Southern Lights (part 1): Photo-Oxidative stress and fruit colour.
June 27th 2016
Farmers in the Southern hemisphere growing soft and top fruit crops have to work harder to get colour, and shelf-life and to reduce physiological disorders. In this article I will discuss the role light plays in this, its effects on plant physiology and what farmers can do agronomically to deal with it.
UV radiation from the sun is necessary for all life on earth, and put to good use by plants during photosynthesis, but too much of a good thing can be dangerous. UV levels and UVA / UVB balance are not uniformly distributed around the earth, and the Southern hemisphere nations of Peru, Argentina Bolivia, Chile, New Zealand and South Africa all receive heightened levels of UV with a higher UVB levels due to the ozone layer being thinner there.
So why does this matter to fruit growers? Photo-oxidative stress generated by UV balance causes heightened production of toxic reactive oxygen species (ROS) during photosynthesis. ROS damage plant cells, ultimately leading to cell death if they build up in plant tissue, therefore plants have mechanisms to deal with them. Namely production of anti-oxidants such as anthocyanins and carotenoids and enzymes such as superoxide dismutase (SOD).
For crops to grow well where there is a high UV level, plants must use anti-oxidants in order to protect themselves. These antioxidants are the main source of colour in fruit crops, with yellow, orange and red pigments coming from carotenoids and red, blue and purple pigments coming from flavonoids. From a practical agronomic point of view this means that colours and flavours that come from anti-oxidants are metabolised by the fruit as they are used to prevent damage from ROS. This means that it can be harder to hold colour in crops like table grapes, cherries, apples, pears and soft fruits.
So what can we do about this? Firstly we need to understand that the problem is not caused by the plant not producing colour, rather it is due to the colour being metabolised. Anything a grower can do to reduce stress will improve the situation (most kinds of stress induce ROS increases, not just UV), so paying attention to minimising stress in general can help.
Secondly we can try and stimulate production of antioxidants. Thirdly we can pay attention to the enzyme systems (superoxide dismutases), which require sufficient levels of enzyme co-factors (copper, manganese and zinc) to properly function. To help growers with this Levity have developed Indra, a fertiliser that can help plants produce higher levels of antioxidants, and ensure proper functioning of antioxidant enzyme systems.
Another approach is to encourage the plant to boost up the functioning of hormones involved in maturation (ABA). ABA levels dictate the timing and speed of synthesis of the colours and sugars produced as fruits ripen. Often the speed of synthesis of colours and sugars is slowed by lack of availability of molybdenum, a nutrient vital for processing of ABA. Because molybdenum deficiencies do not show leaf symptoms growers are often not aware of them.
To help growers stimulate maturation of colour and brix Levity have developed Sulis, a product that can help the plant stimulate maturity processes and help the plant produce better levels of colour and sugar.
To summarise. In many southern hemisphere nations, colour is harder to achieve and maintain in fruit crops, due to high UV causing photo-oxidative stress. If we understand what is causing the problem, we can adjust our agronomy and support the plant by developing products that work with the crop to help them improve production and protection of pigments.
Levity’s science team have developed products to help, Indra to stimulant antioxidants and SOD, and Sulis to stimulate maturity processes. When farmers use these products they can reliably achieve good colour and brix levels.