Leaf age, decline in photosynthesis, and changes in abscisic acid, indole-3-acetic acid, and cytokinin in cotton leaves

Abstract Abscisic acid (ABA) decreases stomatal aperture and has been reported to decrease photosynthesis. Conversely, indole-3-acetic acid (IAA) and cytokinins have been reported to increase stomatal aperture and stimulate photosynthesis. Field experiments were conducted in 1990 and 1991 to determine possible correlations of concentrations of ABA, IAA, and the cytokinin, isopentenyladenosine ([9R]iP), with a decline in photosynthesis that occurs in cotton ( Gossypium hirsutum L.) leaves as they age. Sympodial leaves were tagged 31 July 1990 at the first node of fruiting branches when the flower was at anthesis at that node. Photosynthesis was measured at intervals thereafter during the next 37 days as the tagged leaves became older. Mainstem leaves were selected at 4, 8, 12 and 16 nodes below the apex on 7 August 1991. Photosynthesis rates were determined in full sunlight and the leaves then harvested and analyzed for free and conjugated ABA, free and ester IAA, and the cytokinin, [9R]iP. Free ABA decreased, rather than increased, with leaf age. Although conjugated ABA increased with leaf age, it is though to be sequestered in vacuoles and, therefore, metabolically inaccessible. [9R]iP changed little with leaf age. Ester IAA increased with leaf age and was negatively correlated with photosynthesis. Free IAA decreased with leaf age and was significantly correlated with photosynthesis.