Diurnal fluctuation of leaf solutes and their role in the osmotic adjustment of a johnsongrass (Sorghum halepense (L.) Pers.) cultivar in response to low-water stress

Abstract

A research was conducted to study the role of some solutes (carbohydrates, dhurrin, organic acids, amino acids, and potassium) in the osmotic adjustment of johnsongrass as a possible mechanism of drought resistance in this species and to characterize and quantitate the solutes involved and their diurnal fluctuation in stressed leaves. The role of starch in this process was also studied. It was determined that a mild water stress induced an accumulation of solutes in mature, fully expanded leaves of johnsongrass. Stressed plants accumulated about 50% more total solutes than nonstressed plants. This solute accumulation induced an osmotic adjustment of stressed leaves accompanied with a light tissue dehydration. The drop in osmotic potential ((psi)(,s)) in stressed plants was about -0.30 MPa lower than nonstressed plants and the overall water potential ((psi)(,w)) of stressed plants was about -0.40 MPa lower than nonstressed ones. There was a drop in turgor potential ((psi)(,p)) of about -0.10 MPa indicating a partial turgor maintenance in stressed plants. The principal solutes which accumulated were sucrose, glucose, potassium and proline. Sucrose accumulation contributed about 18% to the total change in osmotic potential; glucose accumulation contributed about 9% to the total change in osmotic potential; proline contributed about 11% and potassium accumulation contributed about 33% to the total change in osmotic potential. Total soluble carbohydrates plus organic acids (ascorbate and citrate) accumulation contributed about 49% to the change in osmotic potential compared to about 18% contributed from total free amino acids. Citrate, ascorbate, dhurrin, and several amino acids increased slightly in the morning to afternoon interval. In general, there was a significant correlation between measured total solute concentration and measured osmotic potential as leaves were subjected to mild stress. Starch accumulated during the dark period in stressed leaves. This accumulated starch was apparently degraded during the light period, causing an increase in sucrose and glucose levels. The persistence of johnsongrass in arid climates could be attributed to its capability for adjusting osmotically and maintaining turgor, resulting in the maintenance of high leaf water potential.