Physiological Responses of Two Contrasting Kiwifruit (Actinidia spp.) Rootstocks against Waterlogging Stress

Rootstocks from Actinidia valvata are much more tolerant to waterlogging stress than those from Actinidia deliciosa, which are commonly used in kiwifruit production. To date, the tolerance mechanism of A. valvata rootstocks' adaptation to waterlogging stress has not been well explored. In this study, the responses of KR5 (A. valvata) and 'Hayward' (A. deliciosa) to waterlogging stress were compared. Results showed that KR5 plants performed much better than 'Hayward' during waterlogging by exhibiting higher net photosynthetic rates in leaves, more rapid formation of adventitious roots at the base of stems, and less severe damage to the main root system. In addition to morphological adaptations, metabolic responses of roots including sufficient sucrose reserves, modulated adjustment of fermentative enzymes, avoidance of excess lactic acid and ethanol accumulation, and promoted accumulation of total amino acids all possibly rendered KR5 plants more tolerant to waterlogging stress compared to 'Hayward' plants. Lysine contents of roots under waterlogging stress were increased in 'Hayward' and decreased in KR5 compared with their corresponding controls. Overall, our results revealed the morphological and metabolic adaptations of two kiwifruit rootstocks to waterlogging stress, which may be responsible for their genotypic difference in waterlogging tolerance.