Silicon augmented resistance against plant stresses

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Different environmental factors such as light, temperature, water, humidity and nutrients work in an integrated fashion for better plant growth. However still certain types of pathogens and insects/pests remains a challenge and a major biological constraints of normal plant growth and development. Hence in order to get maximum benefit in terms of yield and growth better understanding of all these factors and their role is very important. Inadequate supply of these factors is called environmental stress and is classified into abiotic and biotic stresses. The former are due to temperature, drought, salinity, metal toxicity and imbalanced nutrition while the latter include the activity of different pathogen and insects/pests. Among the factors balanced nutrition is of paramount importance and plays a critical role in maintaining plant’s growth and tolerance to different abiotic and biotic stresses. Sixteen mineral elements have so far been recognized essential for growth. However some mineral elements are absorbed in greater amount but are not considered essential. Although, essentiality of such elements is not recognized but these enhance plant growth and include silicon (Si), sodium (Na) vanadium (V) and cobalt (Co). Especially, they work “behind the scene” as activators of many plant functions.

Silicon (Si) is the most important of these non-essential but beneficial elements which has a key role in plant-environment relationships because it can improve plants’ abilities to withstand edaphoclimatic and biological adversities by acting as a “natural anti-stress” mechanism that enables higher yields and a better-quality end product. Si is absorbed by plant roots in its neutral form (H4SiO4) through a passive process regulated by transpiration stream or by an active process, through transporters located in the plasma membrane of root cells. The absorbed Si accumulates in the plant’s oldest tissues, mainly in the walls of epidermal cells which strengthens cell walls and increases the structural rigidity of tissues.          

There is great variability in the genetic ability of crop plants to accumulate Si. Plants, in general, have Si levels ranging from 0.1 to 10% of their total dry matter. Those with leaf levels above 1% of their dry matter are considered accumulators and those with levels below 0.5% are considered non-accumulators. The potential benefits of Si nutrition in plants includes the enhancement of growth and yield, improvement of mechanical properties, reduction of transpiration and resistance to drought stress, salinity, metal toxicity and insect herbivory. In water-deficit environments, Si accumulation in leaves causes the formation of a double layer of cuticle silica which reduces the transpiration rate, decreasing the opening of stoma and limiting the loss of water in plants. Silicon fertilizer application can also alleviate the adverse effects of salt stress on plants by increasing cell membrane integrity and stability through its ability to stimulate the plants’ antioxidant system. Silicon deposited in the cell walls of roots, leaves and stalks as silica gel reduces Na absorption and transport thus alleviating the adverse effects of salt on plant growth. Silicon can also reduce the toxicity of potentially toxic elements, such as Al, within the plant through the stimulation of the antioxidant system, metal ion chelation, immobilization of heavy metals during plant growth or compartmentalization of heavy metals in vacuoles, the cytoplasm or the cell wall.

Increased abrasiveness and hardness of epidermal plant tissues and reduced digestibility as a result of deposition of Si primarily as phytoliths (stones of the plant) within cuticle layer is now the most authenticated mechanisms of Si in enhancing plant resistance to insect herbivores. This mechanism may affect the insect herbivores by direct or indirect action. Direct action mainly results in reduced insect reproduction and growth which leads to an associated lower crop damage. Indirect action of Si on insects usually affect the insect mortality rates which may be due to various factors as reduced or delayed plant penetration by the insects. This leads insect herbivores to remain vulnerable for a greater time period to climatic extremes, bio-control agents and other management techniques such as chemical control.

Decreasing availability of fresh water, increasing salinity/sodicity problem and other environmental stresses decrease agricultural productivity. As these stresses decrease plant growth and yield, plants needs to be well equipped to withstand these stresses and maintain good growth and yield. In this scenario, inclusion of Si in nutrient management of plants seems promising for enhancing crop productivity. Silicon application makes plants more tolerant against these stresses and thereby plants maintain good growth and give economic yields.

Farmers are intensifying land use without proper nutrient management practices which results in depletion of nutrients from soil, which is linked to yield decline. Hence in an intensive cultivation system aiming high productivity, a proper fertilization program is fundamental. Although Si is the second most abundant element in the earth crust after oxygen but its availability to plants is still low because it is present in the form of quartz (SiO2) which is inert. Plants uptake Si in the form of silisic acid Si (OH)2. There is a need to provide plants with soluble source of Si for which different Si fertilizers are being used. In Pakistan, Si is not well known as beneficial element for plant growth. However there is a need to include the Si in fertilizer management of different crops to improve their tolerance against abiotic and biotic stresses.

This article is collectively authored by Asim Abbasi, Dr. Muhammad Sufyan, Hasnain Sajjad, Dr. M. D. Gogi, Dr. Ahmad Nawaz -Department of Entomology, University of Agriculture Faisalabad, Pakistan.


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Published in: Volume 08 Issue 30

Short Link: http://www.technologytimes.pk/?p=17577