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Photosynthetic characteristics of field-grown cotton (Gossypium hirsutum L.) leaves were determined at several insertion levels within the canopy during two growing seasons. Net photosynthesis (Pn), stomatal conductance to CO2 (gs(.)CO2), leaf area expansion, leaf nitrogen and light intensity (PPFD) were recorded at 3-4 day intervals during ontogeny for the main-stem leaf and the first three sympodial leaves at main-stem nodes 8, 10 and 12. During early leaf development, Pn increased to a maximum of 16.5 µmol m-2 s-1 at 79-100% of full leaf expansion depending on leaf position within the canopy. Peak values of Pn were maintained for only a few days and were followed by a period of linear decline to negligible Pn at 60-65 days after leaf unfolding. Analysis of the parameters which contributed to the rise and fall pattern of Pn with leaf age indicated the primary involvement of leaf area expansion, gs(.)CO2, leaf nitrogen and PPFD in this process. The decrease in individual leaf Pn began shortly after anthesis and continued throughout the boll filling period. This suggested a possible inefficiency in the production of carbon (C) by leaves and the C requirement of bolls. Simulations of C allocation indicated that the C demand of the bolls were generally out of phase with the C production by the subtending leaves. The first three individual fruiting positions along the fruiting branch at main-stem node 10 required an average import of 47.3%, 25.7%, and 7.1%, respectively. Bolls at mainstem nodes 8, 10 and 12 required an average C import of 58.2%, 26.7%, and 4.4% to sustain dry weight development when analyzed across fruiting positions. The synchronization of C allocation from leaves to bolls at all fruiting positions was poor and as such poses a potential limitation to yield. |
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©National Cotton Council, Memphis TN |
Document last modified Sunday, Dec 6 1998
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