Response of two Artocarpus species seedlings to flooding stress and evaluation of waterlogging tolerance

【Objective】Hainan has a tropical maritime monsoon climate with distinct dry and wet seasons, with concentrated rainfall from June to October. The rainfall is heavy and frequent. Therefore, seasonal waterlogging has become one of the major threats to crops in Hainan. Woody crops have a large biomass and slow response to waterlogging, which is often overlooked. As waterlogging time prolongs,the harm of waterlogging stress becomes increasingly prominent. This study aimed to investigate the responses of two Artocarpus species to different degrees of flooding stress, and comprehensively evaluate their waterlogging tolerance, so as to provide a theoretical basis for the study on waterlogging resistance of woody food crops.【Methods】Jackfruit (Artocarpus heterophyllus Lam.) and breadfruit [Artocarpus altilis (Parkinson) Fosberg], two Artocarpus species, were used as experimental materials, and the pot flooding test was carried out by the‘double pot method’. The experiment was designed as five treatments, i.e., normal water management (seedlings were flooded for 0 h) and flooded for 24 h, 48 h, 72 h, and 96 h. The plants treated with flooding were watered thoroughly to ensure that the water level exceeded the soil surface by about 2 cm. Afterwards, water was replenished daily to keep the roots in a saturated flooded state. After the first watering of the control plants, a certain amount of water should be replenished every 3 days to maintain the overall soil moisture content at 60% to 80% of the maximum field capacity. The changes of biomass, photosynthetic characteristics and antioxidant enzyme activities of jackfruit and breadfruit in different flooding times were investigated, and the waterlogging tolerance was evaluated by factor analysis and membership function methods.【Results】Flooding for 24 h significantly inhibited the aboveground dry weight and photosynthetic parameters of jackfruit seedlings, and the chlorophyll content also significantly decreased. Stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) increased after flooding for 72 h. The biomass of breadfruit seedlings significantly decreased after flooding for 24 h, and showed a significant increase trend after flooding for 72 h. The chlorophyll α, b, and (α+b) contents reached the lowest values, with decreases of 23.64%, 78.17%, and 31.96%, respectively, compared to the 0-hour flooding treatment. The net photosynthetic rate (Pn), Gs, Ci, and Tr of breadfruit seedlings showed an increasing trend after flooding for 24 h, while photosynthetic parameters and antioxidant enzyme activity showed significant inhibition after flooding for 72 h and 96 h, respectively. With the increase of flooding time, the aboveground and total biomass of jackfruit seedlings increased first and then decreased, the underground and root/shoot ratio increased, the Pn, Gs, and Tr decreased, and the antioxidant enzyme activity increased. The Pn, Gs and Tr of breadfruit seedlings reached their maximum values at 24 hours of flooding and then continued to decrease, while the activities of CAT, POD, and SOD enzyme activity showed an increasing trend and then decreasing, reaching their highest values at 72 hours of flooding with the proportions of increase compared to 0 hours of flooding were 234.49%, 317.68%, and 251.88%, respectively. Correlation analysis was conducted on the waterlogging tolerance coefficients of various physiological indicators, and six waterlogging tolerance evaluation indicators were selected, including Pn, Gs, Tr, CAT, POD, and SOD. Principal component analysis showed that the variance contribution rates of the first two comprehensive indicators were 62.47% and 34.26%, respectively, with a total variance contribution rate of 96.73%, which basically included all the information of the measured indicators. Factor 1 had a significant load on Pn, Gs, and Tr, which was classified as a photosynthetic parameter. Factor 2 had a significant load on CAT, POD, and SOD, classified as antioxidant enzyme activity. Factor analysis showed that the order of waterlogging tolerance of jackfruit and breadfruit was: breadfruit -96 h＞jackfruit -72 h＞ breadfruit -72 h＞jackfruit -96 h＞breadfruit -48 h＞breadfruit -24 h＞jackfruit -48 h＞jackfruit -24 h. Membership function analysis showed that the waterlogging tolerance of two Artocarpus species was from strong to weak: breadfruit - 96 h＞breadfruit - 24 h＞breadfruit - 72 h＞jackfruit - 72 h＞ breadfruit -48 h＞jackfruit -96 h＞jackfruit -48 h＞jackfruit -24 h. Flooding stress significantly inhibited the growth, photosynthetic characteristics and antioxidant enzyme activities of jackfruit. Breadfruit had stronger waterlogging tolerance and response to flooding stress.【Conclusion】Flooding stress sig-nificantly inhibited the biomass, chlorophyll content, photosynthetic properties and antioxidant enzyme activities of the two Artocarpus species. Jackfruit alleviates damage caused by flooding stress through expanding root absorption, pigment regulation and increasing antioxidant enzyme activity, while breadfruit resists waterlogging by enhancing photosynthesis and antioxidant enzyme activity. In the comprehensive waterlogging tolerance score, breadfruit is better than jackfruit, which has stronger adaptive potential in short-term and medium-term flooding environment and can be used as the preferred waterlogging resistant rootstock, while jackfruit is more suitable for planting in areas with good drainage.