Effects of growth substances on the protoplasmic drought tolerance of leaf cells of the resurrection grass, Sporobolus stapfianus

Hydrated leaves of the resurrection grass Sporobolus stapfianus Gandoger are not desiccation tolerant, but moderate to severe drought stress can induce their desiccation tolerance while the leaves remain attached to drying intact plants. Free-cell suspensions prepared from fully turgid leaves of S. stapfianus were also found to be desiccation-sensitive. In this study a sensitive assay was developed to determine the effect of exogenous substances on the protoplasmic drought tolerance (PDT) of cell suspensions. Suspended cells were incubated with a wide variety of individual plant growth substances over a range of concentrations. After the incubation, the protoplasmic drought tolerance at each concentration of each substance was determined osmotically. Brassinolide (BR) and methyljasmonic acid (MJA), applied separately, gave the greatest effect — each improved PDT by approximately 6 MPa, compared with controls incubated without growth substances. Exogenous abscisic acid (ABA) improved the PDT of S. stapfianus only slightly (about 1 MPa). Salicylic acid (SA), 1-aminocyclopropane–1-carboxylic acid (ACC), ethephon, gibberellic acid (GA), kinetin (KN) andN 6 -(2-isopentyl)adenine (2ip) each improved PDT by ca 1.5 MPa. Indoleacetic acid (IAA) had no effect, and zeatin (ZN) had a deleterious effect on PDT. Incubation of suspended cells in combinations of two growth substances (MJA with ABA, ACC, BR, GA, IAA or KN; BR with ABA) produced no evidence of synergism, or in most cases, even of additive effects of the tested substances. ABA, BR and MJA (applied separately) stimulated the synthesis of numerous specific proteins. Each growth substance stimulated the synthesis of differing proteins, with four exceptions. This investigation raises the possibility that BR and MJA may play a role in a chain of events which leads eventually to the induction of desiccation tolerance in leaves of S. stapfianus. The effects on cell PDT observed in this study, while large, were insufficient to account for the remarkable improvement observed in the PDT of S. stapfianus leaves attached to drying plants.