A Method to Assess Freezing Tolerance of Axillary Flower Buds in Japanese Pear by Indexing with Sugar Concentration in Xylem Sap during Winter Season

Abstract

In order to develop a simple method to evaluate freezing tolerance of Japanese pear flower buds, we compared the total sugar concentration in xylem sap of shoots and the lethal temperature 50 (LT₅₀ (°C)) of axillary flower buds on the shoots. We used shoots from the mature ‘Kosui’,‘Niitaka’, and/or ‘Nijisseiki’ trees grown in the following six orchards between December and February from 2011 till 2018: (i) the experimental orchard of the Institute of Fruit Tree and Tea Science, NARO (Tsukuba, Ibaraki, Japan), (ii) the experimental orchard of the Tottori Prefectural Agriculture and Forest Research Institute (Hokuei, Tottori, Japan), (iii) the experimental orchard of the Fukuoka Agricultural and Forestry Research Center (Chikushino, Fukuoka, Japan), (iv) the experimental orchard of the Kumamoto Prefectural Agricultural Experimental Station (Uki, Kumamoto, Japan), (v) a commercial orchard in Hikawa, Kumamoto Prefecture (Hikawa, Kumamoto, Japan) and (vi) the experimental orchard of Kagoshima Prefectural Institute of Agricultural Development (Satsumasendai, Kagoshima, Japan). There was a negative correlation for each cultivar between LT₅₀ (°C) and corresponding sugar concentrations in the xylem sap (‘Kosui’: LT₅₀ (°C) = –0.516A–0.417, ‘Niitaka’: LT₅₀ (°C) = –0.342A–4.55, ‘Nijisseiki’: LT₅₀ (°C) = –0.268A–9.84, where A means the total sugar concentration in xylem sap (mg·mL^–1)). The calibration lines contain some outliers, and a part of them may relate to large changes in temperatures (atmosphere) encountered before sampling. Experiments using the ‘Kosui’ potted tree revealed that sugar concentrations in xylem sap responded within one or two days to the temperature transition whereas LT₅₀ (°C) took more than 10 days, and these differences in responses may have caused these outliers. We also showed that the sugar concentrations in xylem sap were positively correlated with the index shown by refractometers like a portable Brix meter for measuring fruit quality. Thus, although there is a certain limitation on actual application, such as the requirement for a different calibration curve for each variety and the consideration of factors other than the sugar concentration to determine LT₅₀ (°C), we suggest that regularly analyzing the xylem sap can help monitor vulnerability in freezing tolerance for certain orchards and/or years.