|Author: ||M.M. Blanke|
|Keywords: ||abscisic acid (ABA), apple, auxin, CO2, climate change, PEPC, photosynthesis, Rubisco, sink strength, tomato, transpiration|
Within a plant, the “source” may be defined as a photosynthesizing tissue or organ with export of carbon skeletons, the “sink” as one requiring import of carbon, the “sink strength” as the ability of a tissue or an organ to mobilize photo-assimilates, the “sink capacity (or sink size)” as the capacity of a tissue or organ to import and store further compounds from the source(s) and the “sink activity” by the rate of respiration.
Plant growth and development are normally limited by photosynthetic resources, i.e. are “source-limited”. Leaf photosynthesis is normally down-regulated by the sink and the presence of sinks viz fruit prohibits or retards leaf senescence; several sinks can compete for photo-assimilates; after fruit removal, i.e. harvest, the roots become the dominant sink.
In apple, fruit removal led to starch accumulation in the leaf chloroplasts, while the soluble sugars remained largely unaffected.
In tomato, fruit harvest lead to an instant decline in photosynthesis, carbohydrate surplus, release of vacuolar nitrate into the cytoplasm with nitrate assimilation by cytoplasmic nitrate reductase as a consequence.
The sink or presence of fruit increased the stomatal conductance of the leaves, enhanced transpiration, uptake and transport of water and nutrients and evapotranspirational cooling, but also required more water supply for the leaves and the fruit; a water potential gradient enables water flux into the fruit.
Overall, the fruit or sink down-regulated leaf photosynthesis, increased photosynthesis, Rubisco in vivo activity, dark respiration, stomatal conductance (to water), transpiration and water use efficiency (wue) and prevented or retarded leaf senescence, then modified by the environment.
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