Sentence:  Root and stem tumors from sweet clover plants infected with WTV were quick-frozen in liquid nitrogen , embedded in ice , and cut at 3 - 6 | m in a cryostat <WSD>maintained</WSD> at - 16 ` to - 20 ` . 
Meaning of the word marked with <WSD> tags:  keep in a certain state, position, or activity; e.g., "keep clean"