Nonstructural carbohydrates influence water relations in Pinus ponderosa
Non‐structural carbohydrates influence water relations in Pinus ponderosa Aurora Bayless The Sala lab of Plant Physiological Ecology Division of Biological Sciences University of Montana
Why study water relations in Ponderosa pine? • Drought is predicted to increase with climate change • Drought stress mortality is a major driving factor in forest ecology 2, 562 matches Mainsfield search 123, 000 articles google scholar • Forests hold lots of carbon, and influence global cycles • The specific mechanisms behind drought related mortality are still unknown Shinneman et al. 2016
Photosynthesis makes carbohydrates Non‐Structural Carbohydrates (NSC) Sugars are hydrophilic Structural Carbohydrates (cellulose)
Making sugar requires water loss. Water loss increases xylem tension.
Xylem tension is dangerous! Plants must prevent water loss or the water column will break leading to catastrophic hydraulic failure
Plants can close stomata to prevent water loss However, closing stomata to prevent water loss prevents photosynthesis carbon starvation
Two main hypotheses for the mechanisms of drought stress mortality: • 1) hydraulic failure, mortality is caused by xylem cavitation • We know this • 2) Carbon starvation happens as a result of stomatal closure (Mc. Dowell, et al. 2008). • We don’t know the threshold yet
NSC is not used as expected drought control Drought does not deplete starch Galvez et al. 2011. Tree Physiol. Sala & Hoch 2009. PCE Wiley et al. 2015. Tree Physiol. Palacio et al. 2008. Funct. Ecol.
Why so much NSC? Metabolism Reservoir Osmotic role NSC may help retain and move water
• It is critical to know how NSC affects hydraulic function to predict mortality. • First we need to know if there is a threshold for carbon starvation • Then, we can ask how carbon starvation may drive the hydraulic failure to critical thresholds and vice versa
Research hypotheses: • 1) Non‐Structural Carbohydrates are related to hydraulic function. • 2) NSC below a certain threshold causes mortality.
7 CONTROL TREATMENT
Non‐Structural Carbohydrates decreased in the dark 8 Roots 6 NSC (%) Stems Needles 4 2 0 CI 1 2 3 # weeks in dark 4 Stepwise NSC decline with dark exposure 5 6
Non Structural Carbohydrates decreased in the dark Whole Plant NSC (%) 8 6 4 2 0 CI 1 2 3 # weeks in dark 4 Stepwise NSC decline with dark exposure. Weeks 1‐ 3 and weeks 4‐ 6 have similar NSC 5 6
Hypothesis 1 • Non‐Structural Carbohydrates are related to hydraulic function.
Pre‐Dawn Water potential
Water Potential (Y) Decreases with dark exposure despite irrigation ‐ 1 Y (MPa) Increasing water stress 0 ‐ 2 ‐ 3 ‐ 4 CI 1 2 3 # weeks in dark 4 5 6
High water stress despite irrigation is related to declining NSC 1 3 2 4 5 6 C
Water stress occurs at about 40% NSC relative to controls 1 3 2 4 5 6 C
Positive carbon assimilation occurs after one and four weeks of dark exposure 6 A (mmol m‐ 2 s‐ 1) 4 2 0 ‐ 2 ‐ 4 CI 1 2 3 4 5 6 # weeks in dark Declining NSC after four weeks of dark exposure stimulates C assimilation. This could be a compensation mechanism to make up for lost carbohydrates, because of shade.
Increasing water stress Higher water stress despite irrigation related to declining NSC
Increasing water stress Higher water stress despite irrigation related to declining NSC‐depleted plants with low water stress tend to maintain positive photosynthesis. This indicates that Carbon is needed to retain water.
Hypothesis 2 • 2) NSC below a certain threshold causes mortality.
In well watered groups the first mortality occurs after three weeks of dark exposure 90 80 70 % Mortality 60 50 40 30 20 10 0 0 1 2 Weeks of Dark 3 4 5
Below 40% NSC relative to controls plants are unable to maintain hydraulic function and mortality increases
What does this mean for the future? • Drought is a major issue, and we need to model it to understand mortality and Carbon cycles. • How is this research important for land managers, modelers, forest managers, et cetera? • We need to start considering the interactions of hydraulic function and Non Structural Carbohydrates in order to model future tree mortality.
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Thank you! Sala Lab Physiological Plant Ecology • Sean Pinnell (MS Student) • Anna Sala (PI) • Beth Roskilly (MS student) • Gerard Sapés (Ph. D Student)
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