Photosynthesis in leaves and siliques of winter oilseed rape (Brassica napus L.) |
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Authors: | L H Gammelvind J K Schjoerring V O Mogensen C R Jensen J G H Bock |
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Institution: | (1) Plant Nutrition Laboratory Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark;(2) Agrometeorology and Bioclimatology, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark |
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Abstract: | The rate of photosynthesis and its relation to tissue nitrogen content was studied in leaves and siliques of winter oilseed
rape (Brassica napus L.) growing under field conditions including three rates of nitrogen application (0, 100 or 200 kg N ha-1) and two levels of irrigation (rainfed or irrigated at a deficit of 20 mm).
The predominant effect of increasing N application under conditions without water deficiency was enhanced expansion of photosynthetically
active leaf and silique surfaces, while the rate of photosynthesis per unit leaf or silique surface area was similar in the
different N treatments. Thus, oilseed rape did not increase N investment in leaf area expansion before a decline in photosynthetic
rate per unit leaf area due to N deficiency could be avoided. Much less photosynthetically active radiation penetrated into
high-N canopies than into low-N canopies. The specific leaf area increased markedly in low light conditions, causing leaves
in shade to be less dense than leaves exposed to ample light.
In both leaves and siliques the photosynthetic rate per unit surface area responded linearly to increasing N content up to
about 2 g m-2, thus showing a constant rate of net CO2 assimilation per unit increment in N (constant photosynthetic N use efficiency). At higher tissue N contents, photosynthetic
rate responded less to changes in N status. Expressed per unit N, light saturated photosynthetic rate was three times higher
in leaves than in silique valves, indicating a more efficient photosynthetic N utilization in leaves than in siliques. Nevertheless,
from about two weeks after completion of flowering and onwards total net CO2 fixation in silique valves exceeded that in leaves because siliques received much higher radiation intensities than leaves
and because the leaf area declined rapidly during the reproductive phase of growth.
Water deficiency in late vegetative and early reproductive growth stages reduced the photosynthetic rate in leaves and, in
particular, siliques of medium- and high-N plants, but not of low-N plants. |
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Keywords: | Brassica napus canola oilseed rape nitrogen nutrition irrigation photosynthesis photosynthetic nitrogen use efficiency |
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