Dennis P. Murr

Influence of Salicylic Acid on H2O2 Production, Oxidative Stress, and H2O2-Metabolizing Enzymes (Salicylic Acid-Mediated Oxidative Damage Requires H2O2)

We investigated how salicylic acid (SA) enhances H2O2 and the relative significance of SA-enhanced H2O2 in Arabidopsis thaliana. SA treatments enhanced H2O2 production, lipid peroxidation, and oxidative damage to proteins, and resulted in the formation of chlorophyll and carotene isomers. SA-enhanced H2O2 levels were related to increased activities of Cu,Zn-superoxide dismutase and were independent of changes in catalase and ascorbate peroxidase activities. Prolonging SA treatments inactivated catalase and ascorbate peroxidase and resulted in phytotoxic symptoms, suggesting that inactivation of H2O2-degrading enzymes serves as an indicator of hypersensitive cell death. Treatment of leaves with H2O2 alone failed to invoke SA-mediated events. Although leaves treated with H2O2 accumulated in vivo H2O2 by 2-fold compared with leaves treated with SA, the damage to membranes and proteins was significantly less, indicating that SA can cause greater damage than H2O2. However, pretreatment of leaves with dimethylthiourea, a trap for H2O2, reduced SA-induced lipid peroxidation, indicating that SA requires H2O2 to initiate oxidative damage. The relative significance of the interaction among SA, H2O2, and H2O2-metabolizing enzymes with oxidative damage and cell death is discussed.

Changes in Activities of Antioxidant Enzymes and Their Relationship to Genetic and Paclobutrazol-Induced Chilling Tolerance of Maize Seedlings

The potential role of antioxidant enzymes in protecting maize (Zea mays L.) seedlings from chilling injury was examined by analyzing enzyme activities and isozyme profiles of chilling-susceptible (CO 316) and chilling-tolerant (CO 328) inbreds. Leaf superoxide dismutase (SOD) activity in CO 316 was nearly one-half that of CO 328, in which the high activity was maintained during the chilling and postchilling periods. Activity of glutathione reductase (GR) was much higher in roots than in leaves. CO 328 also possessed a new GR isozyme that was absent in roots of CO 316. Ascorbate peroxidase (APX) activity was considerably lower in leaves of CO 328 than in CO 316, and nearly similar in roots. Paclobutrazol treatment of CO 316 induced several changes in the antioxidant enzyme profiles and enhanced their activities, especially those of SOD and APX, along with the induction of chilling tolerance. These results suggest that increased activities of SOD in leaves and GR in roots of CO 328, as well as SOD and APX in leaves and roots of paclobutrazol-treated CO 316, contribute to their enhanced chilling tolerance.

Modulation of phospholipase D and lipoxygenase activities during chilling. Relation to chilling tolerance of maize seedlings

Phospholipase D (phosphatidylcholine choline hydrolase, EC 3.1.4.4) and lipoxygenase activities (linoleate: oxygen oxidoreductase, EC 1.13.11.12) were analysed in chilling-susceptible and chilling-tolerant maize (Zea mays L.) seedlings in relation to their chilling tolerance. Before chilling, phospholipase D activity in the leaf microsomal fraction was 2-fold higher in the chilling-susceptible CO 316 when compared to that of paclobutrazol-treated, chilling-tolerant CO 316. Phospholipase D activity in the heavy membrane fraction comprising chloroplast and mitochondria showed a 3 to 4-fold increase during chilling and post-chilling periods, albeit with temporal variations. An increase in cytosolic phospholipase D activity was observed only in CO 316 and CO 328. Also, phospholipase D activity in the heavy membrane and cytosolic fractions of roots increased in CO 316 during chilling and post-chilling periods. Lipoxygenase activity was low in both leaves and roots of CO 328. Paclobutrazol treatment of CO 316 did not appear to affect the level of cytosolic lipoxygenase activity in roots during the early part of chilling and post-chilling, but increased the activity in leaves. The results suggest that during chilling and post-chilling periods, temporal variations in phospholipase D and lipoxygenase activities in both leaves and roots of CO 316, CO 316P and CO 328 could result in differential metabolism of phospholipids. Enhanced degradation coupled with low levels of turnover could lead to development of chilling injury in the tissue.

