Introduction

The pollen tube, which carries the male gamete, plays an essential role in the fertilisation of the egg cell during pollination and thus final yield of commercial crops.(1) Pollen tubes are the fastest growing cells in nature with elongation rates of > 20 μm min− 1. Their growth tempo, final length and the physical condition of the tube are influenced by several factors i.e. temperature, the availability of nutrients (calcium and boron), carbohydrates (sucrose), as well as phytohormone activity.(2)

Unfavourable weather conditions are often encountered during this important developmental stage in fruit production. The impact of these conditions can be mitigated either by utilising available options to reduce the negative impact, or by adapting management practices in advance to ensure optimum fertilisation.

Factors affecting pollen tube elongation

Pollen tube elongation is affected by various phytohormones and signalling pathways, markedly the interplay between auxin, gibberellic acid, brassinosteroids and polyamines.(3,4,5) Sufficient levels of these phytohormones in the plant will support this sensitive process.

Auxin (IAA – Indole-3-Acetic Acid) stimulates pollen tube growth and influences changes in the tube shape.(3) As such, it is well-known for its role in cell expansion in plants. Auxins, like IAA, affect cell elongation through a mechanism known as the ”acid-growth theory”.

According to this theory, auxin increases the activity of PM H+-ATPase. This releases H+ ions into the cell wall, causing acidification, influencing cell wall rigidity and allowing the cell to stretch and grow.(6)

Furthermore, brassinosteroid treatments resulted in a significant increase in pollen tube length as well as pollination, in a speciesdependent manner.(4) Thus, the application of both phytohormones (auxin and brassinosteroids) enhanced pollination.

Several signalling pathways are involved in the regulation of pollen germination and pollen tube elongation. Exogenously applied polyamines strongly affect pollen maturation, pollen tube emergence and elongation.(5)

The effect of KELPURA™ on pollen germination and tube elongation

KELPURA™ (Reg. no. M442, Act 36 of 1946) is a natural kelp extract produced from Ecklonia maxima through a proprietary extraction process that concentrates and preserves various bioactive components.

Kelp extracts show auxin-, gibberellin- and brassinosteroid-like activity, which impact positively on pollen germination and pollen tube elongation.(7)

KELPURA™ contains quantified amounts (via LC-MC analysis) of phytohormones, including IAA as a bioactive hormone and Indole-2-acetamide (IAM) as an IAA precursor. KELPURA™ also contains brassinosteroids such as brassinolide and castesterone (Figure 1), as well as high levels of polyamines (11 mg/L), which are required for optimum pollen tube elongation.

When applied to kiwi pollen grains, KELPURA™ positively influenced pollen germination and pollen tube elongation when incubated for 3 hours at 20 ⁰C (Figure 2), confirming previous findings.(7) KELPURA™ also stimulated the germination of cherry pollen (Figure 3).

The effect of increased pollen germination and tube elongation can be observed in the increased reproductive buds (Figure 4) and flowers per shoot in blueberries (Figure 5), as well as final fruit yield in greenhouse-grown bell peppers (Figure 6).

Conclusion

KELPURA™ is a natural source of auxins, brassinosteroids and polyamines which can increase pollen germination and pollen tube elongation. Improved pollen germination and pollen tube elongation are positively correlated with improved crop set as well as final yield.

A small intervention to ensure a positive impact on future crop production.

Fertilizer Group 3 | Reg. No. M442 | Act 36 of 1947

References:

1. Zheng, RH., Su, S., Xiao, H., Tian, HQ. 2019. Calcium: A Critical Factor in Pollen Germination and Tube Elongation. International Journal of Molecular Sciences 20(2): 420.
2. Caser., M. 2017. Pollen Grains and Tubes. Module in Life Sciences, Elsevier.
3. Wu et al. 2008. IAA stimulates pollen tube growth and mediates the modification of its wall composition and structure in Torenia fournieri. Journal of Experimental Botany 59(9): 2529.
4. Genç., AC. 2016. The effect of brassinosteroid on pollen germination and tube growth in three Dianthus species. Celal Bayar University Journal of Science 15(4): 371.
5. Benkő, P., Jee, S., Kaszler, N., Fehér, A., Gémes, K. 2020. Polyamines treatment during pollen germination and pollen tube elongation in tobacco modulate reactive oxygen species and nitric oxide homeostasis. Journal of Plant Physiology 244: 153085.
6. Rayle, DL., Cleland, RE. 1992. The acid growth theory of auxin-induced cell elongation is alive and well. Plant Physiology 99: 1271.
7. Ovalle, A., Atenas, C., Larraín, P. 2019. Application of an Ecklonia maxima seaweed product at two different timings can improve the fruit set and yield in ‘Bing’ sweet cherry trees. Acta Horticulturae 10: 319.