By Muhammad Sabir, Dr Syed Iqrar Hussain & Saif Ullah
Plant growth is a function of different environmental factors such as light, temperature, water, humidity and nutrition. In addition, it is greatly influenced by different pathogens and insects/pests resulting in low yield.
Better understanding of these factors and their role is pre-requisite to gain maximum benefits in terms of growth and yield. Inadequate supply of these factors is called environmental stress and is classified into biotic and abiotic stresses. The former are due to different pathogen and insects/pests while the latter include temperature, drought, salinity, metal toxicity and imbalanced nutrition.
Nutrition is the most important among all and plays a crucial role in maintaining plant’s growth and tolerance to different biotic and abiotic stresses. Sixteen mineral elements have so far been identified essential for growth. Some mineral elements are absorbed in greater amount and are not considered essential. Although, essentiality of such elements is not recognised but these enhance plant growth and include silicon (Si), sodium (Na) vanadium (V) and cobalt (Co).
Silicon (Si) is the most important of these non-essential but beneficial elements. Plants’ uptake of Si is often greater than nitrogen and potassium in tissues. Despite high concentration in plants, Si still is striving for the status of essential element. Among terrestrial plants, only the horsetails have been conclusively shown to require Si as an essential nutrient. Depending upon the Si concentration in tissue, plants are classified as silicon accumulators and non-accumulators.
Normally most dicots are silicon non-accumulators (except cucumber and watermelon) and have Si concentration less than 0.5 per cent while most of the grass family members (particularly rice and sugarcane) are Si accumulators (having Si concentration more than five per cent).
However, within plant species or even cultivars, tissue levels of Si vary in relation to soil Si availability. Silicon enhances growth by improving tolerance against pathogens/insects, salinity, water-logging, metal toxicity, nutrient deficiency/toxicity.
Although Si improves plant tolerance against both biotic and abiotic stresses but its role against abiotic stresses is important. Silicon plays an important role in alleviating salt stress in plants, decreases sodium uptake and its translocation to the shoots of salt sensitive plant by decreasing transpiration.
Silicon plays a very important role in drought tolerance because silicon fed plants maintains higher leaf water potential. This is assumed to be due to the formation of silica-cutical double layer on the epidermis. In addition, endodermal tissue, which plays an important role in water transport across the root, accumulates large amounts of silicon in mature drought-tolerant plants.
Plants uptake soluble Si from the soil and deposit it in apoplast in the epidermal cell walls thus restricting penetration of fungal germ tube into the epidermis. In fact, Si improves plant resistance against fungi by improving its defensive mechanisms. Plants, which are fed Si accumulate different phenolic compounds like fungitoxic which kills the fungal hyphae that penetrate the cells. In addition to inhibiting fungal diseases, silicon ameliorates mineral imbalances and other diseases caused by abiotic stresses in plants.
Silicon alleviates metal toxicity by co-precipitation in the cell wall. Several studies have shown that Si can reduce or prevent manganese (Mn) and iron (Fe) toxicity and may also have beneficial effects on aluminum (Al) toxicity. Silicon does not seem to affect Mn uptake, but rather Mn distribution in plant tissues.
When Si concentration in tissue is low, Mn tends to be distributed unevenly and accumulates to toxic levels in spots in leaves while adequate Si supply cause homogeneous distribution of Mn thereby preventing its accumulation in leaves. Silicon alleviates an otherwise detrimental nutrient imbalance between zinc and phosphorus.
Furthermore, in sugarcane, there is evidence that Si may play an important role in protecting leaves from ultraviolet radiation damage by filtering out harmful ultraviolet rays. Silicon has the potential to significantly decrease the susceptibility of certain plants to both biotic and abiotic diseases.
Furthermore, in plants such as rice, Si fertilisation may even increase growth and yield in addition to reducing disease severity.
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.
Silicon availability to plants is low in the soil. Although Si is the second most abundant element in the earth crust but 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 fertilisers 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 fertiliser management of different crops to improve their tolerance against abiotic and biotic stresses.
Among abiotic stresses, salinity and drought are most prominent in Pakistan and affect agricultural productivity to a greater extent. Different technologies are being adopted to cope with these stresses. In addition to all conventional techniques, plant tolerance against these stresses should also be improved. The use of Si may enhance plant tolerance against these stresses and increase growth and yield. Different surveys and research studies should be carried out to identify different crop species and cultivars within the species which are responsive to Si in terms of yield increase.
