No abstract

The factors affecting postharvest color losses in Concord grapes were investigated, with emphasis on the extraction, partial purification and characterization of naturally occurring enzymes that may be involved. Color losses were accelerated in Concord juice by the crude enzyme extracted from the grapes. Catechol was oxidized more rapidly than other o-dihydroxyphenols, although pyrogallol, caffeic acid and dl-DOPA were good substrates for the crude polyphenoloxidase (PPO). The optimum activity was in the pH range 5.9-6.3 and at temperatures 25-30°C. Kinetic data for the crude enzyme showed a Michaelis constant of 67 mM of catechol at pH 6.0. Effective inhibitors were Na, S, 0,, dithiothreitol, phenylhydrazine, cysteine and others. Peroxidase (PRO) activity of the crude extract was very low in ripe Concord grapes. Anthocyanin pigment degradation by the crude enzyme extract was dependent on maturity of the grapes. The most rapid degradation of pigment occurred when crude enzyme from ripe grapes was added.

The loss of non‐elemental N from plant foliage was verified, and the significance of this loss was investigated. Soybean (Glycine max (L.) Merr.) leaf vapors were condensed (–50 C) using a calibrated test tube tra in a closed system. Subsequent N analysis of this condensate revealed water soluble N forms. Since the pyrochemiluminescent technique that was used for N analysis does not detect elemental N, the N present must be in chemically bound forms; preliminary research indicates the N in both oxidized and reduced forms. Significant losses of N from the determinate cultivar ‘Davis’ were positively correlated to temperature and to transpiration rate under field conditions. These losses, which like transpiration followed a diurnal trend, were greater in early vegetative growth stages than during flowering and pod‐fill. The amount of N lost is significant under the high temperatures of field Conditions and may account for much of the reduction in potential seed yields under this stress condition.

Evaluations were made of the amounts of water soluble, non‐elemental N that is lost, along with transpiration water vapors, from plant foliage. A pyro‐chemiluminescent analysis of transpirational vapors, condensed in a closed system containing an attached leaf, had revealed that plants do give off non‐elemental N with transpira. tional water. In this study N loss, as well as transpiration rates, was ascertained for several crop and weed species: corn (Zea mays L.), cotton (Gossypium hirsutum L.), sorghum [Sorghum bicolor (L.) Moench], soybeans [Glycine max (L.) Merr.], common cocklebur (Xanthium pensyl‐vanicum Wallr.), entireleaf morningglory (Ipomoea heder‐acea var. integriuscula Gray), ivyleaf morningglory [1. hederacea (L.) Jacq.], jimsonweed (Datura stramonium L.), johnsongrass [Sorghum halepense (L.) Pers.], and palmer amaranth [Amaranthus palmeri (S.) Wats.]. Transpiration and N loss both were measured at 35 C and 2 days later at 28 C in field‐grown plants. With the exception of johnsongrass, all species exhibited significantly higher rates of N loss at 35 than at 28 C. However, only cocklebur and palmer amaranth had significantly higher transpiration rates at the higher temperature. In this evaluation the weed species lost geater quantities of N and water than did the crop plants.

Evaluations were made to differentiate the forms of water soluble, non‐elemental nitrogen lost with the trans. piration water vapors from plant foliage. Chemiluminescent detection of chemically bound N, with O2 present as a reactant‐carrier gas, and then absent and replaced with N2 during the pyrolysis of transpiration samples, distinguished the oxidized from the reduced N forms in the transpirate. The majority of the N in several crop and weed species was in the reduced state; however, at least 5% of the N in all species tested was in an oxidized form. The percent of loss of oxidized N was fairly constant in cotton (Gossypium hirsutum L.), ‘Forrest’ beans [Glycine max (L) Merr.], and palmer amaranth [Amaranthus palmeri (S.) Wats.] over the solar day. Older soybean leaf tissue lost a higher percentage of oxidized forms than did younger tissue. Both younger and older soybean tissue evolved greater total amounts of N than foliage in the middle of the plant canopy; yet the rate of transpiration was maximal in the middle foliage. Water‐stressed soybeans evolved greater quantities of oxidized N forms than did irrigated soybeans. The N loss increased in ‘Davis’ soybeans and transpiration decreased in the cultivar ‘Forrest’ as a result of water stress.