Abstract: Extreme drought events have triggered widespread forest dieback globally, and thus the investigation of tree responses to lethal droughts has become a hot topic in forest ecology. However, research on the mechanisms of tree responses to lethal droughts is not yet consistent. Here, we examined seedlings of Rhododendron decorum, a widespread species in high-altitude regions of Southwest China, to quantify shifts in non-structural carbohydrates (NSC), photosynthesis, respiration, and water status under lethal drought. The results showed that: (1) During progressive drought intensification, total NSC and soluble sugar concentrations in leaves declined, whereas their concentrations in roots increased. Stem NSC and soluble sugars exhibited an initial increase and then a marked decrease, while starch concentration fluctuated in all organs. (2) Lethal drought changed the allocation pattern of total NSC in the leaves and roots, with NSC preferentially allocated to roots. (3) As drought intensified, leaf physiological traits exhibited either a gradual decline followed by a sharp drop, or a pattern of slow decrease-transient rebound-abrupt decrease. The results indicate that drought significantly suppresses carbon metabolism in plants, and that there is a trade-off in the response of non-structural carbohydrates in leaves and roots to lethal drought, reflecting a strategy of prioritizing carbon allocation to roots. This study provides a theoretical basis for understanding the physiological mechanisms of Rhododendron species in response to extreme drought.

Key words: Rhododendron decorum, drought stress, carbon balance, non-structural carbohydrate