Photosynthetic traits and plant-water relations of two apple cultivars grown as bi-leader trees under long-term waterlogging conditions

Waterlogging has been one of the major problems in plant cultivation worldwide. As its frequency and severity are increasing in the era of climate change, waterlogging tolerance of fruit cultivars is an important factor that should be considered before plantation. In this study, we investigated leaf morphological traits, photosynthetic responses, and plant-water relations of the two most popular apple cultivars in Korea, Hongro (medium maturing) and Fuji (late maturing), under long-term waterlogging conditions. Experimental bi-leader trees grafted on a single M.9 rootstock were subjected to two irrigation regimes (well-irrigated and waterlogged) for 75 days. Waterlogging stress adversely affected most of the morphological, physiological, and anatomical characteristics as well as the plant-water relations in both cultivars; the negative effects of waterlogging were more pronounced in Hongro than in Fuji. Both cultivars experienced a significant decrease in leaf size, whereas leaf length was reduced significantly in Hongro and leaf width in Fuji. Predawn leaf water potential (Psi(PD)) decreased with waterlogging, but midday leaf water potential (Psi(MD)) was more significantly decreased in Hongro than in Fuji under waterlogging. Net photosynthetic rate (P-n) and stomatal conductance (g(s)) were significantly reduced in both cultivars; the reduction was greater in Hongro than in Fuji. A more pronounced reduction in vessel density, vessel diameter, and stomatal density (S-d) was observed in Hongro than in Fuji, resulting in greater reduction in daily sap flow and hydraulic conductivity in Hongro than in Fuji. These results indicate that Fuji was better adapted to waterlogging, as indicated by fewer changes in morphological, physiological, and anatomical responses when compared with those of Hongro. Our data suggest that the lower reduction in vessel density, vessel area, and vessel diameter in the Fuji cultivar under waterlogging are major factors that improve plant-water relations and photosynthetic performance, which in turn determine the leaf morphological characteristics and vegetative growth under prolonged waterlogging.