T.M. Thiyagarajan, S. Rageshwari, L. Ramazeame, C. Partheeban
More than genetic resistance of the plant, application of appropriate nutrient at the right time, in the right dose based on the available form plays a crucial role in plant health. Factors that influence the crop health is yet to be understood in a holistic manner.
Healthy food for human beings can come only from healthy crops. Modern agronomy, plant breeding, agro chemicals such as pesticides and fertilizers, and technological improvements have helped in food sufficiency, but at the same time have caused widespread ecological damage and negative human health effects. Cultivated crops face several health issues with biotic and abiotic stresses which vary with weather, micro-climate, soil and ecology and the plant biochemistry.
A holistic understanding of the crop health in relation to the above-ground and below-ground environment and the plants’ biochemical composition is essential to provide decisive healthcare to the crops.
Soil health is the key
A better soil health environment is a condition where there is good drainage with adequate air circulation for the roots to function without suffocation; nutrient supply to meet crop needs throughout growing season which varies with the crop growth stage; favourable chemical status that does not affect the functioning of the roots and nutrient availability; soil biology that supports nutrient availability; soil organic carbon that forms the basis for the survival of microbes; and soil temperature.
Mere understanding of the soils by conventional soil testing is no longer sufficient to keep the crops healthy. We still do not test the soils for their heath and more research is required to address this lacuna.
Microbial population in the rhizosphere is the greatest reservoir in this global ecosystem. Nearly 20 percent of the photosynthates are released into the soil as root exudates. The exudates mainly comprise of sugars, amino acids, flavonoids, aliphatic acids, proteins and fatty acids. They serve as a nutrient source for the plant as well as for microbiome near the rhizosphere. Pathogen activity increases, when the requirement goes from deficiency to sufficiency. Besides, soil edaphic and biological factors play a major role in deciding the amount and composition of root exudates. The soil bacteria are attracted towards the root zone and it converts ammonium nitrogen to nitrate. Phosphorous is sparingly soluble in most soils and is often the limiting nutrient in natural ecosystem. There is some evidence that the soil bacteria can support in making phosphorous available to the plants by solubilizing the water-insoluble forms while mycorrhizal fungi help in mobilizing the immobilized nutrients from the soil.
Beneficial micro-organisms can enhance the nutrient uptake and make nutrients more available by inﬂuencing root architecture. VAM-infected Lavendula plants growth increased 8.5 times over non-infected plants. The primary cause of mycorrhizal-enhanced growth appears to be enhanced uptake of nutrients, especially phosphorous. Plant health can be successfully improved by manipulating the rhizosphere by (i) increasing the suppressiveness of the soil, decreasing the suppressive of pathogens and by inflecting host immune system. A study on increased activity of microbiome in the rhizosphere soil carried out by De Boet and his co-workers in 2003, revealed that it directly increased the competition for iron, the limiting nutrient and thereby suppressed diseases such as fusarium wilt of carnation and radish, caused by soil borne pathogens. PGPF (Plant growth promoting fungi) and PGPR (Plant growth promoting rhizobacteria) present in the rhizosphere has an ability to induce resistance to plants against pest and pathogens. Studies reveal that, plants selectively attract micro-organisms thereby increase the density of few species where, it also restricts different diversity of micro-organisms.
Microclimate influences crop health
Microclimate refers to the climate just above and within the crop canopy and in the soil root zone that can be influenced by management practices. The best crop microclimate is one that provides the most favourable growing environment and that maximizes crop productivity. Mulching produces new microclimate in the original soil, reduces transmittance of solar radiation; soil temperature and evaporation, helping in water conservation. The biological activity in the soil is also influenced.
The microclimate of a cropland is influenced by the moisture and temperature available in the soil and air; the presence of dew and frost; humidity; wind speed etc. This can affect the plant growth and germination, soil respiration, the vigour of soil biotic life, nutrient cycling, and the occurrence of pests and diseases. Microclimate management is well known in protected cultivation but is only to a limited extent in field crop cultivation.
Microclimate has been poorly studied in relation to crop health and there is no adequate research on microclimate management in relation to crop health.
