Heat is bad for strawberries

We look at both the yield and the strawberry fruit size in the NZBerryfruit Propagator variety trials which are held in Auckland and Christchurch each year, and every year we note a dramatic decrease in fruit size in the Auckland trial by early January.  This season we have been pondering how temperature affects yield, and reading up on the research that details the effects of high temperature on strawberry fruit production.
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High heat conditions are bad for strawberry production because:

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1. Flower initiation suffers—plants produce smaller flowers which in turn grow into smaller fruit.  They also produce fewer flower clusters overall, leading to fewer berries.
2. Pollination suffers—pollen viability is lower, meaning fewer seeds are successfully fertilized, and there are more deformed fruit (knobbly berries)
3. Faster ripening means that berries haven’t had as long to fill out before being picked, which translates into smaller berries
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For those interested in the nerdy details, including what temperatures constitute too hot for strawberries, read on.

For those who want to skip to the bottom line, what can we do about heat?  This is an area we could learn more about, but some general ideas are below.
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1. Evaporative cooling: possibility for outdoors and indoors, but effectiveness depends on ambient humidity
2. White versus black plastic mulches
3. Greenhouse coatings—shading, and special coatings that let through photosynthetically-active wavelengths while screening out heat (infrared) wavelengths.
4. Ground cloth coverings under a tunnel structure could play a role—white versus black, grass versus ground cloth.
5. Cooler growing environments can have a competitive advantage when it comes to summer heat conditions.
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Back to the nerdy details of how temperature affects strawberry yield.  Turns out that it’s multi-faceted, and there’s a penalty to high heat at every instance.
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Flower primordia development:

Flower primoridia are developing deep in the strawberry crown, and their embryonic development is influenced by temperature.  Primorida grow more pistils (the female part of the flowe) when they develop slowly under cooler temperatures.  Pistils are important to fruit size because each strawberry pistil contains an ovary, and every fertilized ovary makes a seed which produces auxin, the plant hormone that causes the berry to swell in size.  So the more seeds, the bigger the berry can potentially grow.

In a Korean study, double the number of seeds developed when plants were grown at a cool 16°C day/11°C night versus a moderate temperature of 24°C day/19°C night.  In fact, for every 1°C increase between the cool and the warm growing conditions, berries lost 1g in fruit size, because they had fewer seeds.

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Pollination:

To better understand the following discussion about heat and flower biology, here’s a quick review of the steps in pollination:

Pollination happens in stages and each stage has to be successful to get a live embryo.  First, mature pollen lands on the stigma.  Second, the pollen grain germinates and grows a tube down to the ovule.  Third, the genetic material from the pollen grain joins with that of the ovule and…. Voila! Fertilization happens and an embryo can grow.

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Heat is bad for pollination in several ways.

1. Pollen viability: 
   In a Japanese study, pollen grown at 23°C day/18°C night was 80-85% viable (alive).  Bump that temperature up to 30°C/25°C, and then only 30-75% of the pollen grains were alive.  The low percentage represents a heat sensitive variety, while the higher percentage is a variety that is more tolerant to heat.
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2. Pollen tube growth: Pollen grains must germinate and grow a long pollen tube (can be 200x the length of their diameter) in order to reach and fertilize an ovary.  The hot temperature regime halved the number of pollen grains that managed to germinate.  Those that did manage to germinate, grew shorter pollen tubes. Fertilization generally happens within 24 hours of pollination, but if pollen is struggling to germinate and grow, it can result in fertilization failure, where the seed is not viable and produces no auxin to stimulate growth of the berry flesh.  The more unviable seeds, the smaller the resulting berry.
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3. ‍Knobbly berries: When pollination has been problematic, fruit can develop misshapenly.  We’ve been contemplating the various reasons for “cat-faced” or knobbly berries during our mirid bug evaluations this season.  More on this in a subsequent article.

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In another Japanese study, strawberry fruits maturing from flower to ripening in hot conditions (30°C day/25°C night) were only 50% the size of those ripening at more moderate temperatures (23°C day/18°C night).

We are all familiar with how hot temperatures makes fruit ripen faster.  In the heat, fruit went from flowering to red ripe in 21 days, where the more moderate temperatures took 28 days.  When fruit ripens faster, there’s less time for fruit to grow in size during the ripening process.

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Number of inflorescences:

In this same study, over the 3 months there were 15-30% fewer inflorescences developing on plants grown in hot conditions than in the more moderate temperature regime.  In the runner-growing world, growing plants in hot conditions favours runner growth over flower production, a good situation for plant production but not for fruit growing.

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Temperature differences between Auckland and Christchurch:

The following graph got us thinking about the climatic differences between our two trial regions. (‘Mean’ signifies ‘average’, rather than ‘nasty’)

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Temperatures are cooler in Christchurch.  Sure, we have some hot days, but nights are generally cool even in summer.  Perhaps Auckland’s hotter climate explains why average fruit size in the Auckland trial peaks at 40+ gram/berry in mid November, but is averaging 10-12 grams by January.  In Christchurch the 40g berry size happens around Christmas, but then tails off to smaller (17g) in February, before it gets bigger again in the autumn.

Interestingly, the berry size never dipped as low in Lincoln as it did in Auckland, and the temperatures averaged 2-3 degrees cooler in Lincoln.

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References:

Mori, 1998.  Effect of temperature during flower bud formation on achene number and fresh weight of strawberries.

Ledesma et al, 2007.  Effect of high temperature stress on the reproductive growth of strawberry cvs. ‘Nyoho’ and ‘Toyonoka.’

Ledesma and Sugiyama, 2005.  Pollen quality and performance in strawberry plants exposed to high-temperature stress.

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