Studies of the use of antitranspirants on grain sorghum

Abstract

The possibility of suppressing transpiration of grain sorghum by means of etomatal closing antitranspirants was investigated. Attention was paid to three possible beneficial effects. (1) Improvement of water use efficiency by shifting the transpirationphotosynthesis balance in favour of the latter. (2) Conservation of soil water with special reference to the evening out of water use over the growth cycle in the field. (3) Avoidance of water stress in plants, either directly by decreasing; the lag between water loss and absorption, or indirectly through water conservation.Of three chemicals tested, Phenyl Mercuric Acetate (PMA) was found to be the most suitable. It was, however, less effective on sorghum than on sone other species, which were also tested, this being attributed to the ability of sorghum plants to continue transpiring at a relatively high rate with reduced stomatal aperture. The concentration of FMA (2 X 10-4 M), which induced a significant reduction in transpiration, appeared to be toxic to sorghum, reducing dry matter accumulation more than water loss. It was demonstrated in a glasshouse experiment that reduction of transpiration of plants, subjected to increasing soil moisture tension, results in water conservation and in prolongation of growth. The effectiveness of the antitranspirant however, diminishes once the plants begin to exert their own control of transpiration. Contrary to the glasshouse results, PMA was not able to induce a significant water conservation in the field. Experiments comparing antitrampirants effectiveness in the glasshouse and out of doors showed that low wind speeds could bring about significant stomatal closure, thereby reducing the usefulness of artificial stomatal closure. The sensitivity of stomata to low light intensity, during cloudy datfs, also reduces the value of antitranspirants. Sorghum is capable of maintaining a high internal water statue up to ard during the onset of wilting, and therefore the application of antitranspirants during this period will not he of any benefit to developmental processes dependent upon high water status. This ivas shown it. the case of flowering in relation to water supply. It was concluded that stomatal closing anti transpirante are not likely to be beneficial to grain sorghum. Comparative observations however, suggested that they may be of more use with some other crop species.