Using nanotechnology to enhance plant growth

Abstract

Efficient and effective delivery of fertilisers, herbicides, pesticides and growth regulating compounds to plants is the subject of much ongoing research. The objective of this research was to develop nano-formulations for delivery of compounds to plants. Two formulations were developed: the first was solution-based focused on encapsulation of the active ingredient in a nanoemulsion. Nanoemulsions should be ideal for facilitating transfer of compounds to plant leaves as their size correlates well with the nanoscale surface features of leaves, achieving significantly greater total contact area between the oil droplets and the leaves. The second nano-formulation was solid-state based, focused on locating the active ingredient within the tubules of a nanotube clay.

For proof-of-concept two synthetic plant hormones, N-phenyl-N‘-(2-chloro-4-pyridyl)urea (CPPU) or forchlorfenuron, a synthetic cytokinin, and 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic auxin, were chosen for encapsulation. CPPU is a phenylurea derivative that shows strikingly strong cytokinin-like activity in plants, including delaying senescence. It is highly water insoluble, but soluble in organic solvents. It is widely used in a variety of crops, particularly kiwifruit and table grapes. Delivery of CPPU safely, efficiently and at the appropriate dosage is a priority as overdosing or incorrect timing of application causes detrimental effects on fruit firmness and other quality attributes. Auxins are also a group of plant hormone. 2,4-D is a synthetic auxin which has been widely used at high concentrations as a herbicide, at medium concentrations for fruit thinning, and at low concentrations promotes root initiation, but at even lower concentrations promote root elongation. Consequently, careful control of dosage is required to obtain the desired effect.

The nanoemulsion system developed was water/polysorbate 80/glycerol/soybean oil. The active ingredient, CPPU, was incorporated into the nanoemulsion via the oil phase in a pre-concentrate which was then crash diluted to yield the final nanoemulsion. Nanoemulsions are created only when the concentrate is located in the bicontinuous or oil-in-water microemulsion regions of the phase diagram. The droplet size of the nanoemulsions was measured using dynamic light scattering with droplets ranging in size from 30 – 100 nm. The CPPU-loaded nanoemulsions were stable for more than three days.

To determine if the nanoemulsion was an effective delivery system, a leaf senescence bioassay was conducted to test the senescence-delaying effect of the CPPU-loaded nanoemulsions when applied to explants. The nanoemulsions were applied directly to the leaves of dwarf bean explants. Chlorophyll was extracted from the leaves and measured spectrophotometrically before and several days following treatment. The CPPU-loaded nanoemulsions enhanced the effectiveness of CPPU in delaying leaf senescence compared with the control experiments, including direct application of CPPU. A >10-fold reduction in CPPU concentration was achieved.

The second delivery method was a solid-state preparation, using halloysite clay nanotubes loaded with 2,4-D. A rooting bioassay using mung bean explants was used for proof of concept. Application of 2,4-D nanotubes to the cut end of a young stem, without roots, stimulated root formation compared to controls after 10 days and at a lower applied concentration. The retardation of root elongation, relative to controls after 13 days, potentially indicated continued slow release of the active ingredient from the nanotubes.

Results obtained from this research indicate that nano-formulations have the potential to deliver biologically active compounds to plants in the horticultural and agricultural sectors at effective concentrations lower than in current usage.