ARS | Publication request: CHANGES IN APOPLASTIC SUGARS IN RESPONSE TO SECOND PHASE COLD HARDENING

Submitted to: International Fructan Conference
Publication Type: Other
Publication Acceptance Date: July 8, 1996
Publication Date: N/A

Technical Abstract: Cold hardening is essential if winter cereals such as wheat, barley, rye and oat are to survive most winter conditions. Two phases of cold hardening have been described. Phase one occurs at temperatures above freezing and phase two at temperatures below freezing but before freezing injury occurs. One explanation for the additional hardening acquired during second phase hardening is that sugars accumulate in the apoplast resulting in a decreas in the freezing point of the apoplastic solution. To test this hypothesis a hardy winter oat cultivar, Wintok, was grown in growth chambers at 15 C day and 5 C night with 12 hours light/day for 5 weeks. Plants were then subjected to the first phase of hardening which was 3 weeks at 2 C under the same lighting conditions as before. The second phase of hardening consisted of 2 days at -3 C. This treatment resulted in the LT-50 of Wintok oat being reduced from -12.5 C before to -16.5 C after second phase hardening. Apoplastic fluids were extracted from crown tissues after each phase of hardening. Cellular rupture, assessed by comparing the osmolality and malate dehydrogenase activity in the apoplast to that of whole tissue extracts, was always less than 3%. Fructan, sucrose, glucose and fructose were separated by HPLC and detected with a refractive index detector. Apoplastic sugars increased 3.5-5.8 fold from 1st to 2nd phase of hardening and resulted in 50% of the total glucose and fructose residing in the apoplast. Apoplastic fructans increased 3.7 fold from the 1st to 2nd phase of hardening. The composition of fructan isomers DP3 to 5 was nearly identical to that found in the ground tissue. Assuming a constant apoplast volume, this increase in sugar could partially explain the increase in freezing tolerance in oat crowns due to second phase hardening.