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  • Andrew Turnbull

After the Drought & Heat Wave


So, rain and lower temperatures have arrived, or arriving, and your grass is starting to recover. Is there anything we can do to help the grass plant?


First, we need to understand what happens to a grass plant when under drought and heat stress. Put simply, photosynthesis slows down, or stops, due to damage to the photosynthetic mechanisms caused by lack of water, intense light and high heat. Carbohydrates are the product of photosynthesis and used as the fuel to power the processes involved in plant health and growth. Carbohydrates reserves are stored by the plant during the cooler spring and early summer when carbohydrate production from photosynthesis exceeds consumption. Symptoms of stress, such as slow growth and loss of colour, occur when carbohydrate reserves are rapidly drained in keeping the plant alive and healthy when photosynthesis slows or stops during periods of stress.


Applying non-ionic wetting agents/surfactants helps water to penetrate the soil/thatch surface and plant roots are able to take up vital moisture. Recovery will be slow and weak, however, if there is no or little available carbohydrates, no matter how much fertiliser is applied. This is where applying treatments that contain carbohydrates can relay help in the recovery of you


r lawn. But, if you are ready for a GOOD READ, let’s talk about fertilisers.


Fertilisers

Nitrate, urea and ammonia are the most common sources of nitrogen in synthetic fertilisers – produced by extracting hydrogen from natural gas, hence this year’s high fertiliser prices.


Grasses transport most of the nitrate (NO3-) absorbed by their roots to the leaves where it is reduced to ammonium (NH4) and assimilated into amino acids. These amino acids can then be transported from the leaves to all parts of the plant, including the roots, where they support cell division and growth. NO3- in the leaves functions as a "signal molecule" that diverts sugars, made by photosynthesis, from transport to the roots to the synthesis of amino acids in the leaves.




The process of NO3- reduction and assimilation into amino acids involves 4 separate reactions;

  • Nitrate (NO3-) to nitrogen dioxide (NO2-)

  • NO2- to Ammonium (NH4+)

  • NH4+ to an amino acid: glutamine

  • Glutamine to two glutamate molecules

Reactions occur in the leaf and use the same energy source as CO2 assimilation that have very high energy requirements. NO3- metabolism requires a lot of energy and if photosynthetic energy, in the form of carbohydrates (complex sugars), is used to assimilate nitrogen there will be less carbohydrates available for transport to roots.


The anatomy of a grass plant dictates that transport of sugars and amino acids from leaves to roots must pass through the crown at the soil surface, where leaves are initiated and their growth supported, be


fore they can reach the root tips where root growth occurs.

When the roots demand carbohydrates and nutrients for growth, hormones send a signal for nutrients to be moved from the leaf back to the root, but they stop at the crown. This is the area from which new leaf tissue is being made and there is a high demand for carbohydrates and nutrients, resulting in most of the nutrients destined for the root being captured and used in leaf production. Hence the lack of root growth in summer.


In short, NO3- present in the leaves directs plant resources toward leaf and shoot growth at some expense from root growth. That scenario alone explains how NO3- favours turf leaf production and retards root growth. Bioactive molecules in seaweed help to overcome this barrier by ‘forcing’ the movement of nutrients into roots.


While NH4+ can avoid the negative signaling problem of NO3-, it presents some problems of its own that can reduce root growth. Root growth is fundamentally driven by photosynthetic energ


y obtained from the leaves, and anything that diverts this energy from the roots (high NO3- in leaves) will depress root growth and compromise turf quality. For example, fertilising with nitrogen in winter depletes carbohydrate reserves, and heavy fertilising of new turf diverts energy into leaf growth with little root growth.


Too much applied nitrate and ammonia causes increased leaf growth but little root growth. High NH4+ concentrations can reduce root growth by 30% but not as severely as high NO3- . This can be explained by the fact that NH4+ is more readily absorbed by root than is NO3-. Also, once absorbed, NH4+ is rapidly assimilated because it can easily become toxic if accumulated in root cells. Rapid NH4+ uptake by roots will likely divert much available energy (sugars) in the roots to support NH4+ assimilation into amino acids and not to growth. These excess amino acids will be transported to the leaves where leaf growth will be stimulated.


What do carbohydrates do?

Carbohydrates are utilised as an energy source (fuel in the tank) for:


the maintenance and growth of plant tissue and micro-organisms

  • the assimilation of polysaccharides such as starch and fructosans that serve as storage compounds

  • cellulose and lignin that are used as structural components in cell walls

  • proteins, enzymes and other protoplasmic constituents.


Where do Carbs in plants come from?

Carbohydrates are the product of photosynthesis and used as the fuel to power the processes involved in plant health and growth. Carbohydrates reserves are stored by the plant during the cooler spring and early summer when carbohydrate production from photosynthesis exceeds consumption. Symptoms of stress, such as slow growth and loss of colour, occur when carbohydrate reserves are rapidly drained in keeping the plant alive and healthy when pho


tosynthesis slows or stops during periods of stress, such as;

  • Extremes of heat & cold

  • Low soil moisture

  • Insufficient nutrients

  • Low light/shade

  • High light intensity

CONCLUSION - Healthy turf growth only possible by maximising carbohydrate production through photosynthesis, e.g. reducing shade, providing light, or application of carbohydrate treatments.


Which Carbohydrates To Use?


Molasses

  • Quickly available C for micro-organism assimilation

  • Increased take-up of nutrients from the soil

  • Quickly available energy for plant nutrient assimilation

Seaweed



  • Enhances the plant’s ability to take up nutrients from the soil

  • Assists the transfer of major and minor elements to sites of synthesis

  • The increased permeability of vegetal cell membranes allows a more rapid assimilation of the trace elements contained in seaweed.

Applied carbohydrates can enter plants through stomata on the underside of leaves, but mostly through the crown where it is directed to growing areas. Carbohydrates help to direct nutrients to growth areas, importantly past the crown and into the roots – hence increase in root mass. Applied carbohydrates do not add to the plants’ reserves, but it can more readily use it so keeping the reserves for when required.


Benefits of carbohydrate nutrition

  • Quicker establishment of new turf

  • Reduced input of synthetic fertilisers

  • Reduced pesticide applications

Grass does not need a carb-free diet!




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