Authors: Madhu Choudhary*, Kana Ram Kumawat, Ravi Kumawat and Rajni Verma
Department of Plant Breeding and Genetics, S.K.N. Agriculture University, Jobner-303329, Jaipur (Rajasthan), India
*Corresponding Author E-mail: anamikaz0129@gmail.com
Abiotic stresses generated by mineral salts affect a considerable proportion of the arable land (20%) and they rank second after moisture stress. Nutrient absorption by plant roots follows Michaelis-Menten relationship. Mass flow of ions is usually able to provide the required amounts of N, S, Ca, and Mg but not of P, K and some minor elements. When plant performance is adversely affected by deficiency or excess of essential nutrient in soil, it is called mineral-stress. Plants respond to mineral stress by enhancing their root growth relative to that of shoot and by increasing the availability of P, Zn and Mn particularly in deficient soils.
Resistance to Mineral Deficiency Stress
Genotypes that show markedly lower detrimental effects of mineral deficiency than do other genotypes of the same species are called mineral deficiency resistant genotypes and the phenomenon is known as mineral deficiency resistance. Such genotypes are often referred to as ‘mineral-efficient’ genotypes. These genotypes are usually able to absorb and/or transport to the appropriate sinks more quantities of the deficient mineral than are other genotypes. This strategy is known as avoidance of mineral deficiency as it prevents deficiency of the concerned mineral in the relevant tissues of plants. In contrast, tolerance to mineral deficiency refers to a lower level of injury to essential life processes in some genotypes than in others when their tissues contain a comparable and a lower than critical concentration of the given element; this phenomenon seems to be rare.
Mechanisms of Mineral Deficiency Resistance
Generally, mineral-deficiency resistant genotypes perform as well under deficiency conditions as do susceptible genotypes under non-stress conditions. The various mechanisms that confer resistance to mineral deficiency are as follows:
- Mineral Redistribution In this strategy, minerals present in older, senescing leaves are mobilized to younger tissues. Redistribution of N, P and K is well-documented. In some cases, redistribution of Mg is also known. Curiously, mobility of Cu is reported to decrease under Cu deficiency stress.
- Efficient Mineral Uptake Mineral uptake is enhanced by acidification of rhizosphore due to secretion of organic acids by plant roots. In addition, roots of Fe-efficient genotypes release H+ ions in the rhizosphere, which reduces Fe3+ to the more available Fe 2+. The availability of different genotypes to acidify the rhizosphore may be differentially affected by temperature.
- Increased Mineral Transport In calcareous soils, Ca competes with or inactivates Fe; this causes iron chlorosis in susceptible genotypes. Fe-chlorosis resistant genotypes avoid interference due to Ca and apparently show an enhanced transfer of Fe from cortical root cells to xylem e.g. in maize variety WF9. In addition, transport of minerals from roots to appropriate sinks is also involved in deficiency resistance. Low mobility in phloem appears to be a problem in case of elements like B, Ca and Mn. Genotypic difference for mobility of at least B and K are known.
- Increased Root/Shoot Ratio Apparently, the signal for enhanced root mass is perceived much before mineral deficiency causes cessation of photosynthesis. Most likely, this signal involves root cytokinins. Extensive formation of root laterals under stress tends to support this conclusion. The available evidence reveals that genotypes with larger root system are better at avoiding mineral deficiency stress than are those having a smaller root system. It may be pointed out that the yield increase in high yielding varieties is due largely to improved harvest index, which is likely to restrict the dry matter allocated for root growth in high yielding varieties. Therefore, in such crops where root size is an important mechanism of deficiency resistance, high yields are likely to be opposed to mineral deficiency resistance.
- Increased Root Hair Density/Length Root hair density or length is positively associated with P and K uptake. However, there is no evidence that root hair formation or length is promoted by mineral deficiency stress.
Ample within species genetic variation exists for resistance to mineral deficiency stress and in many crops, cultivated varieties were found to be resistant. In many cases, e.g. P efficiency in several crops, the trait was reported to be polygenic in nature with low to high heritability. Such expression may involve nonspecific, indirect effects on mineral nutrition, such as, those of root size, growth duration, growth rate and plant size. Similarly, the cases of heterosis in mineral absorption may be the secondary effect of heterosis on growth and development. Nitrogen nutrition was studied in various primitive and modern varieties of rice but appeared to be comparable in their N-efficiency. Thus modern rice varieties yield more simply because of their ability to avoid lodging and mutual shading at high N supply (Blum, 1988).
