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

Abstract

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.

Photosynthesis of individual field-grown cotton leaves during ontogeny

“…The reasons for these differences cannot be specifically addressed by this study, but intuitively may be due to mutual leaf shading. Bunce (1988) has pointed out that plants which produce successive leaves on a vertical stem (i.e., cotton and soybean) face a serious dilemma when grown in dense stands. Increased light competition by upper canopy leaves can reduce the photosynthetic capacity of lower leaves through a possible accelerated senescence phenomenon (see Field, 1987).…”

“…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). While the involvement of irradiance and temperature in the modification of leaf Pn and gs'CO2 has been extensively investigated, it is generally recognized that certain biochemical factors can also moderate leaf Pn particularly during leaf ontogeny (Evans, 1983;Ford and Shibles, 1988;Wells, 1988).…”

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

“…Differences among cultivars in the exposure of developing leaves to light, which occurred for plants in stands but not for plants grown in isolation, is the probable explanation of the differences in maximum photosynthetic rates. Several studies have shown that shading of developing leaves can result in lower maximum photosynthetic rates at maturity (Bowes et al 1972, Bunce 1988, Lugg and Sinclair 1980. It was not coincidental that in stands, cultivars with large leaves had more shading of their own developing leaves.…”

“…Five cultivars of Glycine max (L) Merr., Amsoy, Clark, Hodgson, Lincoln, andMandarin were grown in 1987 and1988 in the field at Beltsville, Maryland, USA. These cultivars were chosen because they have a range of leaf sizes and photosynthetic rates.…”

“…For example, although ribulose bisphosphate carboxylase content and photosynthetic rate were positively correlated (r = 0.79) among soybean cultivars (Hesketh et al 1981), ribulose bisphosphate carboxylase content was only weakly related to leaf area (r = -0.46). Another possible explanation of the negative relationship between leaf size and photosynthetic capacity is that genotypes with large leaves may have more severe mutual shading, which could reduce their photosynthetic capacity (Bunce 1988). I have studied five soybean cultivars grown under field conditions in stands and as isolated plants to determine to what extent differences in maximum photosynthetic rates may be related to differences in leaf size, and to differences in exposure of developing leaves to light.…”