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NADH-specific and NAD(P)H bispecific nitrate reductases are present in barley (Hordeum vulgare L.). Wild-type leaves have only the NADH-specific enzyme while mutants with defects in the NADH nitrate reductase structural gene (nar1) have the NAD(P)H bispecific enzyme. A mutant deficient in the NAD(P)H nitrate reductase was isolated in a line (nar1a) deficient in the NADH nitrate reductase structural gene. The double mutant (nar1a;nar7w) lacks NAD(P)H nitrate reductase activity and has xanthine dehydrogenase and nitrite reductase activities similar to nar1a. NAD(P)H nitrate reductase activity in this mutant is controlled by a single codominant gene designated nar7. The nar7 locus appears to be the NAD(P)H nitrate reductase structural gene and is not closely linked to nar1. From segregating progeny of a cross between the wild type and nar1a;nar7w, a line was obtained which has the same NADH nitrate reductase activity as the wild type in both the roots and leaves but lacks NADPH nitrate reductase activity in the roots. This line is assumed to have the genotype Nar1Nar1nar7nar7. Roots of wild type seedlings have both nitrate reductases as shown by differential inactivation of the NADH and NAD(P)H nitrate reductases by a monospecific NADH-nitrate reductase antiserum. Thus, nar7 controls the NAD(P)H nitrate reductase in roots and in leaves of barley.

A disorder of unknown aetiology appearing as silvering on squash (Cucurbita pepo) leaves has become prevalent in south Florida. Typical symptoms, observed after 2 weeks of exposure to adults of Bemisia tabaci, the sweet potato whitefly (SPWF) include vein clearing followed by silvering of whole leaves. Fruits from affected plants are blanched or streaked longitudinally. The silverleaf syndrome was not mechanically transmissible when crude extracts of symptomatic leaves were used as inoculum. Two sizes of double‐stranded RNA (dsRNA) of c. 4.6 and 4.2 kilobase pairs (kbp) were consistently observed in leaf extracts of caged, whitefly‐infested, symptomatic plants. In contrast, caged plants not exposed to whiteflies showed no symptoms and contained no detectable dsRN A. In addition, squash plants exposed to SPWF colonies from California neither developed silverleaf symptoms nor contained any dsRNA. Double‐stranded RNA could also be readily detected in whitefly‐infested yellow summer squash and zucchini from the field. However, asymptomatic eggplant and watermelon infested with SPWF from nearby fields did not contain detectable dsRNA. Dot‐spot hybridization assays using an RNA‐specific probe detected homologous sequences in both adults and nymphs of SPWF that induced silverleaf symptoms but not in adults and nymphs of SPWF that could not induce silverleaf symptoms. These results, along with the non‐endogenous nature of the detected dsRNA suggest that the dsRNA is or is associated with the causal agent of whitefly‐mediated leaf silvering in squash.

Study of the stomata and leaf epidermis of eight Lycopersicon species has revealed a relationship between frequency of stomata, stomatal size and some morphological leaf characteristics, and resistance to bacterial leaf spot caused by Xanthomonas campestris pv. vesicatoria. Significant correlations were found between the stomatal frequency in both adaxia! and abaxial leaf epidermis and the number of bacterial lesions per leaf area (/-= 0-70 and 0-68, respectively) and the frequency of stomata in both adaxial and abaxial epidermis and disease incidence (percentage of diseased plants) {r -0-^1 and 080, respectively) for all the Lycopersicon species and one hybrid tested. A correlation was also found between disease incidence and number of bacterial lesions per leaf area (/'^O-SS). The length and width of stomata were correlated with frequency of stomata in adaxial and abaxial epidermis (/• = 0-85, 0-75. 0-89 and 090, respectively). The stomatal width was correlated with the number of bacterial lesions per cm-(/• = 0'82; P = 0-0065). Scanning electron microscopic studies of the leaf surface and stomata indicated that other morphological features such as the raised stomatal complex in L. hirsutwn and persistent, hydrophobic waxy coating of the epidermis in L. peruvianum may also be relevant in disease response.

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