Dynamics in nonstructural carbohydrates of Fraxinus mandshurica seedlings during a progressive shade

Key message

Tissues were of different importance as NSC storage site; NSC concentration and NSC mass under C limitations were better indicators of the C balance at the organ and whole-plant level, respectively.

Abstract

Non-structural carbohydrates (NSC) are a critical carbon (C) source of trees when the C demand overweighs the supply of newly assimilated C. However, it remains uncertain regarding the remobilization and allocation of stored NSC when a C limitation occurs. Here, we manipulated the C balance of Fraxinus mandshurica seedlings by a progressive reduction of light availability from ambient conditions to moderate shade (30% of full sunlight for 3 weeks) and then to deep shade (0.1% for another 6 weeks). We measured the NSC concentration and mass in six tissues, and C-sink activities (including leaf respiration, stem respiration, biomass and its allocation) every three weeks. We found that, compared with ambient conditions, moderate shade only reduced the NSC concentration in fine roots, while deep shade reduced those in almost all of the tissues especially for starch concentrations. Likewise, the NSC mass in each tissue investigated in moderate shade was not significantly different from that in ambient conditions, but that in deep shade was consistently lower. The difference in NSC mass between the deep shade treatment and ambient was greater than that in NSC concentration. Across the organs, roots were a more critical compartment for NSC storage and remobilization than stems; both living barks and woods were important NSC storage tissues. The NSC mass fraction of stems was higher in deep shade than in ambient conditions. The decrease in NSC under deep shade significantly contributed to the lower leaf and stem respiration. Our findings highlighted the important role of NSC storage and remobilization in seedling growth and survival under deep shade.