effect on the uptake of Mo and Se. In view of the facts, better understanding of the nature and site of interaction of S with N, P, Zn and B in soils and plants will help in enhancing the fertilizer use efficiency as well as in improving the quality of the produce.