Seasonal Metabolic Investigation in Pomegranate (Punica granatum L.) Highlights the Role of Amino Acids in Genotype- and Organ-Specific Adaptive Responses to Freezing Stress.

Parisa Yazdanpanah,Parisa Jonoubi,Mohammad Reza Vazifeshenas,Homa Rajaei,Mohammad Reza Ghaffari,Mehrshad Zeinalabedini,Somayeh Abdirad

doi:10.3389/fpls.2021.699139

Abstract

Every winter, temperate woody plants have to cope with freezing stress. Winter hardiness is of crucial importance for pomegranate survival and productivity. A comparative morphological and metabolic study was conducted on the stems and buds of 15 field-grown mature pomegranate genotypes in seven time-points during two developmental cycles. Seasonal changes of frost hardiness, as determined by electrolyte leakage method, and metabolite analysis by HPLC and GC revealed the variability in frost hardiness and metabolic contents result from genetic background and organ, as well as seasonal condition. Morphological adaptations, as well as metabolic remodeling, are the distinct features of the hardy genotypes. Larger buds with a greater number of compressed scales and the higher number of protective leaves, together with the higher number and content of changed metabolites, especially amino acids, seem to provide a higher frost resistance for those trees. We recorded two-times the change in metabolites and several-times accumulation of amino acids in the stem compared with buds. A better potential of stem for metabolome adjustment during the hardening period and a higher level of tolerance to stress is therefore suggested. High levels of arginine, proline, glutamine, and asparagine, and particularly the accumulation of alanine, tryptophan, and histidine are responsible for excellent tolerance of the stem of tolerant genotypes. With regard to the protective roles of amino acids, a relation between stress tolerance and the level of amino acids is proposed. This points both to the importance of amino acids in the winter survival of pomegranate trees, and to the evaluation of frost tolerance in other plants, by these specific markers.

Highlights

Pomegranate (Punica granatum L.) is an economically valuable fruit tree that originates from the region spanning Iran to the Himalayas in northern India but is cultivated in many parts of the world today (Holland et al, 2009)
The changes of frost hardiness were recorded throughout the pomegranate annual cycle, we selected LT50 values of winter months, i.e., December and January of 2 successive years for representing the maximum frost tolerance of the two organs of each genotype (Figures 1B,C)
The metabolite quantification represented different metabolic adaptations among genotypes and between the stem and bud, indicating genotype- and organ-specific responses which lead to different levels of frost tolerance

Summary

Introduction

Pomegranate (Punica granatum L.) is an economically valuable fruit tree that originates from the region spanning Iran to the Himalayas in northern India but is cultivated in many parts of the world today (Holland et al, 2009). Iran is one of the main producers and exporters of pomegranate fruit in the world (Erkan and Dogan, 2018), its production has recently declined due to winter frost damage. In most pomegranate growth and cultivation areas in Iran, minimum winter temperatures may fall to −20◦C or even lower. Severe winter frost and unprecedented temperature drops in 2007 and 2016, with the temperature down to −23 and −20◦C for 3 and 2 weeks, destroyed 36,931 and 35,000 ha of pomegranate orchards in Iran, respectively. The screening and selection of hardier genotypes could be of considerable help in coping with frost problems, enabling producers to reduce severe damage and huge economic losses

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