Boron Nutrition in Rice Plants
Boron nutrition is essential for rice growth and yield. It improves plant water relations and increases the total chlorophyll content in rice plants. It also enhances the kernel quality and reduces panicle sterility for rice plants.
Rice is a staple food crop providing more than 20% of the daily energy needs of half of the world’s population. Globally, it meets 21% of the energy and 15% of the protein requirements for humans. Rice is primarily grown in lowland (puddled-transplanted) and upland conditions (rain-fed cultivation without standing water).
Even though rice is grown in many developed countries, it is more common in low and low-middle-income countries, making up 19% of all cropland. It is increasingly grown without standing water to save water. With escalating wages, labour scarcity, water shortages, and nutrient mining, micronutrient deficiencies (especially Boron) are becoming a more serious problem.
Boron nutrition is quite essential for plant health. Its soil deficiency reduces crop yield, degrades grain quality, and makes crops more susceptible to disease. However, B requirements differ between plant species. They are affected by various other factors such as soil type, soil moisture, pH, and so on.
Boron deficiency has been discovered in rice-growing regions. Boron-deficient rice is yellowish or white. It causes panicle sterility in rice due to pollen germination and changes in cell wall pectin in pollen tubes. It also results in reduced grains per panicle.
On the other hand, B toxicity has also been observed with symptoms such as inter-veinal chlorosis in older leaves and dark brown elliptical spots on affected plant parts.
The Boron Effect in Plant Biology
Boron is a vital nutrient that plays many roles in plant physiological processes. Its primary function is its structural role in cell walls and the maintenance of plasma membrane functions. Plants are far more sensitive to B deficiency during their reproductive stage than during their vegetative phase.
BOR1, a B-specific transporter, is actively involved in B transport regulation. Its messenger RNA resides in roots and shoots. Furthermore, B is required for tissue growth, membrane function, pollen germination, cell division, and assimilate partitioning.
Boron Requirements by Rice Plants
Plant species have different requirements and responses to applied Boron. Its need for reproductive and vegetative growth is relatively high in monocots, including rice, for normal pollen and grain development. Anthers require > 20 mg/kg compared with 3 mg/kg in the flag leaf. Significant reductions in reproductive and vegetative growth occur below this concentration. Rice in Pakistan responded favourably to 0.5–1.0 kg B/ha applied at panicle initiation.
Factors Affecting the Availability of Boron in Rice
Factors like high pH, alkalinity, organic matter, soil adsorptive capacity, amounts and types of minerals in the soil, soil water contents etc., affect Boron availability to plants.
Soil pH is the most significant factor determining Boron availability. Soil soluble B content (0.10–0.45μg/g) is inversely related to soil solution pH. As a result, increasing the pH of the soil solution reduces Boron availability to plant roots, resulting in severe B deficiency in rice crops, particularly in calcareous soils. Soil adsorptive capacity increases at pH corresponding to boric acid pKa, i.e., nearly 9.1.
Rice grown in saline soils and with high-sodium irrigations suffers from B-deficiency because of an inverse relation between soil B-availability and a high sodium adsorption ratio (SAR). The effect of excess B is significantly reduced with increasing salinity.
Soil Organic Matter
Organic matter content varies across rice production systems. For example, organic matter decomposes faster than in anaerobic systems in aerobic rice production. It accumulates in the soil due to its slow decomposition in submerged conditions. Wetland rice production absorbs around 11–20% more C than aerobic rice production increasing soil fertility, essential for the system’s long-term sustainability.
Diagnosing Boron Deficiency in Rice
Boron is an essential mineral that helps improve rice crop growth and development. Therefore, the correct diagnosis is required to measure B deficiency.
Various methods such as visual deficiency symptoms, soil testing, and plant analysis are used for diagnosis. Furthermore, knowledge of the soil and other factors known to exacerbate B deficiency aids in accurately diagnosing its deficiency.
In Pakistan, the dilute hydrochloric acid (HCl) method has been developed to measure boron availability in acidic soils. It is also effective for predicting boron deficiency in alkaline and calcium carbonate soils. Critical concentrations of boron deficiencies in rice at flag leaves are 6mg B/kg.
Managing Boron Deficiency in Rice
Boron application to rice production systems on low B soils increases yield while alleviating boron deficiency. However, the efficacy of various application methods and B sources may differ.
Fertilization with Boron Enhances Yield and Grain Quality
Rice crops were once thought to be resistant to B deficiency. Recent research, however, has revealed that B deficiency in rice soils is a significant cause of low kernel yields.
According to Dunn et al. (2005), B fertilizer in rice increased yield significantly. In the 1970s, B fertilizer in farmers’ fields in Pakistan (Punjab) increased rice yields by 14%. However, boron in soils and rice plants was not observed until 1967.
Field experiments in rice-growing areas of Punjab, Pakistan, over multiple locations and years in low B calcareous soils (0.21–0.42 mg B/kg) with low organic matter (0.8–1.8%) revealed B deficiency as a widespread nutritional problem.
Boron fertilization increased paddy yields in several rice genotypes, attributed to decreased panicle sterility and an increase in panicle size.
Boron fertilization improves crop yield and rice grain quality in deficient soils. Soil-applied B improved milling return and head rice recovery, grain, and cooking properties. Furthermore, it enhanced desirable cooking attributes such as the quality index, kernel elongation ratio, bursting (during cooking), and alkaline spreading value. Better grain filling was attributed to better cooking quality in rice with Boron nutrition. As a result, adequate Boron nutrition of rice plants appears to be a requirement for achieving optimal yields and quality rice.
Discussion and Future Directions
A soil boron deficiency threatens rice yields. New rice varieties with higher yield potentials have been developed. Still, soil boron deficiency remains a significant threat to crop production. Rice is an important staple food for billions of people worldwide. Fertilizer applications help produce more rice, but there are many factors affecting the amount of B in rice grains. Research should focus on finding new sources of B, developing better ways to apply B, and studying how B affects human health.