Abstract: [Objective] Larix principis-rupprechtii, a deciduous conifer tree species, is an important forest component and timber resource in subalpine of North China. This species is mainly distributed in subalpine regions of Shanxi province where droughts have occurred frequently in recent decades. In this study, drought effects on seedling growth and physiological responses of L. principis-rupprechtii were carried out to analyze its drought resistance and growth strategy at early stage. This study intends to provide a basis for afforestation, especially for seedling management and protection of this species. It can also efficiently predict regeneration of the species under the drying climate. [Method] A potted experiment with one-year-old seedlings of L. principis-rupprechtii was carried out with four water regimes by controlling soil water content using weighing method. Light drought (LD) was treated as 50%-60%, moderate drought (MD) as 35%-50%, severe drought (SD) as 20%-35%, and control (CK) as 70%-80% of field water carrying capacity, respectively. For each treatment, length of needles, stems and primary roots of the seedlings were measured, and lateral roots of each seedling were counted and measured to analyze drought effects on elongation growth of vegetative organs. Physiological indexes, including needle chlorophyll contents, antioxidants, i.e. SOD, POD activities and malondialdehyde (MDA) contents, and osmotic substances, i.e. proline, soluble sugar and protein were also measured to analyze seedling physiological changes. Then drought resistance of the one-year-old seedling was analyzed and drought stress threshold was estimated on the basis of above indexes. [Result] Elongations of needles and stems of L. principis-rupprechtii seedlings were significantly restrained by both the moderate and severe drought stress, but not significantly by light drought. The formation and elongation of lateral roots were promoted by all drought stresses. There was close correlation between primary root length and needle length under soil drought. Contents of total chlorophyll and chl a were not changed by light drought, but constantly decreased with increasing drought intensity. The Chl b content was decreased by severe drought while chl a/chl b was not significantly affected by drought. With increased drought level, activities of SOD and POD were both significantly promoted and contents of MDA and proline were remarkably increased, but soluble protein contents significantly decreased. Soluble sugar content of stems was continually rising by increased drought, while the sugar contents of needles and roots were increased by light drought but decreased by moderate and severe drought. The antioxidant system and proline contents of three organs (root, stem and needle) were closely related.[Conclusion] Light drought had no or only slight effects on the elongation growth and physiological indexes of seedlings, while moderate and severe drought had significant negative effects. One-year-old seedlings of L. principis-rupprechtii were of some drought resistance, fulfilled by the adaptive strategy that reduced the aboveground growth and gave priority to the root growth. The utility of antioxidant system and proline accumulation were also efficient responses to alleviate the seedling impacts by drought. Morphological change of needles was less distinct and less than physiological changes. No needles turned yellow or shed during the trail of 120 days. Carbon uptake of needles and carbon storage of roots were both affected by moderate and severe drought, which would lead to carbon starve and even to death of seedlings. It is estimated that the drought stress threshold of the relative water content of soil is less than 35% of field water carrying capacity. Mean monthly rainfall of May in Shanxi is roughly equal to the moderate and severe drought in this trail, thus seedling growth of L. principis-rupprechtii is substantially affected by the spring drought. The effect would be strengthened by the drying climate in future, saying the rainfall reducing by 30% of the present amount.

Key words: Larix principis-rupprechtii, seedling, drought, drought resistance