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Effect of photoperiod on photosynthetic down-regulation and carbon gain in Allium fistulosum.
van Gestel, Natasja1, Nesbit, April1, Haley, Luke1, Peffley, Ellen1, Tissue, David1, 1 Texas Tech University, Lubbock, Texas
ABSTRACT- We investigated the effect of increased source activity (24-h continuous light) on the growth, photosynthesis and biochemistry of onion (Allium fistulosum) by growing plants for 56 days in a 12-h (control) or 24-h photoperiod. Plants grown in 24-h light exhibited decreased leaf net photosynthesis (A) and photosynthetic capacity (Amax) compared with plants in 12-h light, indicating photosynthetic down-regulation. The reduction in A and Amax for leaves of 24-h plants was due to reductions in leaf N and increased soluble sugar. The reduction in leaf N suggested that less N was available to produce Rubisco. We suggest that the accumulation of sugars is due to the inhibition of respiration in 24-h plants, thereby allowing carbohydrates to accumulate in the leaf and initiate a feedback signal to reduce photosynthesis. These results demonstrate that carbohydrate utilization in respiration affects photosynthesis the following day. On a specific leaf area basis, 24-h plants generally gained more carbon than 12-h plants because they had a positive A over the entire 24-h period. However, 24-h plants produced much less total leaf area, because 24-h plants may have allocated their carbon to other functions (e.g. root exudates, volatile carbon emissions, growth of other plant organs, and other physiological processes). Therefore, on a whole plant basis, 24-h plants gained less carbon than 12-h plants. In conclusion, increased source activity (i.e. continuous light) was detrimental to plant growth due to reductions in photosynthetic capacity and changes in carbon allocation.
Key words: Photosynthesis, photoperiod, Allium fistulosum, Biochemistry