Advances in research on cold resistance in kiwifruit

Abstract:Kiwifruit (Actinidia Lindl.) is amongst the most important fruit crops to be successfully grown and marketed in the world. Actinidia chinensis Planch is one of the few species of genus Actinid- ia that are domesticated and cultivated for fruit production. It possesses less cold hardiness and is culti- vated in limited areas. Another commercial species Actinidia arguta (Sieb. & Zucc.) Planch. ex Miq. is regarded as highly cold resistant, which can withstand a temperature up to -30 °C. Actinidia eriantha is another commercial species that has very poor cold resistance. Adverse climatic conditions can lead ki- wifruit plants to the freezing injury, which is one of the most important abiotic stresses in horticultural crops. Freezing injury can disrupt biosynthetic activity of plants by disturbing the normal function of their physiological processes. Low temperature can dramatically reduce the area and production of kiwi- fruit worldwide, which results in a dramatical reduction or destroying the orchard. In China and world- wide, there are many freezing injuries in kiwifruit every year, bringing out tremendous economical loss. In recent years, the issue of cold resistance of kiwifruit has attracted much attention and researchers have made great progress in it. In this review, we systematically summarize the causes of freezing inju- ry, methods of cold resistance identification, investigation of freezing damage in main cold years, identi- fication and evaluation of cold resistance in main cultivars and wild resources of kiwifruit, physiologi- cal and molecular mechanism of cold resistance and methods for cold prevention in kiwifruit. Further- more, the future research fields of kiwifruit cold resistance were prospected. Among them, previous ob- servations have indicated that kiwifruit may incur serious freezing injury in spring on young shoots and flower buds, in autumn on fruits and leaves and in winter on main arms, canes, and dormant buds. In spring and summer, very mild frosts can cause damage to young shoots, flower buds and flowers. In au- tumn, mild frosts can cause premature leaf abscission and damage to fruit before harvest, resulting in the fruit unsuitable for storage and marketing. In winter, severe frosts can result in considerable damage to arms and canes of dormant vines. A comprehensive understanding of the seasonal changes can help us prevent and decrease the freezing damage to kiwifruit. Cold resistance, defined as the ability of plants to withstand sub-freezing temperatures without sustaining significant damage, is an important cri- terion for evaluation of the cultivation potential of a species or cultivar for breeding or selection pur- pose. Past research work conducted by researchers from various countries on cold hardiness of commer- cial kiwifruit cultivars have provided us suitable temperature for their planting. Some other cultivars are reported to be harder than commonly planted cultivar like‘Hayward’, but so far limited information is available about their cold hardiness of specific cultivars or species of Actinidia. A deep understanding about the cold hardiness of different kiwifruit varieties will help us in their plantation and breeding pro- grams. Cold injury in plants can cause lipid peroxidation and an increase in electrolyte leakage. As a re- sult, poor cold resistant material shows an increased electroconductivity and malonaldehyde (MDA) contents than strong cold resistant ones. Meanwhile, cold tolerant varieties accumulates higher amount of soluble sugar, soluble protein, superoxide dismutase (SOD) than cold sensitive varieties in order to protect plant cells under cold stress. Previously, these physiological indices have been used frequently to evaluate cold hardiness of kiwifruit plants. The stem structure implied the ability of cold resistance and the thickness of collenchyma, the ratio of collenchyma thickness to cane diameter, and the ratio of xylem thickness to cane diameter could be used as a morphological index to evaluate the cold resistance of kiwifruit. There is very little report available on molecular mechanism of cold resistance in kiwi- fruit. Over expression of CBF transcription factor regulated by ICE gene can impart the cold resistance in kiwifruit. Moreover, MYB transcription factors also play an important role in cold hardiness of kiwi- fruit. The product of sugar synthesis and starch degradation will provide energy for kiwifruit to with- stand low temperature, the ability of degradation of starch is different in different varieties, and key genes in different varieties related to these metabolic pathway need to be researched. The development of omics technology gives us great opportunity to study the mechanism of cold resistance in detail. Ki- wifruit as a newly-developed commercial fruit has a good development prospect. Until now, studies on many aspects of kiwifruit have already been reported. Cold stress is a main limiting factor adversely in- fluencing kiwifruit industry worldwide. In the future, we suggest using advanced scientific method to forecast weather change to guide and prevent freezing damage to kiwifruit plants. In addition, extensive research on molecular mechanism should be carried out in order to dig out the excellent cold resistance genes that will provide theoretical basis for the introduction and breeding of cold resistant varieties.