Table 1: Genetic Control of Mineral Deficiency Resistance
| Characteristics | Crop | Genetic control | Remarks |
| B-inefficiency | Tomato | Btl | Causes brittle stem |
| B-inefficiency | Celery | Single recessive gene | - |
| Cu-efficiency | Rye | Long arm of 5R chromosome | Associate with hairy peduncle (Hp) |
| Fe-efficiency | Soybean | Fe | Minor gene involved |
| Fe-efficiency | Tomato | Fer | Minor gene involved |
| Fe-inefficiency | Maize | Ysl | Mutant; unable to use Fe3+ ions |
| K-efficiency | Wheat | Ku | Enhances K uptake from sodic soils |
| Mg-inefficiency | Celery | Single recessive gene | |
| Mn-inefficiency | Barley | Polygenic or single dominant gene (?) | Introduction from Egypt |
| Mn-efficiency | Barley | ? | Mutant (Ɣ-irridiation) |
| P-efficiency | Several | Polygenic | Association with root attributes |
| P-efficiency | Maize | Chromosome 9 | - |
| P-efficiency | Rye | Three chromosomes | - |
| P-sensitivity | Soybean | Np | np lines show several splotching and chlorosis |
(1) Cultivated varieties, (2) land races or desi (local) varieties (3) mutants and (4) related species. In some cases, mineral-deficiency resistance has been found in cultivated varieties, especially in those, which were bred in/adapted to areas that are deficient in specific minerals. For obvious reasons, utilization of such resistance in breeding programmes is the easiest. Local varieties adapted to mineral deficiency are good genetic resources for resistance. Similarly, related species adapted to problem soils may also be useful sources.
Selection Criteria
Selection for mineral deficiency resistance may be based on (1) visible deficiency symptoms, (2) mineral contents of plant tissue, (3) biochemical assay and (4) yield; these features are to be evaluated under the specific deficiency stress.
- Visible Deficiency Symptoms Deficiencies of different elements produce recognizable visible symptoms (Table 2). But these symptoms are often affected by plant genotype unrelated to deficiency resistance by the environment. In addition, they appear only when the mineral concentration is below a certain threshold, while in many marginal situations deficiency would exist without producing visible symptoms. Deficiency symptoms can be supplemented or replaced by carefully selected chemical or biochemical tests but they are far more convenient, easier and cheaper to score. Table 2: Visible deficiency symptoms of different mineral nutrients
| Nutrient | Deficiency symptom |
| B | New leaves first affected; leaves crinkled and brittle; stem dark bronze in colour, hollowed and finally decays |
| Ca | Young leaves first affected; yellowish specks initially; leaf tip tightly curled; sticky liquid on dead leaves |
| Cu | New leaves first affected; yellow to bronze colour first appears at tip |
| Fe | New leaves first affected; intense yellowing of intervenous tissues, which necrose rarely |
| K | Older leaves first affected; bronze to yellowish brown necrotic lesion |
| Mg | Older leaves affected first; light yellow discolouration between veins; margins curled; dark necrotic spots; reddish or purple in colour |
| Mn | Older leaves first affected; tissue b/w veins light in colour, later turns necrotic |
| Mo | Older/younger leaves; mottled and curled; lighter coloured intervenous region |
| N | Older leaves first affected; general yellowing |
| P | Older leaves first affected; leaves dark green to dark purple |
| S | First in new leaves; homogeneous light yellow to yellow |
| Zn | Small yellow new leaves; stem elongation inhibited; rosette-like plants in some cases |
The content of concerned element may be determined in the target tissues of plants growing under mineral deficiency. Mineral contents are affected by growth, tissue age and various environmental factors. Chemical analysis may prove useful in plant selection if various problems related to plant age; sampling and the specific mineral are accounted for.
- Biochemical Tests Activities of certain enzymes and accumulation of specific biochemical’s are good indicator of the intensity of deficiency of specific elements (Table 3). The concerned element may either be a component of active enzyme or it may be mobilized by the concerned enzyme. These tests are highly specific and are useful in selection criteria. These tests can detect range of deficiency levels much before the visible symptoms appear. Table 3: Some biochemical assays (enzyme activities or biochemical accumulation) that serve as good indicators of the status of specific elements
| Enzyme/biochemical | Activity increases with | Crop | Remark |
| Phosphatase | P-deficiency | - | Satisfactory |
| Pyruvate kinase | K content* | Barley | Correlated well with efficiency |
| Peroxidase | Fe content* | Oats, citrus, tomato | Useful indicator |
| Pentose accumulation | Mn deficiency | - | Colorimetric assay for pentose |
| Ascorbic acid oxidase | Cu content* | Several | Useful indicator |
| Carbonic acid anhydrase | Zn content* | Several | Sensitive and useful |
Yield under deficiency stress appears as an obvious selection index for resistance to mineral deficiency. However, heritability of yield declines markedly when yield decreases due to various stresses. Therefore, yields of different genotypes should be determined both under mineral deficiency stress and non-stress conditions and selection criteria should be based on mean yield under stress as well as the ratio between the yields under stress and non-stress conditions.
Criterion of Mineral Deficiency Environment
Selection for mineral deficiency resistance may be carried out
- In the field
- In a greenhouse using 2a) soil-filled pots 2b) hydroponics
References:
1. Blum, A. (1988). Plant breeding for stress environments. CRC Press, Inc., Boca Raton, Florida.
2. Singh, B. D. (2015). Plant breeding principles and methods. Tenth Revised Edition, Kalyani Publishers, New Delhi.
3. Singh, P. (2010). Essentials of plant breeding. Fourth Revised Edition, Kalyani Publishers, New Delhi.
About Author / Additional Info:
I have completed M.Sc. (Ag.) degree from S.K.N. Agriculture University, Jobner.