Changes In Seed Mitochondial Respiration And Scavenging Enzymes Activities Of Reactive Oxygen Species During Dehytration

Seeds have been categorized into orthodox, recalcitrant and intermediate group depending on their storage behavior. Orthodox seeds can be dried without damage to low moisture contents. Recalcitrant seeds, in contrast, cannot survive desiccation below a comparatively high moisture content. These two categories are not just of theoretical interest, but also have great practical importance. There is a continuum of seed recalcitrance in nature,namely,low recalcitrance,intermediate recalcitrance and high recalcitrance,difference of which is sensitivity degree to drying.Desiccation sensitivity of intermediate seeds is between orthodox seeds and recalcitrant seeds. In the present study, Zea mays (cv. Nongda 108) seeds (orthodox), Antiaris toxicaria Lesch seeds and Nephelium lappaceum L. seeds (recalcitrant) were used as experimental material. Mitochondria were purified by differential centrifugation followed by separation on a continuous Percoll gradient. Based on their different desiccation tolerance degree, changes in desiccation tolerance, in activities of cytochrome c oxidase (CCO) and NAD+-malate dehydrogenase (MDH), in respiratory rate and pathways of mitochondria, and mechanisms of scavenging enzymes of reactive oxygen species (ROS) and TBA-reactive product content during dehydration were studied. This is the first report of changes in respiratory rate and pathway of mitochondria during dehydration of seeds or axes. The main results are as follow:Z. mays axes gradually lost desiccation tolerance with dehydration, the water content (WC) at which survival percentage decreased by 50% were approximately 0.11g g -1. Survival of A. toxicaria axes with the initial water content of 1.11 g g -1decreased to zero when axes were dehydrated to water content of 0.34 g g -1. And survival of N. lappaceum seeds with the initial water content of 0.47 g g -1 decreased to zero when axes were dehydrated to water content of 0.27 g g -1.CCO and MDH activities of Z. mays axes mitochondria declined gradually with dehydration. However, the CCO latencies decreased by 7% when water content of axes dropped to 0.11 from 1.67 g g-1. CCO activities and latencies and MDH activities of A. toxicaria axes mitochondria declined significantly with dehydration, decreased by 59.4 and 27.3, 53.4%, respectively, when water content of axes dropped to 0.34 from 1.11 g g-1. CCO activities and latencies and MDH activities of N. lappaceum seeds decreased more significantly with dehydration, decreased by 82.4% and 76.1%, respectively, when water content of N. lappaceum seeds dropped from o.47 g g -1 to 0.27 g g -1.The more desiccation sensitivity of the seeds studied, the higher their basic respiration and respiratory rate. And the basic respiration and respiratory rate decreased with dehydration. As for the proportion of oxygen consumption pathway in respiratory state III during dehydration, the proportions of Cyt c pathway and AOX pathway of mitochondria from fresh Z. mays seeds were 10.2% and 3.8%, respectively. And then the proportions of Cyt c pathway decreased to 2.8% and at the same time the proportion of AOX pathway was 0 at 0.10g g-1. Similarly, the proportions of Cyt c pathway and AOX pathway of mitochondria from fresh A. toxicaria axes were 32.5% and 58%, respectively. With dehydration to 0.34 g g-1, the Cyt c pathway did not work, however, the proportion of AOX pathway increased to 66.7%. For N. lappaceum seeds, the proportions of Cyt c pathway and AOX pathway were 72.6% and 25.9%, respectively. When dried to 0.40 g g-1 water content, the Cyt c pathway did not work. However, when desiccated to 0.45 g g-1 and 0.27 g g-1, the proportions of AOX pathway were 43.5% and 10.4%, respectively.With decrease of water content, for Z. mays seeds, activities of both SOD and GR also decreased, whereas the activities of DHAR increased. And the content of TBA-reactive products of mitochondria of Z. mays seeds decreased firstly, and then increased and decreased dramatically again during dehydration. But, Activities of SOD, GR and DHAR ofmitochondria of A. toxicaria axes increased during the early phase of axis dehydration, and then decreased; but their activity peaks were different during dehydration. Peak of SOD activity was at a water content of 0.4 g g-1 of axes, and those of GR and DHAR were at the water content of 0.75 g g-1 of axes. Activities of CAT and APX, however, could not be detected. TBA-reactive products of mitochondria of A. toxicaria axes increased and then decreased during axis dehydration.In conclusion, according to the results above, we could infer that (1) the higher desiccation sensitivity of seeds is, the more of the proportion of Cyt c pathway of mitochondria with initial water content is, and (2) during further desiccation, the proportion of Cyt c pathway of mitochondria decreased gradually until Cyt c pathway does not work; (3) mitochondria of seeds or axes, which are sensitive to desiccation, are subjected to ROS produced during dehydration.