Photosynthetic Rate Control in Cotton

Abstract

The relationship between single leaf photosynthesis and conductance was examined in cotton (Gossypium hirsutum L.) across a range of environmental conditions. The purpose of this research was to separate and define the degree of stomatal and nonstomatal limitations in the photosynthetic process of field-grown cotton.Photosynthetic rates were related to leaf conductance of upper canopy leaves in a curvilinear manner. Increases in leaf conductance of CO2 in excess of 03 to OA mole per square meter per second did not result in significant increases in gross or net photosynthetic rates. No tight coupling between environmental influences on photosynthetic rates and those affecting conductance levels was evident, since photosynthesis per unit leaf conductance did not remain constant. Slowly developing water stress caused greater reductions in photosynthesis than in leaf conductance, indicating nonstomatal limitations of photosynthesis.Increases in external CO2 concentration to levels above ambient did not produce proportional increases in photosynthesis even though substomatal or intercellular CO2 concentration increased. The lack of a linear increase in photosynthetic rate in response to increases in leaf conductance and in response to increases in external CO2 concentration demonstrated that nonstomatal factors are major photosynthetic rate determinants of cotton under field conditions. Definition of the order of limitations in the photosynthetic process is a prerequisite to genetically increasing the photosynthetic rate of crop species. Although stomatal conductance has often been proclaimed as the principal limitation to photosynthetic C02 assimilation (1,5,7,8,16)

Trocas gasosas e condutância estomática em três espécies de gramíneas

“…Os valores máximos de g (gmáx) ocorreram, respectivamente, para trigo, arroz e milho, em irradiâncias ao redor de 450, 600 e 1.500 u.mol m s" , mostrando que as duas espécies C3 (arroz e trigo) foram mais responsivas em relação a /, sob condições naturais. Padrões semelhantes de respostas de g ao incremento de / já foram observados para outras espécies C3 e C4 (Downes, 1971a;Turner & Begg, 1973;Raschke, 1979;Hutmacher & Krieg, 1983;Ceulemans et ai.,1989). A irradiância para que gmáx seja atingido varia marcadamente -2 1 (100 a 2.000 ^imol m s ), dependendo da espécie, da idade da folha e do pré-condicionamento das plantas (Turner & Begg, 1973;Schulze & Hall, 1982;Dai et ai.,1992).…”

“…A ausência de resposta de A ao incremento de g, acima dos valores de saturação, indica que outros mecanismos, além de abertura estomática, estão controlando a assimilação de CO2 (Hutmacher & Krieg, 1983).…”

Leaf Photosynthesis and Conductance of Selected Triticum Species at Different Water Potentials

“…Recently, however, it has been shown that reductions in mesophyll photosynthetic capacity and reductions in stomatal conductance under water deficits may, to a significant degree, be coupled (7). As a result, leaf C,2 is relatively stable and reduced stomatal conductance is not considered the major cause of reduced photosynthesis under water deficits (7,14). Stomata may still, however, play an important role in determining and explaining differences in the photosynthetic rate and water use efficiency between given plant species.…”

Modulation of Water Stress Effects on Photosynthesis by Altered Leaf K+

“…biochemical) limitation to CO2 uptake in leaves of water-stressed plants (15) indicate that nonstomatal factors can contribute substantially to decreased photosynthetic rates during water stress. Several groups of investigators have attempted to characterize the mechanism(s) mediating inhibition of chloroplast metabolism by water stress in subcellular studies (2,6,16).…”