Image caption Soybean - an important world crop

A pair of widely used chemicals in the form of tiny "nanoparticles" have been shown to spread throughout a crop plant or affect growth and soil fertility.

The use of nanoparticles is increasing, yet their environmental impact is poorly understood.

A report published in PNAS shows that nanoparticles present in exhaust gases and some fertilisers adversely affect soybean growth and surrounding soil.

The nanoparticles harmed bacteria that the plant relies on for growth.

A nanoparticle is defined as a particle that has at least one diameter that is less than 100 nanometres (nm). A nanometre is a length measurement that exists at the microscopic end of the size spectrum - you can fit one million nanometres into one millimetre.

Image caption Nanoparticles - illustrated in this computer generated image - are just millionths of a millimetre in size

Nanoparticles - also known as nanomaterials - are manufactured for use in an array of applications such as cosmetics, material coatings and as a fuel additive. They are being increasingly investigated for use in medical applications such as drug delivery and release.

Whilst many of their effects have been well documented, some of their mechanisms of action are not fully understood. Concern has arisen that widespread long-term nanoparticle use may "trickle down" into the environment, sparking unforeseen effects on plant or animal, or even human, health.

In the current study, a team led by Prof Patricia Holden from the University of California tested the effects of two commonly used nanoparticles for their effects on the growth of soybean.

Soybean is a crop of huge commercial importance. Globally, it is the fifth-largest crop and is the largest producer of edible oil and plant protein, such as tofu.

The researchers focussed on the effects of zinc oxide and cerium oxide nanoparticles. Zinc oxide is a common component of cosmetics and ultimately ends up as a contaminant of solid waste generated by sewage treatment. This waste is widely used as an organic fertiliser.

Cerium oxide is used in some diesel fuels to improve combustion and reduce particulate emissions.

A prior report in Environmental Science and Technology studied the effect of these same nanoparticles on soybean when the plants were grown hydroponically - without soil - but the team behind the new paper suggest that a fuller picture is to analyse soybean grown in more natural conditions.

Image caption Bacteria living in the roots of the soybean help provide it with essential nutrients

They grew soybeans in a greenhouse in the presence of increasing amounts of the nanoparticles, monitoring the plants' growth. In addition, the accumulation of the nanoparticles in different parts of the plant was also scrutinised.

The plants grown in the presence of zinc oxide nanoparticles actually grew slightly better than control plants grown in the absence of nanoparticles. However, zinc built up in the edible parts of the plants, which included the leaves and the beans.

Zinc itself is an important dietary supplement and zinc deficiency is a large public health concern in the developing world, but zinc oxide nanoparticles have been shown to be toxic to mammalian cells grown in the laboratory.

However, effects in humans remain to be examined fully, and the researchers used comparatively low levels of zinc oxide in their experiments.

Soybean growth was significantly stunted when the plants were cultivated in the presence of high levels of cerium oxide nanoparticles.

The cerium was able to enter the plants' roots. Soybeans are members of a group of plants called legumes. The roots of these plants host bacteria that turn atmospheric nitrogen into a form that the plant can use for growth - so-called nitrogen fixation.

The cerium nanoparticles seemed to completely inhibit the bacteria's ability to fix nitrogen.

Commenting on the wider toxicity of nanoparticles, Prof Vicki Stone from the Heriot-Watt University, Scotland, said: "Nanomaterials are not 'equally hazardous' or conversely 'equally safe'.

"Effects seem to depend upon their physical and chemical characteristics - this is what scientists are aiming at better understanding, so that in future they can predict toxicity or safety based on these characteristics."

The authors concluded that the build-up of manufactured nanomaterials in soils may compromise soil-based crop quality and yield and may necessitate the greater use of synthetic fertilisers.