Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 19, 1999
Publication Date: N/A
Interpretive Summary: A problem of increasing severity in North America and Canada is there are longer periods of drought and high temperatures apparently due to the changing climate. The economic importance of drought and elevated temperature stress imposed on crop plants has been demonstrated during the 1999 growing season in the mid-Atlantic states. Record low rainfall and prolonged elevated daily temperatures during the late spring and summer of 1999 has already resulted in considerable lost crop yield and income in that area. Beltsville scientists are studying the mechanisms of oxidative stress in crop plant leaves induced during periods of drought stress. Current studies reported in the research literature indicate that leaves of drought stressed plants in high light are subject to the production of elevated internal levels of elevated strong oxidants such as hydrogen peroxide and superoxide free radicals. These internally generated strong oxidants cause severe, irreversible leaf damage. However, Beltsville scientists are showing that during water stress, crop plants such as soybean and spinach can maintain levels and high redox status of ascorbic acid (vitamin C) which is an essential factor involved in leaf cell protection against elevated hydrogen peroxide levels. This study is important to soybean geneticists and breeders. It is also important to other plant physiologists studying the enzymatic pathway of ascorbic acid biosynthesis.
The purpose of the present study was to examine the influence of severe water (drought) stress on the leaflet levels of ascorbic acid (ASC), dehydroascorbate (DHA) and the ASC: DHA redox status in soybean and spinach cultivars. The soybean cultivars (cv Essex and cv Forrest) and the spinach cultivar (cv Nordic) were grown in high light growth chambers (1000-1200 micromol m-2.s-1) or in the greenhouse during May, June, and July 1999. The culitvars were kept well watered up to 25-29 d PE at which time they were subjected to drought for 4.5 to 7.5 days. On designated days, leaf water potential (WLf) was measured and leaf disk samples of finite area were taken in the period 1230 and 1330 hrs. Leaf disk samples were immediately frozen in liquid N2; leaves were extracted and ASC and DHA levels were measured. For the soybean cultivars, low WLf values, e.g. -3.00 to -3.95 MegaPascals (MPa) were accompanied by only slight decreases in ASC levels and slight increases in DHA levels. In some cases ASC level were not different, or were even increased by as much as 1.2 times, in leaves of water stressed soybeans. The mole fraction of ASC remained at 98 to 99 mol % of the total vitamin C (ASC+DHA) indicating that most of the total vitamin C remained reduced even at low water potential. In spinach plants grown continuously in the greenhouse and subjected to water stress (-1.8 to -2.6 MPa), leaf ASC decreased as much as 40%, but the ASC remained at 98-99 mol % of the total vitamin C. It is concluded that during water stress, enzymes of the ascorbate-glutathione cycle in leaf cells, as well as 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.