Non-structural carbohydrates vary by species, organ, and season in woody plants of the Sonoran Desert
Non-structural carbohydrates (NSCs) play an important yet not fully understood role in the complex relationships between plant growth, resource allocation, and survival under persistent stress. For example, recent evidence suggests that NSC storage and transport may have important implications for xylem embolism repair and recovery of hydraulic function following daily and seasonal drought cycles. Therefore, understanding seasonal patterns of NSC storage may have important consequences for predicting landscape-scale patterns of plant productivity and mortality in response to climate change. While NSCs have been quantified in tropical, temperate, and semiarid trees, less is known about seasonal variations and species differences in NSCs of desert species. In the Sonoran Desert, plant water stress varies seasonally and usually peaks before the onset of the summer monsoon, which could affect seasonal variation in NSC. In June (pre-monsoon), August (monsoon), and October (post-monsoon) of 2014, leaf and twig samples were collected from seven widely distributed woody species, and additional wood samples were extracted from the trunks of three of these species. Sugar and starch fraction was determined colorimetrically following hot ethanol extraction; a second extraction protocol was used on the October samples and compared to the original protocol using a mixed-effects modeling framework.
Across all species, organs, and seasons, total NSC averaged 51.28±2.49 mg per g oven-dry tissue (total=sugars + starch, mean ± 1 SE). Soluble sugars ranged from 6.49±0.55 mg/g (Prosopis velutina, October) to 64.03±8.27 mg/g (Lycium fremontii, August) across all organs. Similarly, starch ranged from 1.57 ±0.43 mg/g (Ambrosia deltoidea, August) to 35.65±17.21 mg/g (Celtis pallida, August). Across organs and seasons, Parkinsonia microphylla had the highest total NSC (68.39±9.18 mg/g) whereas A. deltoidea had the lowest (35.53±4.46 mg/g). Total NSC was highest in August (67.20±3.64 mg/g) and lowest in October (25.96 ±4.55) for all species and organs. Likelihood ratio tests from repeated-measures ANOVAs found that soluble sugars varied among individual plants and between protocols (P<0.001), but starch did not vary among these random factors (P=0.16). Furthermore, including all two-way interactions explained significantly more variation than the main effects alone for both soluble sugars (P=0.009) and starch( P<0.001). Our results show that drought-deciduous species such as A. deltoidea, C. pallida, and P. microphylla maintain different proportions of sugars and starches in response to seasonal water stress. In summary, NSC can vary greatly among species and seasons, with implications for understanding whole-plant function in water-limited environments.