Don’t miss boron in your foliar spray program during flowering

Written on 06/30/15

Boron deficiency occurs in most of the agricultural soils, especially soils with a low pH, sandy texture or low organic matter.  Soils with high pH, high clay minerals and low organic matter may also show boron deficiency due to the high Boron adsorption capacity. 

State of Boron in Soils across Western Canada 

In Western Canada, soil samples from analytical labs, report that 65%, 32% and 44% of the soils tested from Alberta, Saskatchewan and Manitoba respectively, are significantly deficient in boron. In addition, the results also show that for high yielding growers, approximately 80%+ of the soils will show an agronomic response to a Boron application. 

Figure 1. Boron levels in Western Canada (2014).  The critical threshold level is 1.0 ppm. 

Boron Mobility - How does Boron move within the plant? 

We need to be aware that even if soils have high Boron levels, plants may still develop boron deficiency irrespective of soil conditions. The reason is, for most crops, Boron cannot be sufficiently transported into the reproductive organs (shoot tips, buds, flowers, seeds, etc) due to the plants very poor phloem* mobility. Boron is the second worst nutrient for phloem mobility, with the worst being calcium. 

As a result, Boron is transported mainly to the reproductive parts through the Xylem*. However, xylem transport of nutrients depends on the transpiration capacity of these plant parts, and the reproductive organs have a very low rate. This explains why, actively growing reproductive organs with low transpiration capacity are often at risk of a Boron deficiency, even when there are high Boron concentrations in the leaves. 

*Phloem and xylem are the two transportation channels for water and nutrients in plants. 

Role of Boron in Pollination 

The physiological demand for Boron is much greater during the reproductive growth stage when compared to the vegetative growth stage for a number of reasons. One of the key reasons is the role of Boron in pollen viability, pollination, and seed setting. 

With up to 95% of the Boron existing in cell walls, this nutrient becomes essential for proper growth, delivery of photo-assimilates and development of the reproductive parts. Adequate Boron nutrition is vital for pollen size and viability. In Boron deficient plants, empty pollen grains and impaired germination of pollen grains have been well documented.   Secondly, flowering time and number of flowers per plant are also reduced under low boron supply. 

For pollination to occur, the pollen tube (one of the fastest growing organs in the plant) and the cell wall of the pollen tube requires high amount of Boron for its stability and function. Subsequently, the pollen tube elongation process is highly sensitive to low Boron. Boron deficiency causes  failure of pollen tubes to elongate and does not enter into the embryo. As expected, such impairment in pollen tube elongation is associated with severe impairments in fertilization and thus in seed setting (see related pictures below). 

Figure 2. Boron deficiency in peas and canola    


Boron deficiency causes 

i) reduction in  root  elongation and shoot tip growth, 

ii) failure of flowers to set seeds, 

iii) fruit abortion and 

iv) highly susceptibility to diseases 

Both Western Canadian data (below), plus several published reports show that seed yield is severely depressed due to Boron deficiency although biomass production is not affected and the leaves do not develop symptoms of Boron deficiency. These results highlight the importance of boron in pollen viabilty, pollination and seed setting.  

How to fix Boron deficiency: 

Soil applied Boron, will increases in the Boron levels in the leaves, but may not be transported to the adequate level to the reproductive plant parts due to low transpiration capacity of the reproductive plant parts. 

A properly timed foliar spray of Boron, is of great importance to maintain and guarantees better pollination and seed production.  Growers should be aware that flowers may fail to set seed even without visible leaf symptoms of B deficiency or even high boron concentrations in the leaf samples. 

Foliar spray of boron may also improve seed boron concentrations. Published reports show that seeds with low boron germinate poorly and develop abnormal seedlings as shown in soybean.