Biochemical regulation during the mid-day decline of photosynthesis in field-grown sunflower

Robert R. Wise, Denise H. Sparrow, Adriana Ortiz-Lopez, Donald R. Ort

Research output: Contribution to journalArticlepeer-review


Sunflower (Helianthus annuus L.) was raised according to common agricultural practices except that drought was imposed from the 20th day after planting in order to accentuate the natural daily cycling in leaf water potential (Ψleaf). The nature of the processes involved in regulating assimilation on a diurnal time scale was then investigated using techniques of gas exchange analysis. Although Ψleaf decreased from -0.5 to -1.1 MPa between predawn and mid-afternoon, autoradiographs of recent 14C-photoassimilation in leaves showed no evidence for inhomogeneities in stomatal apertures at any time off the day. Only by severing the tap root and allowing the shoot to dry rapidly could 'patchiness' be induced. A significant diurnal decline in assimilation (A) (from ∼ 31 to 25 μmolCO2 m-2 s-1) was accompanied by declining leaf conductance but, since there was no significant change in intercellular [CO2] (Ci), it is unlikely that A was being limited by the supply of CO2 for carboxylation. Instead, a decrease in the initial slope of A/Ci curves taken over the course of the day showing a decreased apparent affinity of the CO2 reductive processes for CO2, indicating chloroplast-level control. By briefly treating the leaf with intercellular CO2 levels near the CO2 compensation point, the relationship between A and Ci could be significantly shifted. These experiments revealed a strong biochemical element contributing to the lowre photosynthetic rates that develop over the course of a day in field-grown sunflower. In addition, it was seen that rapid adjustments could be made in the level of the biochemical control and that they were probably initiated by changes in the intercellular CO2 levels.

Original languageEnglish (US)
Pages (from-to)45-52
Number of pages8
JournalPlant Science
Issue number1
StatePublished - 1991


  • Compositaceae
  • Helianthus annuus L
  • drought
  • leaf water potential
  • photosynthesis
  • regulation
  • sunflower

ASJC Scopus subject areas

  • Genetics
  • Agronomy and Crop Science
  • Plant Science


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