Getting Nitrogen right – the key to good blueberry production

All crops have their own peculiarities, but none more so than blueberry, a crop that has both metabolic and physiological differences that influence nitrogen use and its effect on growth habit. Here David Marks of Levity Crop Science discusses how this impacts the way blueberry pants process and respond to nitrogen, and what lessons we can learn from this for better production.

Blueberry roots are different.

Blueberry and other ericaceous crops (including cranberry) do not produce root hairs. This is unusual and impacts upon their physiology and the best practice for agronomy.

 The root systems of blueberry are shallow, the larger diameter roots serve mainly to anchor the plant and transport water, whilst the finer ‘feeder roots’ do most of the work regards taking up and transporting nutrients. These feeder roots are short lived, typically having a lifespan of less than 130 days.

 So how does this impact on the crop? Well the roots die off quickly, so for good crops it is of paramount importance that the crop is managed for maximum root growth, and that root growth is continuous throughout the growing season. Secondly and maybe less well understood is the effect on plant growth hormone production. The main site of biosynthesis of the cytokinin hormone in plants is root hair. Blueberry (and cranberry) do not have root hairs, this makes them les capable of cytokinin synthesis than most plants. Cytokinin synthesis has a big effect on plant growth habit (more on this later).

Blueberry nitrogen metabolism is different

Blueberry plants are less capable of processing nitrates than other plant species due to an inability to efficiently utilise the nitrate reductase enzyme system that plants use to convert nitrate N into protein in order to grow. This is a function of having evolved to be adapted to a low nitrate environment. This makes use of conventional fertilisers highly inefficient.

Here we see the mechanism plants use to metabolise nitrogen, nitrate N is converted to protein using a nitrate reductase enzyme system. Blueberry and other ericaceous plants have a low capability to operate this enzyme system and therefore if they take up nitrates they can accumulate faster than they can be processed.

This is the reason that blueberry growers are often advised to avoid the use of nitrate fertiliser, and instead to use ammonium or ureic N sources. However this does not solve the problem as nitrogen is not environmentally stable. Nitrogen uptake by crops bears little resemblance to nitrogen inputs as it changes between application and uptake.

In most cases crops a. Only take up between 30 and 50% of the nitrogen that is applied (with the rest ending up in the environment), and b. Take up the majority of nitrogen applied in the nitrate form (regardless of what form it was applied as).

 In the field then if  blueberry farmers apply N in non-nitrate forms, the reality is that the plants are still receiving more of that N as nitrate than any other form, limiting the efficacy of nitrogen fertiliser on blueberry farms.

What are the consequences of excess nitrate on blueberry?

The effect of excess N on blueberry is well known, but is perhaps not well understood. Once we understand that the non-nitrate N applied to blueberry is mostly taken up as nitrate we can better understand why excess N is problematic.

Nitrates are taken up by roots, but have to be processed into protein in leaves. After uptake plants use transporter proteins to move them to the leaf, and nitrate reductase enzymes to convert to protein. In all plants growth hormone synthesis is strongly linked to nitrogen metabolism. The synthesis of the growth hormone auxin is increased when nitrate concentrations in leaves increases.

Blueberries lack the capacity of other plants to process nitrate, so leaf build-up is rapid following soil nitrogen application (whatever the form applied). This leads to excessive nitrate hormone production. This excess auxin encourages flushes of growth which can be detrimental to the crop.

Blueberry physiology can create a high Auxin:Cytokinin ratio

 Earlier we explored the unique root structure of blueberries, which do not produce root hairs. Roots are the main site of synthesis for the cytokinin hormone, and due to this lack of root hair blueberries have a reduced capacity to produce cytokinins compared with most plant species.

 The speed at which plants grow is a function of total growth hormone production, but where plants allocate that growth (growth partitioning) is a function of the relative abundance of two hormones (auxin and cytokinin). If auxin production is high compared to cytokinin plants allocate more growth to shoot development or ‘vegetative growth’. Conversely if plants have a better cytokinin to auxin ratio they allocate growth differently, with more emphasis on reproductive growth (fruit development).

Blueberries have a problem. Due to lack of root hairs they synthesis relatively little cytokinins, and due to inability to process nitrate they are susceptible to excess auxin production when exposed to nitrates. This physiological ‘double whammy’ makes presents unique challenges to growers of blueberry. As getting nitrogen wrong makes them vulnerable to excess vegetative growth and a drop in yield.

All plants react to nitrogen form in the same way, if nitrates predominate they get increasingly leggy and more resource goes into vegetation than grain/fruit/ tuber development. Blueberries however are particularly susceptible to this effect, and this is why N recommendations are low, best practice is ‘little an often’ (reduces the fraction taken up as nitrate), and timing is important. However if no N is used yield go down, so getting the balance better is a clear path to better yield.

 So how can we use this understanding to improve production?

Blueberries respond badly to nitrate, but applying urea or ammonium sulphate is just a different way of applying nitrate (the crop wastes most of it, and takes up the rest as mostly nitrate). So switching out nitrate for conventional non-nitrate fertiliser is not the answer.

However there are ways we can improve, by using more advanced stabilised amine nitrogen we can give blueberry the right balance of N for best growth and yield. Levity Crop Science are the world leaders in Stabilised Amine Nitrogen fertiliser development, their LimiN technology is used to provide amine nitrogen that stays in that form.

Lono products supply stabilised amine N and have been scientifically proven to change growth partitioning (where plants allocate growth) in many crops. Lono applied in small doses either foliar or via drip changes the growth habit of crops, giving shorter plants with more branching, root development and investment in flower and fruit production rather than the classic nitrate driven N response.

 This approach is perfect for blueberries, as it allows growers to produce better root growth (vital in blueberry), more lateral branching (lateral shoots give more fruit than upward shoots), and encourage fruit development rather than extra ‘leaf flush’. Furthermore small amounts can be used, as all nitrogen is taken up and used unlike conventional nitrogen applications.

Use of stabilised amine as a nutrient source for blueberry crops gives a far more favourable growth habit, encouraging shorter plants with active root production and a higher yield potential.

Summary

Blueberry growers have a tough job, as the crop has a unique physiology. Blueberry plants do not produce root hairs, and can’t process nitrates effectively making them poor producers of cytokinin, and overproducers of auxin if exposed to excess nitrogen in conventional formulations.

This physiology makes blueberries prone to ‘flushes of growth’, and limits potential for improving fruit yield via fertilisation. Growers have been avoiding application of nitrate on blueberry for good reason, but switching to urea or ammonium sources have provided nitrate anyway due to poor environmental stability.

 Lono offers a way to get the most out of this tricky crop, supplying nitrogen in a form that puts the growth in the right place, producing plants with a good growth habit (better lateral branching, with thicker shorter stems that support better fruit capacity) and focusing the crops resource on fruit development.

Recommendations

• Lono

  Apply Lono at 5L per Ha foliar or drip, commencing at bud break and making further applications at three week intervals. This will keep roots actively growing, encourage a good growth habit, and ensure the plant focuses on fruit production.

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