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A paper published this year in the New Zealand Journal of Botany outlines effective strategies for sexual propagation (via seeds) and asexual propagation (via cuttings) of maire tawake/swamp maire (Syzygium maire). We spoke with Jayanthi Nadarajan about the design of the study, the strategies with the best results and the applications of this research.

Maire tawake, also known as swamp maire (Syzygium maire), is a wetland tree endemic to New Zealand. The species faces threats on all fronts, from habitat destruction to declining remnant populations. On top of that, maire tawake is highly susceptible to myrtle rust.

“Myrtle rust is killing young plants and infecting the seeds as well,” says Jayanthi Nadarajan, researcher at Plant & Food Research and co-author of the recent study funded by Ngā Rākau Taketake. “Infected seeds rarely germinate, leading to very little succession. If there is no next generation, it will lead to population extinction eventually.”

To protect this species, researchers, conservation groups and communities have shown an interest in large-scale replanting initiatives. However, propagation hadn’t been fully explored in this species, and it’s not always as simple as sowing seeds in the ground.

“We had been asked by local groups how to germinate seeds – they had tried but the seeds hadn’t germinated at all,” says Jayanthi.

Jayanthi and her team looked to nature for clues as to how these seeds usually germinate. They found answers in ones that had been dropped by birds.

“These seeds had been digested,” says Jayanthi. “They looked really clean with seed covering removed, and they were happily germinating.”

In the lab, the team removed the fleshy, pulpy exterior around the seed before trying to germinate them. When they did, germination improved significantly. The paper reports about 94.5% germination success.

Another factor that impacted germination was seed maturity.

“If seeds are collected when they are not fully matured, they’re not going to germinate as well,” says Jayanthi.

Luckily, maturity is fairly easy to gauge in maire tawake – the seeds change colour as they age and will turn bright red when they are fully mature.

The team found that propagation is also possible using softwood cuttings of new, actively growing stems. Researchers tested the efficacy of root production for two stem diameters (a thin stem class of 1 – 2 mm and a thick stem class of 3 – 5 mm). Some of the stems from each group were treated with exogenous indole-3-butyric acid (IBA), a compound known to trigger root growth.

The take home message from this experiment was that 1 – 2 mm cuttings worked best by far (63% success) and applying IBA does improve root production, bumping success of rooting in thin stems up to 75%. The ideal concentration of IBA varies by stem diameter, which is outlined in the paper.

Seed propagation and cutting propagation are both useful in different contexts. Seeds are a genetic mix of two parent plants while cuttings are clones. Propagating from seed is great for maintaining the genetic diversity of a population. Cloning doesn’t increase genetic diversity and instead maintains existing traits.

“Resistance to disease (like myrtle rust) is a desirable trait,” says Jayanthi. “If you want to multiply a disease-resistant tree, then cuttings are very useful.”

Having two propagation methods also serves a human purpose.

“It gives people options for achieving their goals based on the expertise and facilities they have available,” says Jayanthi.

Jayanthi believes that there are multiple audiences who may be able to use this new knowledge about maire tawake propagation.

“Maire tawake is a New Zealand endemic species,” says Jayanthi. “Propagation knowledge will be useful to largescale replanting programmes, national and international botanic gardens, international researchers struggling with propagation of other Myrtaceae, and our local communities who would like to plant these trees on their own land.” 

Article details: Jean Carlos Bettoni, Karin van der Walt, Juliana Aparecida Souza, Andrew McLachlan & Jayanthi Nadarajan (2023): Sexual and asexual propagation of Syzygium maire, a critically endangered Myrtaceae species of New Zealand, New Zealand Journal of Botany, DOI: 10.1080/0028825X.2022.2158110

Jenny Leonard

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