Drought shifts internal carbon partitioning and use of recent photosynthates in black spruce trees: From bud to mature shoot

Background/Question/Methods

Springtime bud-break and shoot development induces substantial carbon (C) costs in trees, temporally altering the internal canopy source-sink relationships. Drought stress impedes C translocation delaying shoot development and potentially increasing the total C costs associated with foliar development. We studied effects of drought and re-hydration on shoot development and C use, in 10-year old Picea mariana [black spruce] trees to identify and quantify key morphological/ physiological processes. Trees were subjected to one of two treatments in a growth chamber; well-watered control (C) or drought and re-hydration (D). We monitored changes in morphological, biochemical (osmolality, [chlorophyll], [nitrogen], [C] and [non-structural carbohydrates (NSC)]) and physiological (rates of respiration (R_d) and light-saturated photosynthesis (A_sat)) processes during shoot development. Further, to study functional compartmentalization and use of new assimilates; we ¹³C-pulse labeled shoots at multiple development stages (at individual branch level), and measured isotopic signatures of leaf respiration, NSC pool and structural biomass.

Results/Conclusions

Overall shoot development was delayed by drought. Water deficit during shoot expansion resulted in longer, yet more compact shoots with on average greater (63%) needle osmolality compared to the shoots on the control-trees. The positive non-linear relationship through time between xylem water pressure potential and needle osmolality suggests osmoregulation occurs independent of developmental stage. Development of the photosynthetic apparatus was delayed, as shoots on C-trees broke-even (A_sat > 0) 14 days prior to D-shoots. Average values of R_d decreased with shoot maturation, generally smaller in D-trees, ranging from 224.8 to 12.8 and from 96.8 to 12.5 nmol g^-1 s^-1, in treatment C and D, respectively. ¹²C:¹³C isotopic patters, indicated that the internal C partitioning and use was dependent on foliar developmental stage and treatment. Shoots on D-tress respired a greater proportion of recently fixed C; this was especially true during early stages of shoot development. Mean residence time of C was dependent on rates of respiration more so than A_sat. In conclusion, temporary periods of water deficit inhibits C translocation from older organs delaying new shoot development (increasing C input) in black spruce, entailing shift in the internal C partitioning and use of recent carbon ensuring/prioritizing substrate availability for growth and mainten