When leaf temperature becomes too warm, plants switch from photosynthesis dominant to photo-respiration dominant, and begin ‘consuming themselves’.

The threshold for C3 photosynthetic pathway plants is a leaf temperature of 78 degrees Fahrenheit (25.5 C). For C4 plants, the threshold is 86 degrees Fahrenheit (30C) leaf temperature.

Leaf temperature and air temperature are not the same. The healthier plants become, the better they are at cooling themselves. There are several mechanisms in play, topics for future blog posts. It is clear that plants with a waxy sheen on the leaf surface can have a leaf temperature as much as 8-10 degrees cooler than plants that lack nutritional integrity in the same climactic conditions.

When this threshold is crossed and photo-respiration becomes the dominant process, a few important shifts occur:

  • photosynthesis/sugar production drops or stops completely
  • plants consume the limited available sugar supply
  • plants consume any free/available lipids as an energy source (these are abundant in high energy plants, very low in plants getting nutrition from soluble ions instead of from living microbes.)
  • once the supply of available sugars and lipids has been used as, plants begin consuming their own proteins as an energy source.

80% of the nitrogen (proteins) contained in plants is in the form of enzymes. Breaking these down further weakens the plants ability to recover quickly. Protein catabolism also leads to the formation of ammonium, which is a requirement for  spider mite infections.

When plants experience periods of high heat stress, one of the best management strategies is to provide them with a surplus of energy in the form of sugars, oils, and sometimes proteins, to avoid the stress consequences of catabolism.

Foliar applications of sugars and sometimes vegetable oils can produce a tremendous crop response. In the past several weeks heat stress period, growers have reported some remarkable crop responses within 24 hours from foliar applications, both when applied proactively as a preventative, and during and after the heat stress. Rejuvenate inclusion in the foliar mix seems to consistently deliver clear visual results.