The effect of gibberellic acid on apparent photosynthesis and dark respiration of simulated swards of Pennisetum clandestinum Hochst

Abstract

Gibberellic acid (GA3 at 40 g/ha equivalent was applied to simulated swards of Pennisetum clandestinum (kikuyu grass) at the fourth leaf stage in a glasshouse held at a 25°C maximum and 15° minimum diurnal temperature cycle. Continuous 24-hr recordings of apparent photosynthesis and dark respiration in relation to normal variation in solar radiation were made on the whole swards by using an open chamber system monitored by infrared gas analysis. No effect was noted until 9 days from spraying, by which time the GA3 application stimulated the apparent photosynthetic rate of the sward and reduced the dark respiration rate. These effects became greater with time. The stimulation of apparent photosynthesis increased as the level of radiation increased. At 21 days from spraying, while large increases in apparent photosynthesis and reductions in dark respiration rate were being recorded, a botanical separation of the swards was made. GA3 increased the total dry weight (tops plus roots) of the sward and the dry matter above soil level. The latter effect was due to a greater amount of stem dry matter in the GA3-treated sward. GA3 reduced the dry weight of senescent leaf blade and leaf sheath tissue. GA3 had no effect on leaf area index. Area per leaf was increased, but GA3 stimulated apical dominance reduced the number of leaves per sq metre of sward. It is postulated that stimulation of apparent photosynthesis by GA3 is related to leaf age and a change in canopy structure through an improvement in leaf dispersion.