Boron uptake is closely related to the boron concentration of the soil and the rate of water transpiration by rice plants. This means that rice plants can be particularly susceptible to boron toxicity.
When there is a large concentration of boron in the soil, boron can accumulate in leaf margins and leaf tips. Excess boron causes stunted grain filling but normal leaf growth. Rice varieties with a large boron requirement are less susceptible to boron toxicity.
What causes boron toxicity in rice?
Boron toxicity can be caused by one or more of the following:
- Large boron concentration in the soil because of:
- Boron-rich groundwater and hot temperatures (arid regions, very deep tube-wells, or wells in areas affected by geothermal activities)
- Boron-rich parent material, including some marine sediments or plutonic rocks and other volcanic materials such as tuff)
- Overuse of borax or large applications of municipal waste (compost)
Where is boron toxicity most likely?
Boron toxicity is most common in arid and semi-arid regions, but has also been reported in rice in other areas. Soils prone to boron toxicity include the following types:
- Soils formed on volcanic material where the use of irrigation water pumped from deep wells is common. These wells typically contain a large boron concentration (eg IRRI farm, Los Banos, and Albay, Philippines)
- Some coastal saline soils
What does boron toxicity look like?
Rice plants with boron toxicity will show the following symptoms:
- Initially, brownish leaf tips and dark brown elliptical spots on leaves
- Two to four weeks later, dark brown elliptical spots appear on the discolored areas
- The spots turn brown and dry up
Necrotic spots are most prominent at panicle initiation. Some varieties exhibit discoloration only at leaf tips and margins. Vegetative growth is not markedly reduced. The extent of yield reduction on high-boron status soils varies among varieties and is not clearly related to the severity of typical necrotic symptoms.
Toxicity levels in rice plants
Critical toxicity limits of boron in leaves have to be interpreted with caution:
- There is a steep concentration gradient of boron within a leaf blade, from low values at the leaf base to high values at the leaf tip.
- Critical toxicity levels in field-grown rice are lower than those of plants grown in the greenhouse because of boron leaching from leaves due to rain.
- The effect on yield differs significantly among rice varieties.
Toxicity levels in irrigation water
Boron concentration is hazardous at > 2 mg boron L -1.
Boron toxicity may become more severe in the dry season. Boron concentration in deep-well irrigation water is larger when there is little rainwater to dilute it in irrigation water and/or leach boron from the soil and rice plants.
Toxicity levels in soil
The critical toxicity limits of boron in the soil are:
- > 4 mg boron kg-1: 0.05 N HCI
- > 5 mg boron kg-1: hot-water soluble boron
- > 2.5 mg boron L -1: Soil solution
How to prevent boron toxicity
There are a number of ways to prevent boron toxicity in rice, including:
- Variety choice: Choosing boron tolerant rice varieties, such as IR42, IR46, IR48, IR54, and IR984-54. Boron toxicity tolerant varieties can yield up to two times / ha-1 more than susceptible varieties.
- Water management: Use surface water with a low boron content for irrigation. Groundwater must be monitored regularly if used for irrigation. If the boron concentration is too great, dilute with water from a different source containing a small concentration of boron.
- Soil management: Plow when the soil is dry so that boron accumulates in the topsoil. Then, leach with water containing a small amount of boron.
How to treat boron toxicity
Leach with low boron irrigation water if percolation is sufficient and a suitable water source is available.