Springtime Drought Shifts Carbon Partitioning of Recent Photosynthates in 10-Year Old Picea mariana Trees, Causing Restricted Canopy Development

Abstract

Springtime bud-break and shoot development induces substantial carbon (C) costs in trees. Drought stress during shoot development can impede C uptake and translocation. This is therefore a channel through which water shortage can lead to restricted shoot expansion and physiological capacity, which in turn may impact annual canopy C uptake. We studied effects of drought and re-hydration on early season shoot development, C uptake and partitioning in five individual 10-year old Picea mariana [black spruce] trees to identify and quantify dynamics of key morphological/physiological processes. Trees were subjected to one of two treatments: (i) well-watered control or (ii) drought and rehydration. We monitored changes in morphological [shoot volume, leaf mass area (LMA)], biochemical [osmolality, non-structural carbohydrates (NSC)] and physiological [rates of respiration (Rd) and light-saturated photosynthesis (Asat)] processes during shoot development. Further, to study functional compartmentalization and use of new assimilates, we 13C-pulse labeled shoots at multiple development stages, and measured isotopic signatures of leaf respiration, NSC pools and structural biomass. Shoot water potential dropped to a minimum of −2.5 MPa in shoots on the droughted trees. Development of the photosynthetic apparatus was delayed, as shoots on well-watered trees broke-even 14 days prior to shoots from trees exposed to water deficit. Rd decreased with shoot maturation as growth respiration declined, and was lower in shoots exposed to drought. We found that shoot development was delayed by drought, and while rehydration resulted in recovery of Asat to similar levels as shoots on the well-watered trees, shoot volume remained lower. Water deficit during shoot expansion resulted in longer, yet more compact (i.e., with greater LMA) shoots with greater needle osmolality. The 12C:13C isotopic patterns indicated that internal C partitioning and use was dependent on foliar developmental and hydration status. Shoots on drought-stressed trees prioritized allocating newly fixed C to respiration over structural components. In conclusion, temporary water deficit delayed new shoot development and resulted in greater LMA in black spruce. Since evergreen species such as black spruce retain active foliage for multiple years, impacts of early season drought on net primary productivity could be carried forward into subsequent years.

Warming and elevated CO2 induced shifts in carbon partitioning and lipid composition within an ombrotrophic bog plant community

“…Furthermore, results from δ 13 C measurement showed that the fraction of new C decreased with the needle age (Y0 > Y2 > Y4; Fig. 7), indicating that recent photosynthates were strongly invested in structural compounds to build Y0 needle biomass, as also reported for young spruce needles (Schadel et al, 2009;Jensen et al, 2021), as opposed to allocation to metabolic storage compounds in existing older cohorts. We therefore expected to observe a large amount of newly assimilated carbon (80-90%) in the Y0 needles considering their active metabolic stage (Sachse et al, 2015;Jensen et al, 2021).…”

“…7), indicating that recent photosynthates were strongly invested in structural compounds to build Y0 needle biomass, as also reported for young spruce needles (Schadel et al, 2009;Jensen et al, 2021), as opposed to allocation to metabolic storage compounds in existing older cohorts. We therefore expected to observe a large amount of newly assimilated carbon (80-90%) in the Y0 needles considering their active metabolic stage (Sachse et al, 2015;Jensen et al, 2021). Surprisingly, our results show lower fractions of newly assimilated C in the Y0 needles which increased with warming and maximized at 70% (in plant tissues and cuticular lipids; Fig.…”

“…While the primary aim of our study was to understand the impact of warming and elevated CO 2 concentration on secondary metabolism and carbon investment across taxonomic groups, we also evaluated carbon assimilation and investment patterns of Picea mariana to new growth vs maintaining existing cohorts in relation to overwintering, multiyear retention and temperature responsiveness. We initially hypothesized that conifers exposed to warmer temperatures and elevated CO 2 concentration would display an alteration in their leaf chemistry in older needles as a result of stored carbon from older needles being reallocated to developing newly emerged needles (Jensen et al, 2015(Jensen et al, , 2021. Indeed, in the Y0 needles, we observed significantly higher nitrogen concentration compared with older cohorts, with increasing temperature (Fig.…”

“…Indeed, in the Y0 needles, we observed significantly higher nitrogen concentration compared with older cohorts, with increasing temperature (Fig. 6b), indicating that P. mariana was strongly investing in developing Y0 cohorts compared with older cohorts (Heinrich et al, 2015;Jensen et al, 2021) when exposed to warmer temperatures. Furthermore, results from δ 13 C measurement showed that the fraction of new C decreased with the needle age (Y0 > Y2 > Y4; Fig.…”

“…While environmental changes may have lasting impacts on species composition (McPartland et al, 2019;Norby et al, 2019), its effects are often first seen in plant C allocation of NSC to secondary metabolites for defense, osmoregulation, respiration, or growth (Ward et al, 2019;Hartmann et al, 2020;Jensen et al, 2021). Analysis of secondary metabolites such as lipids across plant taxonomic groups offer opportunities for examining leaf metabolic processes and physiological traits under changing environmental condition (Heinrich et al, 2015;Sachse et al, 2015;Desalme et al, 2017).…”