Measurements and modeling of photosynthesis in field crops

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This study investigated the basis of the negative relationship between leaf size and photosynthetic rate per unit of area among five cultivars of soybeans. Exposure of developing mainstem leaves to light, and sizes and light saturated photosynthesis rates of those leaves at maturity were compared in cultivars grown in field plots for two years at Beltsville, Maryland, USA. Plants were grown both in stands at 2.5 cm by 1 m spacing and as isolated plants. While cultivar differences in leaf size were large and consistent in both planting arrangements, significant cultivar differences in light saturated photosynthetic rates were found only in plants grown in stands. Similarly, leaf size was significantly correlated with specific leaf weight only for plants grown in stands. The mainstem apex and developing mainstem leaves experienced more severe shading in large-leaved cultivars than in small-leaved cultivars when plants were grown in stands. Thus, cultivar differences in photosynthetic capacity were probably a consequence of differences in the exposure of developing leaves to light.

Section: Introductionmentioning

confidence: 62%

Section: Introductionmentioning

confidence: 99%

The effect of leaf size on mutual shading and cultivar differences in soybean leaf photosynthetic capacity

Bunce

1990

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“…Net photosynthesis (Pn), stomatal conductance to COz (gs'CO2), and various other photosynthetic characteristics are influenced by a number of environmental and physiological factors (e.g., Bunce, 1986;Hsiao, 1973;Schulze and Hall, 1982). Although these effects are often highly species dependent, recent studies indicate that the conditions under which a plant develops can exert a significant influence on the photosynthetic capacity of its leaves (Bunce, 1985(Bunce, , 1988; Gordon et al, 1982;Schulze, 1986).…”

Section: Introductionmentioning

confidence: 99%

Photosynthesis of individual field-grown cotton leaves during ontogeny

Wullschleger

Oosterhuis

1990

Photosynthetic characteristics of field-grown cotton (Gossypium hirsutum L.) leaves were determined at several insertion levels within the canopy during the growing season. Single-leaf measurements of net photosynthesis (Pn), stomatal conductance to CO2 (gs·CO2), substomatal CO2, leaf area expansion, leaf nitrogen, and light intensity (PPFD) were recorded for undisturbed leaves within the crop canopy at 3-4 day intervals during the development of all leaves at main-stem nodes 8, 10, and 12. Patterns of Pn during leaf ontogeny exhibited three distinct phases; a rapid increase to maximum at 16-20 days after leaf unfolding, a relatively short plateau, and a period of linear decline to negligible Pn at 60-65 days. 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, leaf nitrogen, PPFD, and gs·CO2 in this process. The response of Pn and gs·CO2 to incident PPFD conditions during canopy development was highly age dependent. For leaves less than 16 days old, the patterns of Pn and gs·CO2 were largely controlled by non-PPFD factors, while for older leaves Pn and gs·CO2 were more closely coupled to PPFD-mediated processes. Maximum values of Pn were not significantly different for any of the leaves monitored in this study, however, those leaves at main-stem node 8 did possess a significantly diminished photosynthetic capacity with age compared to upper canopy leaves. This accelerated decline in Pn could not be explained by age-related variations in gs·CO2 since all leaves showed similar changes in gs·CO2 with leaf age.

“…Another mechanism found to be operating in other experiments with the cultivar Kent Bunce 1986), decreased photosynthetic capacity because of shading of lower leaves, could also have been operating in these experiments. However, it could not be separately identified here, because in the present experiment young developing leaves were shaded nearly as soon in the season as leaves of adjacent plants overlapped.…”

Section: Discussionmentioning

confidence: 90%

“…Photosynthetic capacity was inversely related to the leaf area index above a developing leaf in ryegrass swards (Woledge 1979), as has also been found in deciduous trees (Jurik 1986). As for the second possibility, shading leaves at all nodes below that of a developing mainstem leaf in soybeans reduced the maximum photosynthetic capacity of the developing leaf (Ward and Bunce 1986). In controlled environments this effect was found only under a very limited set of conditions.…”

Section: Introductionmentioning

confidence: 99%

Mutual shading and the photosynthetic capacity of exposed leaves of field grown soybeans

Bunce

1988

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Light-saturated photosynthetic rates at air levels of carbon dioxide were measured about weekly in upper canopy leaves of two soybean cultivars grown at stand densities of 40 and 100 plants per square meter. Early in the season, when leaf area indices differed between stand densities, plants of both cultivars grown at high stand density had photosynthetic rates which averaged 23% lower than plants at low stand density. Later in the season, when there were no differences in leaf area index between stand densities, there were no differences in photosynthetic rates in the cultivar Kent, but rate differences of about 14% persisted in the cultivar Williams. In Williams mainstem leaves emerged into full sunlight later in their development at high than at low stand density. In both cultivars the oldest fully exposed leaves were photosynthetically immature for much of the season, as higher rates could be achieved by lower leaves which were shaded in situ. The results identify shading of young developing leaves and photosynthetic immaturity of fully exposed leaves as factors limiting canopy photosynthesis in soybeans, and indicate cultivar differences in how much high stand density reduces photosynthetic capacity.