Effects of leaf position and nitrogen supply on the expansion of leaves of field grown sunflower (Helianthus annuus L.) |
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Authors: | N Trápani A J Hall |
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Institution: | (1) F.E.V.A., Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martín 4453, 1417 Buenos Aires, Argentina |
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Abstract: | Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field
conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LAmax) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small.
Specific leaf nitrogen (SLN, g N m-2) fell quite rapidly during the initial leaf expansion phase (LA < 35% LAmax) but leveled off during the final 65% increase of leaf area. This pattern held across leaf positions and N supply levels.
Leaf nitrogen accumulation after 35% LAmax continued up to achievement of LAmax; reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for
expansion during the final phase. LER in the quasi-linear expansion phase (35 to 100% of LAmax) was strongly associated with SLN above a threshold that varied with leaf position (mean 1.79±0.225 g N m-2). This contrasts with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a
threshold of 0.3 g N m-2; in the present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m-2. Thus, once the area of a leaf exceeds 35% of LAmax, expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves
during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels. |
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Keywords: | Helianthus annuus leaf expansion leaf photosynthesis specific leaf nitrogen sunflower |
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