Frost Follows the Plow In Wyoming? Crops and Soils Are Reservoirs of Biological Ice Nuclei.

External ice-nucleating particles are triggers of frost damage to crops. This is because most plant tissues do not freeze at 0°C, but can supercool to -5°C or colder. A common source of external ice nuclei (IN) are ice-nucleating proteins produced by several species of bacteria. These ice nucleation active (INA) bacteria all belong to three closely-related genera (Pseudomonas, Pantoea and Xanthomonas) and are natural colonizers of plant surfaces. INA bacteria can trigger freezing as warm as -1.5°C. To increase sensitivity and minimize bias we used quantitative PCR to directly count the ina genes that code for the protein. In leaf washings of eight crop species growing at the University of Wyoming Agricultural Experimental Station (SAREC), INA bacterial numbers varied widely. Alfalfa had 5 ×10^3 /g fresh leaf while cereals had >10^8 /g leaf. Leaf washings of wheat, barley and smooth brome grass contained high numbers of both INA bacteria and IN: barley leaf washings had nuclei active at -2.0°C and contained 25,000 IN/g leaf active at -3.5°C. INA bacteria persisted in stubble. We also tested for nuclei in surface soils from under three different crop management systems. All soils contained numerous IN, with activity starting at -4.5 to -5.5°C. Organic-rich soil from beneath an unmanaged pasture contained 10^6 IN/g dry soil at -6.0°C. Heat treatment significantly reduced activity and digestion with hydrogen peroxide destroyed almost all IN active at warmer than -15°C, suggesting they were organic. Both ina genes and high-temperature IN were detected in air above crops. Results reveal that some crops, border species and crop residue management practices produce concentrated reservoirs of high-temperature IN. These reservoirs may act as seed sources of IN that ultimately increase plant sensitivity to frost in spring. At times reservoirs may also contribute significant numbers of biogenic IN to atmospheric precipitation processes.