Preharvest factors that predispose apples to superficial scald

Abstract Preharvest factors during fruit ontogeny greatly affect fruit quality at harvest, modify fruit response to various treatments, development of physiological disorders, and retention of fruit quality at the end of storage period. The development and severity of superficial scald in apple fruit is proportional to the amount of anti-oxidants in the peel and the extent of α-farnesene oxidation. The extent of metabolism of these compounds is also influenced by weather, orchard management, and tree characteristics and nutrition before harvest. Understanding the influence of preharvest factors on α-farnesene metabolism will help in the development of a reliable predictive method for scald susceptibility at harvest prior to long-term storage.

A modified total oxyradical scavenging capacity assay for antioxidants in plant tissues

The total oxyradical scavenging capacity (TOSC) assay of Winston et al. [Free Radical Biol. Med. 24 (1998) 480] was modified through the development and incorporation of a standard curve, yielding a relatively simple but reliable method to quantify the total water-soluble antioxidant capacity (TAC) of plant tissues. The basis of the TOSC assay is the oxidation of α-keto-γ-methiolbutyric acid (KMBA) by 2,2′-azobis-amidinopropane (ABAP) with the evolution of ethylene as the quantifiable end product. A standard curve was established by using a serial dilution (0–100 μM) of the antioxidant Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) which scavenges peroxyl radicals formed from the thermal homolysis of ABAP, thereby reducing the rate of ethylene evolution. Regression analysis of the standard curve yielded a second-degree polynomial (R2=0.9975) that was used to predict the TAC (μMoles Trolox equivalents) of plant tissues. The precision and utility of the method was tested by determining TAC of skin tissue from ‘Delicious’ and ‘Empire’ apples (Malus domestica Borkh.) at various maturities, following cold storage, and after treatment with 1-methylcyclopropene (1-MCP). The TAC of ‘Delicious’ apples was twofold greater than that of ‘Empire’ apples, and remained relatively constant for up to 60 days in cold storage, then significantly decreased thereafter. This decrease during the latter stages of cold storage may be related to the development of superficial scald in this cultivar. ‘Delicious’ and ‘Empire’ fruit treated with 1-MCP had also a significantly higher TAC at various maturities after 120 days of cold storage. This is the first study to show that a treatment with 1-MCP can potentially maintain some intrinsic functional food quality parameters (i.e. antioxidant levels) in stored fruit. The establishment of a standard curve for use, in conjunction with the TOSC assay, resulted in a precise and highly reproducible predictor of TAC in apple fruit, while also permitting a rapid turnover of tissue samples for analysis.

Water relations of cut greenhouse roses: The relationships between stem water potential, hydraulic conductance and cavitation

Abstract The stems of cut greenhouse roses, Rosa hybrida ‘Samantha’, were fully hydrated and then allowed to dehydrate while measurements were made of water loss, stem water potential, hydraulic conductance and the onset and extent of xylem cavitation (i.e. disruption of xylem water continuity). The inter-relationships of these parameters were followed through the full range of plant water status. Xylem cavitation was initiated at stem water potentials of −0.2 to −0.4 MPa. Rates of cavitation increased as water potential declined. This was associated with a corresponding decline in stem hydraulic conductance. The data indicated that a loss of about 50% of the stem water content at full hydration resulted in a total loss of water-conducting capacity. This corresponded with a stem water potential of about −4.0 MPa. The results indicate that the shelf-life of a cut flower may, in many instances, be relative to the proportion of functional xylem remaining in the stem.

