Browsing by Subject "Photosynthesis"

Root carbon exudation is a critical element of the soil carbon cycle, and how both environmental conditions and plant traits influence exudation remains uncertain. I studied relationships between environmental conditions, plant traits, and carbon exudation in four herbaceous plant species: Asclepias incarnata, Microstegium vimineum, Panicum virgatum, and Scirpus cyperinus. Mature individuals were given short-term factorial light and N treatments, and exudates were collected from 8-hour carbon-free hydroponic incubations. I measured size traits (biomass, leaf area, root length, and root volume), photosynthesis (leaf-level and whole-plant), and tissue N traits (root, stem, and leaf percent N and C:N ratio). Neither light nor N treatments affected exudation, while exudation varied with species and traits. Species alone substantially explained mass-specific exudation (estimated R2 = 0.38). Size strongly predicted both total and mass-specific exudation, interacting with species (estimated R2 = 0.52 and 0.48, respectively). Generally, larger individuals exuded more overall but less per unit mass, although larger M. vimineum plants exuded more per unit mass. Whole-plant photosynthetic rate was weakly related to total exudation (estimated R2 = 0.17), and tissue N concentration moderately predicted mass-specific exudation (estimated R2 = 0.23). Other researchers have found that high light and low nitrogen availability stimulate exudation; my results indicate that this relationship is not straightforward. Plant traits, however, significantly explained variation in exudation, including some variation across species, supporting trait-based analyses of plant species' effects on ecosystem processes.

Terrestrial ecosystems depend on vegetation for many indispensable services including carbon fixation from the atmosphere, food production, and the maintenance of the global water and carbon cycles. As the climate changes, temperature and precipitation patterns shift and extreme climatic events become more frequent. In many areas, droughts are increasing in intensity and frequency, posing a challenge to ecosystem health and food security. Plants depend on water for physiological functioning including photosynthesis. The ability of plants to continue supplying water to the leaves from the soil during droughts depends on the anatomy and structure of its vascular network, the xylem. Droughts cause gas bubbles, or embolisms, to spread within the xylem, blocking water movement.

A combination of modeling water flow in xylem of flowering plants and theoretical considerations derived from graph theory is used to explain the response of different xylem functional types to droughts. An open-source model of plant xylem hydraulics was developed with which it was shown how 'network' effects, such as the spatial distribution of anatomy throughout growth rings, alter the response of Maples to drought.

The xylem of similar flowering plants was further investigated through the model in addition to the the physics of percolation. This was the first instance percolation theory has ever been applied to embolism spread inside xylem. It was shown how embolism spread inside the xylem can be represented by an edge percolation process. The results indicate that an increased connectivity among the conduits in the xylem is a necessary feature in plant organs that are resistant to droughts.

The detrimental effects of droughts on plant water translocation cascade to inhibit photosynthesis. Soil-to-leaf resistance to drought is represented by a vulnerability to embolism curve (VC) that plots the percent loss in plant hydraulic conductivity as water potential declines. The whole-plant VC affects plant CO2 fixation under drought. The results show how different VC shapes give rise to typical isohydric and anisohydric plant responses to drought. To arrive at this conclusion, the calculus of variations is used to integrate plant hydraulics into the trade-off between CO2 fixation and transpiration during a drought.

There is no consensus on climatic warming's effect on phenology, photosynthesis, and leaf nitrogen content in temperate deciduous tree species. A major question is whether or how these trees will utilize a longer growing season. Data on leaf photosynthetic rates, leaf nitrogen content, and leaf phenological status were collected weekly or biweekly in 2012 for Acer rubrum, Liquidambar styraciflua, Liriodendron tulipifera, Quercus alba, Quercus rubra, and Fraxinus americana at Duke Forest in Orange County, North Carolina. Seedlings were grown in open-topped chambers established in 2009 that were maintained at either ambient or 5 degrees Celsius above ambient temperatures. Half the chambers were shaded, half were under gap conditions.

Four of six species had advanced spring phenology with warming treatment, and four of six species delayed leaf senescence with warming treatment. Shade delayed spring phenology later more than earlier in the season, and gap conditions delayed fall phenology. Warming had an inconsistent effect on photosynthetic rate. Two species increased photosynthetic rate with chambered +5C conditions, another decreased photosynthetic rate in unchambered plots. Half of the species studied had no significant correlation between temperature and photosynthetic rate. The timing of photosynthetic decline was simultaneous within species across warming treatments, even in species which delayed visible senescence with warming.

There were major differences between years in terms of the effect of warming on nitrogen content and resorption. In 2011, percent leaf nitrogen was lower in warmed chambers, and nitrogen resorption efficiency was not correlated with warming treatment. In 2012, warmed chambers had higher nitrogen concentration and a wider range of nitrogen resorption values than ambient, and resorption efficiency was positively correlated with warming in 2012. It is possible that these differences between years are driven by differences in availability of soil moisture. 2011 was much drier during the senescent period than 2012. The direction and magnitude of warming on nitrogen was consistent within years, but varied by year.

Warming often causes visible change in the timing of phenology, but the timing of photosynthetic decline was unaffected and changes in nitrogen content and resorption were interannually variable. The benefits of delaying visible senescence are unclear. The effect of interactions between warming and soil moisture on leaf processes must be explored further. It is possible that the delay in visible phenology without a concurrent delay in leaf decline is due to a mismatch in environmental cues under a warmer climate. Because of the lack of obvious benefit to plant carbon uptake with warming, it is possible that there is no true extension of the growing season in fall.