Non-structural carbohydrates (NSC) have been traditionally interpreted as passive reservoirs of carbon that fluctuate depending on photosynthetic supply and metabolic demand. However, the starch and soluble sugar fractions of NSC perform contrasting functions and may therefore be differentially regulated. Starch is a storage compound for future metabolic use, while soluble sugars are typically used for immediate metabolic demands as well as signaling, cold tolerance, turgor maintenance, and phloem transport. A recent meta-analysis of seasonal NSC dynamics found strong depletion of starch during the growing season, but soluble sugar concentration remained relatively constant, supporting the hypothesis that soluble sugars must be kept above critical thresholds. However, the aggregation of NSC data across numerous studies can obscure key intra-season patterns and may not be well-matched to each species’ phenology. Thus, this study evaluates the within and between season variation in leaf NSC in an evergreen desert shrub (Larrea tridentata). From June 2015 to January 2017, Larrea leaves were sampled 51 times (weekly-monthly) on six (2015) or twelve (2016-2017) individual shrubs. We expected greater seasonal fluctuations in leaf starch than in leaf sugar, consistent with starch as a longer-term storage pool and the more immediate roles of soluble sugars.
Consistent with our expectations, we found significant seasonal variation in leaf starch. Leaf starch was lowest (4.7 mg/g) during the monsoon season (July-September), increased during the post-monsoon (October-December), and peaked in the winter (January-March), representing a 4.5-fold difference across seasons. Leaf starch declined steadily from its winter peak through the dry pre-monsoon period (April-June), which was not significantly different from monsoon season values. Conversely, leaf sugar did not vary significantly between seasons; intra-season and inter-season variation in leaf sugar were of a similar magnitude (F3,392 = 1.07, P = 0.36). Therefore, although sugars accounted for the majority (72 to 91%) of total leaf NSC, seasonal variability in total leaf NSC was driven by fluctuations in starch. Total leaf NSC was highest in winter (75 mg/g) and similar during the other seasons (55-58 mg/g). In agreement with the recent meta-analysis, our results show strong seasonal depletion of leaf starch during active growth periods, as monsoon leaf starch represented only 22% of winter values. Year-round, leaf sugar in Larrea did not fall below 50 mg/g, consistent with active regulation above a minimum threshold. These results suggest that high-frequency measurements are needed to disentangle the storage and physiological roles of NSC.