Nutrients decide plant defense mechanism
Appropriate nutrient concentration of a plant has a high impact on improving the resistance of a crop while the surplus nutrients are responsible for inviting beneficial as well as harmful micro-organisms and insects. Nutrients serve as the first line of defence in a plant. Altering the nutrients concentration in a crop can directly trigger the defense mechanism of the plants to pest and diseases.
Nutrients can directly alter the physiological and biochemical condition of the host including assimilation, nutrient uptake, cell wall integrity, and etc. There are many factors that affect the severity of plant disease such as, mineral nutrients, organic amendments, tillage, date of sowing, crop rotation, mulching, pH adjustment, etc. These factors decide the availability of nutrients to the plants and pathogens. For example, root rot caused by Rhizoctonia sp, wilt caused by Fusarium sp and club root diseases can be easily managed by altering the soil pH.
The plant defense mechanism is regulated by plant hormones like salicylic acid (SA), Jasmonates (JAs), and ethylene. These hormones impart resistance to plants against pests and diseases. SA plays a role in enhancing the systemic resistance against pathogens especially biotrophic and hemi-biotrophic pathogens, which colonizes living tissue. JAs and ethylene play a role against necrotrophs, and insects which rapidly kill plant cells to obtain nutrients.
Resistance against late blight of potato can be overcome by increased K application which increases the fungistatic compound arginine in leaves.
Plant nutrients and pest incidence
Nitrogen is one of the most important factors influencing the performance of herbivorous insects. Nitrogen has been found to affect the reproduction, longevity and overall fitness of certain pests. Application of synthetic nitrogenous fertilizer resulted in the more serious insect herbivores occurrence and crop damage by reducing plant resistance. The low nitrogen contents in the plants enhances the resistance of plants against pests, while high nitrogen content leads to vigorous growth along with consequent decrease in resistance. However, the high nitrogen rate resulted in the highest per leaf mean population of jassid, whitefly, and thrips. High nitrogen, increases the reproduction of cotton aphid and high population rate of whitefly. For example, the mustard aphid infestation increased with increasing level of nitrogen.
More understanding of the soils by conventional soil testing is no longer sufficient to keep the crops healthy
High phosphorus level shortened the development time of adult aphids (Macrosiphum euphorbiae). Recent reports showed that, the application of Phosphorus reduced the population densities and damage of pod sucking bugs and Empoasca dolichi.
Potassium fertilizer is negatively associated with occurrence of aphid (Aphis glycines), leafhoppers and mites. Potassium nutrition has a profound effect on improving the attractiveness of plant for insects and pathogens as well as their subsequent growth and development.
Potassium deficiency on its own may not correlate with higher insect attack, but the subsequent impact of potassium deficiency on plants, makes plants more readily attacked by sucking insects. For instance, the increase in potassium level led to accumulation of more phenols which probably contributed to increased insect resistance in some rice cultivars. Moreover, potassium induced changes in rice plant had profound effect on insect- host interactions.
Abiotic stresses affecting crop health
Abiotic stresses that affect crop health include salinity, drought, flooding, metal toxicity, nutrient deficiency, high temperature and low temperature, shade, UV exposure photo-inhibition, air pollution, wind, hail etc. Effect of abiotic stresses on the plants and their management have been studied well.
Understanding the factors that influence the crop health has not been addressed in a holistic manner. Perfect manipulation of nutrient, plant and environment can reduce most of the pest and diseases. More than genetic resistance of the plant, application of appropriate nutrients at the right time, in the right dose based on the available form, plays a crucial role in plant health. Sometimes the rhizosphere reaction can be negative too. Some chemical compounds attracts pathogens, hence, application of correct dosage of nutrients is highly crucial. Along with nutrients, crop amendments can help to improve the microclimate as well as, serve as food source for beneficial micro-organisms. Moreover, plants are capable of recruiting micro-organisms in there rhizosphere according to their genotype. The challenge lies in maintaining the density of microbiome high in rhizosphere.
T.M. Thiyagarajan, S. Rageshwari, L. Ramazeame, C. Partheeban
Faculty of Agricultural Sciences
SRM Institute of Science and Technology
SRM Nagar, Kattankulathur – 603 203
Chengalpattu District, Tamil Nadu.