Carbon use strategies in shoot and acorn growth of two evergreen broadleaf trees unraveled by seasonal carbohydrate measurements and carbon isotope analysis.

Abstract

Woody species have evolved carbon (C) storage processes that meet needs for reserves associated with asynchronies between C supply and demand. However, our understanding of storage dynamics is still elusive in mature trees, especially when reproduction is involved. Integrated analyses of isotope ratios, concentrations and biomass may enhance understanding of stored C fractions' dynamics and roles. Thus, we monitored starch and soluble sugars (SSs), C isotope ratios and biomass, in leaves, twigs and reproductive organs of two mature evergreen broadleaf trees, Quercus glauca Thumb. and Lithocarpus edulis Nakai, for 2 years. During the growing season, no starch was observed in twigs, while constant starch levels were observed in leaves. Increase in SSs for winter hardening was earlier in L. edulis than in Q. glauca, in line with L. edulis acorns' earlier ripening. Decrease in SSs and increase in starch occurred simultaneously in the next spring. In addition, sucrose accounted for <10% of total SSs in leaves of both species, whereas mannose accounted for up to 75% in Q. glauca and myo-inositol up to 23% in L. edulis, indicating species-specific sugar composition. These results indicate that seasonal variation of SSs fraction was more reflective of climatic change and nonstructural carbohydrate storage was less influenced by reproduction. No starch was detected in acorn organs of either Q. glauca or L. edulis except in ripening seeds. The biomass of ripe acorns was 1.7- and 6.4-fold greater than that of current-year twigs in Q. glauca and L. edulis, respectively. Bulk twigs and reproductive organs were ca 1.0‰ 13C enriched relative to bulk leaves, which was lower than in deciduous trees. These results indicate that a new photo-assimilate is the predominant C source for reproductive growth. These findings provide new insights into the dynamics of C storage in relation to reproduction in evergreen broadleaf trees.