Volatile production and fruit quality during development of superficial scald in Red Delicious apples

To evaluate potential differences in quality between normal and scald-developing apples, volatile components and tissue integrity of Red Delicious apples were analyzed. Solid-phase micro-extraction (SPME) of head-space volatiles coupled with GC-MS (gas chromatography-mass spectrometry) revealed the presence of several fatty acyl esters and the terpene, α-farnesene. Development of superficial scald in Red Delicious apples was not associated with any major qualitative changes in volatiles. However, unscalded Red Delicious apples evolved much higher amounts of volatiles such as hexyl acetate, butyl hexanoate, propyl octanoate, hexyl hexanoate and α-farnesene. Proton magnetic resonance imaging of scalded Red Delicious apples revealed an increase in the mobility of water protons or an increase in the ratio of free to bound water in the epidermal layer and peripheral cortical tissue. The results show that scald development may be associated with changes in volatile emission and tissue integrity in Red Delicious apples.

Chlorophyll fluorescence of Delicious apples at harvest as a potential predictor of superficial scald development during storage

Abstract Chlorophyll fluorescence (Fv) of Delicious apples at harvest was evaluated as an indicator of superficial scald development during storage. The Fv of ‘Imperial’ and ‘Sturdeespur’ Delicious apples was determined at three weekly harvest intervals, and the apples were later evaluated for scald intensity after 2, 4 and 6 months of storage at 0 °C. ‘Sturdeespur’ apples had an overall higher incidence of scald than ‘Imperial’ apples. Fv at harvest was found to correlate positively with scald development in ‘Sturdeespur’ apples from Harvest 1. ‘Sturdeespur’ apples with low Fv ( 18) were most likely to develop severe scald. However, no such relationship was observed for ‘Imperial’ apples, or for some ‘Sturdeespur’ apples from harvests 2 and 3. The results of this study suggest that Fv at harvest may be a predictor of scald development.

Effects of paclobutrazol and A-Rest on growth, flowering, leaf carbohydrate and leaf senescence in ‘Nellie White’ Easter lily (Lilium longiflorum Thunb.)

Abstract The influence of paclobutrazol and A-Rest treatment on plant height, flowering, leaf senescence and leaf carbohydrates in ‘Nellie White’ Easter lily were examined. Soil incorporation of granular paclobutrazol was most effective in reducing plant height, followed by soil drench and a top dressing of granular paclobutrazol. The relationship between paclobutrazol concentration and height reduction was linear. However, paclobutrazol spray had no effect on height. A-Rest soil drench was more effective than foliar spray. Paclobutrazol and A-Rest treatments hastened lower leaf senescence. During flower bud development, leaf total soluble sugars were reduced by paclobutrazol incorporation and drench, and A-Rest drench.

Chloroplast membrane organization in chilling-tolerant and chilling-sensitive maize seedlings

Summary Resistance of maize ( Zea mays L.) seedlings to chilling-induced damage and its potential bearing on chloroplast membrane organization have been investigated using a chilling-sensitive inbred CO 316 and a chilling-tolerant inbred CO 328. Paclobutrazol treatment of the chilling-sensitive CO 316 (CO 316P) induced chilling tolerance, also causing several morphological and physiological changes, and served as another means of exploring the relation between chloroplast membrane changes and chilling tolerance. Chilling treatment [6°C/2°C (day/night)] resulted in a reduction of Fv/Fm ratio, photosynthetic pigment levels and an increase in leakage of electrolytes. Within 24 h of exposure of CO 316 to chilling, the granal and stromal membrane showed extensive vesiculation and disruption of the granal array. By contrast, the granal and stromal organization of chilling-tolerant CO 328 and P-treated CO 316 remained nearly intact. Our studies suggest that paclobutrazol treatment can alter the membrane deterioration and disassembly processes of the thylakoid membranes of chilling-sensitive CO 316 to make them functionally similar to the chloroplast membrane of genetically chilling-tolerant CO 328.