Regulatory mechanisms that control photosynthetic responses to developmental processes in Xanthium strumarium

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2008-05

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Photosynthetic adjustments to developmental changes (e.g. flowering, fruiting and senescence) are frequently attributed to a source:sink feedback mechanism. This mechanism may be driven by changes in leaf soluble sugar content that in turn induce changes in the activity of key photosynthetic enzymes. Quantifying plant response and enzyme function in terms of age and nutrient supply can help define the relationship between growth and physiology, as well as elucidating the interactions of plant function from the cellular to whole plant level. This study used the model plant Xanthium strumarium to explore these functional interactions through two separate nutrient experiments focusing on nitrogen and phosphorus before, during, and a change in photoperiod, which was used to transition the plants from the vegetative to the reproductive growth phase. Parameters examined were biomass allocation and amount, elemental content (nitrogen and phosphorus), photosynthetic rate and capacity (Asat, Amax, Vcmax, and Jmax), chlorophyll content, total nonstructural carbohydrate amount (sugar, starch, and total TNC), and the enzyme activities of Rubisco, invertase, and sucrose phosphate synthase. Results indicated that changes in nutrient availability negatively affected enzyme activity and source:sink relationships, and specifically demonstrated that during conditions of nutrient limitation, sink activity was reduced due to limited source activity. Results also indicated that increasing age and changing developmental stage exacerbated nutrient limitation effects as well as causing effects that were independent of nutrient limitation. In both experiments, increasing plant age decreased photosynthetic rate and capacity as well as chlorophyll content and enzyme activity while it increased TNC amounts and overall biomass as well as biomass allocation. Increasing plant age increased root TNC content as well as invertase activity and root allocation. Our results indicate that decreasing nutrient supply and increasing plant age are linked processes that have feedback effects not only on overall plant function, but also on each other.

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