X-ray phase contrast imaging of Vitis spp. buds shows freezing pattern and correlation between volume and cold hardiness

Alisson P Kovaleski,Kenneth D Finkelstein,Jason P Londo

doi:10.1038/s41598-019-51415-2

Abstract

Grapevine (Vitis spp.) buds must survive winter temperatures in order to resume growth when suitable conditions return in spring. They do so by developing cold hardiness through deep supercooling, but the mechanistic process of supercooling in buds remains largely unknown. Here we use synchrotron X-ray phase contrast imaging to study cold hardiness-related characteristics of V. amurensis, V. riparia, and V. vinifera buds: time-resolved 2D imaging was used to visualize freezing; and microtomography was used to evaluate morphological changes during deacclimation. Bud cold hardiness was determined (low temperature exotherms; LTEs) using needle thermocouples during 2D imaging as buds were cooled with a N2 gas cryostream. Resolution in 2D imaging did not allow for ice crystal identification, but freezing was assessed by movement of tissues coinciding with LTE values. Freezing was observed to propagate from the center of the bud toward the outer bud scales. The freezing events observed lasted several minutes. Additionally, loss of supercooling ability appears to be correlated with increases in bud tissue volume during the process of deacclimation, but major increases in volume occur after most of the supercooling ability is lost, suggesting growth resumption processes are limited by deacclimation state.

Highlights

Grapevines (Vitis spp.) produce compound mixed buds that contain both vegetative and reproductive tissue in a primary bud, and predominantly vegetative tissue in secondary and tertiary buds[1]
Recently has X-ray microtomography begun to be used for the exploration of floral development in annual plants[20], but here we demonstrate the use of this technique to study morphological changes in buds of woody perennials
We demonstrate that small gains in volume occur during deacclimation, but increases are much faster once most of the supercooling ability of buds is lost, suggesting that the ability to supercool to low temperatures – be that on its own or as another effect of the processes that control the supercooling levels – in some way limits growth and development in dormant grapevine buds

Summary

Introduction

Grapevines (Vitis spp.) produce compound mixed buds that contain both vegetative and reproductive tissue in a primary bud, and predominantly vegetative tissue in secondary and tertiary buds[1]. Growth resumption under forcing conditions, marked by the appearance of budbreak (e.g., opening of outer scales and emergence of tissue from the bud), is typically used to evaluate the changes in dormancy level that occur during winter[3,4]. Phenological scales for budbreak in grapevine are based on observations of V. vinifera buds[5,6], the most widely cultivated grapevine species, and may incorrectly describe changes that occur in wild species This dormancy transition has been observed through gradual increases in rate of cold hardiness loss (deacclimation rate) that occurs with chill accumulation over the winter season[7], the cold hardiness level per se and dormancy are independent. Low temperatures are the most limiting factor in plant distribution[28], the process through which plants control the supercooling point of buds and other structures remains largely unknown

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