Influence of drought-induced water stress on soybean and spinach leaf ascorbate-dehydroascorbate level and redox status

We examined the influence of water stress (water deficit) induced by drought on the steady state levels of ascorbic acid (ASC), dehydroascorbate (DHA), and the ASC:DHA redox status in leaflets of Glycine max (soybean) and leaves of Spinacia oleracea (spinach). Two soybean cultivars (cv. Essex and cv. Forrest) and one spinach cultivar (cv. Nordic) were grown in high-light growth chambers (approximate to 1000-1200 mu mol m(-2) s(-1)) or in the greenhouse during May, June, and July 1999. The cultivars were supplied with water until approximate to 25-29 d postemergence, at which time one-half of the plants were not watered for a period of from 4.5 to 7.5 d; the other half of the plants were provided water daily and served as controls. On designated days, leaf water potential (Psi(Leaf)) was measured, and leaf disks of constant area were excised in the period between approximate to 1230 and 1330 hours. Leaf disk samples were immediately frozen in liquid N-2, samples were extracted, and ASC and DHA levels were measured and expressed as pmol per gram dry mass per time point. For the soybean cultivars, low Psi(Leaf) values (approximate to-3.00 to -3.95 MPa) were accompanied by slight decreases in ASC levels and slight increases in DHA levels per gram dry mass. In some cases, leaflet ASC levels of water-stressed soybeans were similar to controls or were even increased by as much as 1.2 times. In soybeans, the mole fraction of ASC remained at 93-99 mol% of the total ascorbate (ASC + DHA), indicating that most of the total ascorbate remained in the reduced form even at low water potential. In spinach plants subjected to water stress (-1.8 to -2.6 MPa), leaf ASC decreased as much as 38%, but the ASC remained at 96-99 mol% of the total ascorbate. It is concluded that during water stress, enzymes of the ascorbate-glutathione cycle in leaf mesophyll cells, as well as in the system that generates reductant to support DHA to ASC recycling, e.g., photosynthetic electron transport in chloroplasts, is able to remain active enough to maintain reduction of DHA to ASC.