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Comment by b_tterc_p | Hacker News Reader

At what point does the rice just die from too much heat?

Enzymes involved in photosynthesis break down with heat starting at around 20 degrees Celsius, and by 40 degrees Celsius (104 Fahrenheit) break down almost completely. Plants can use their stores of carbohydrates when photosynthesis is not possible. Given a long enough heat wave, plants will use up their stores of sugar and die, even if they have enough water available. Even if they do not die outright, the yield of crops will be reduced to a fraction of what it should be (autophagy when they should be growing).

Rice is very water intensive, and droughts are likely going to be a much bigger problem for rice crops before prolonged heat waves become normal.

https://sciencing.com/effect-temperature-rate-photosynthesis...

How can that be right? 20c is on the low end of room temperature. So a plant is actually bad at photosynthesising when it’s warm and there’s the most sunlight? Why? Wouldn’t this mean greenhouses are BAD for plants?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC396144/

Is the closest thing to a real source I could find at short notice. I found this surprising as well, I guess it merits some more casual research!

Plants evaporate a lot of water, so their leaves are likely a bit cooler than ambient temperatures.

Yeah, I don't believe this at all. In Florida for instance the daily high never really goes below 20C all year, and the name literally means "flowery". 40C I might believe, but 20? No way.

Yeah I agree. Vast parts of jungle land is consistently around 25-30C, surely the plants there are adapted for that?

And that 20C is in the shade, while foilage is typically exposed directly to the sun, at significantly higher temperatures.

> So a plant is actually bad at photosynthesising when it’s warm and there’s the most sunlight?

Yes. That is one of the big reasons why it is difficult to grow crops in desert regions even with irrigation.

> Why?

Thermal properties of chemical reactions. There is a very narrow range of temperatures at which the chemical reactions needed for plant life and growth can occur. The space of possible chemical reactions are limited by the availability and distributions of various elements and molecules in the environment that the plant has available, so there is not going to be a magic evolution or breeding of heat-tolerant crops that will violate these chemical constraints.

> Wouldn’t this mean greenhouses are BAD for plants?

Greenhouses are not "tropical desert hot houses." They are used to boost temperature in cold climates for the specific crops that are grown. Tomatoes like 25 degrees Celsius, but, for example, lettuce will not grow if it is above 21 degrees Celsius.

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