No abstract

Ladino clover plants were allowed to wilt in the field and in the greenhouse to determine the effects of wilting on soluble nitrogen compounds. Wilting caused a slight decrease in total nitrogen but little change in amount of soluble nitrogen. A large increase in free proline occurred in all wilted leaves. Free asparagine was abundant in unwilted leaves but varied considerably during wilting. Other free amino acids occurred in small amounts. Only traces of glutamine were found.Two light intensities were used in the greenhouse experiment. During severe wilting, the higher intensity stimulated proline synthesis. The results suggest that proline accumulation during wilting is influenced by available carbohydrate.The results of these experiments show that marked changes in free nitrogen compounds occur during wilting. These changes may aid in conserving nitrogen and carbon during moisture stress.

The cyclic hydroxamate, 2,4‐dihydroxy.7‐methoxy‐l,4‐benzoxazin‐ 3‐one (DIMBOA), has been implicated in the resistance of corn (Zea mays L.) to fungi and insects. In our study, bioassay and field experiments tested the relationship between hydroxamate concentration and resistance to corn leaf aphid (Rhopalosiphum maidis (Fitch)).In the bioassay, various concentrattons of DIMBOA were added to an artificial diet fed to corn leaf aphids. DIMBOA concentrations of 0.1, 0.25, and 0.5 mg/g diet caused 5.1, 12.8, and 20.8% mortality, respectively. In field trials, 12 inbred lines were evaluated for corn leaf aphid resistance under natural infestation. Aphid infestation was evaluated at mid‐silking stage with a visual rating scale and index system. Cyclic hydroxamate concentration in a separate set of the same lines was estimated by a rapid procedure based upon the colorimetric reaction of hydroxamates with ferric chloride. Aphid infestation values ranged from 119 to 366. Hydroxamate concentration ranged from 0.08 to 1.48 mg/g fresh weight. A highly significant correlation (r = 0.72) was obtained between aphid infestation and hydroxamate concentration.Eighteen hybrids derived from crosses among 12 inbred lines were also evaluated for corn leaf aphid resistance in the field. Aphid infestation values from high ✕ high, intermediate ✕ intermediate, and low ✕ low DIMBOA lines were generally low, intermediate, and high, respectively. Hybrids derived from crossing high ✕ low, high ✕ intermediate, and intermediate ✕ low DIMBOA lines produced infestation values between the parental extremes.

Field experiments were conducted to determine the relationship between hydroxamic acid, total phenol, and orthodihydroxyphenol concentrations in corn (Zea mays L.) tassels and resistance to the corn leaf aphid [Rhopalosiphum maidis (Fitch)]. Eleven corn inbreds were evaluated for aphid resistance soon after tassel emergence using a visual rating scale and index system. The corn tassels were harvested and analyzed for hydroxamic acid, total phenol, and orthodihydroxyphenol content. A highly significant negative correlation (r =-0.63) was obtained between aphid infestation and hydroxamic acid concentration. Poor correlations were found between aphid infestation and both the total phenol and orthodihydroxyphenol concentrations. Seven corn inbred lines were further evaluated for corn leaf aphid resistance by artificial infestation in the greenhouse. A highly significant correlation (r = 0.90) was found between the greenhouse and field results.

A cyclic hydroxamate (2,4‐dihydroxy‐7‐methoxy‐l,4‐benzoxazin‐3‐one; DIMBOA) recently has been implicated in the resistance of maize (Zea mays L.) to fungi and insects. In our study, a set of 13 inbreds was used to determine the relationship of concentrations of DIMBOA and resistance to northern corn leaf blight (Helminthosporium turcicum Pass).DIMBOA concentration was estimated by the colorimettic procedure of Hamilton in plants 60 to 75 cm in height. Mean concentrations of DIMBOA ranged from 0.06 mg/g to 0.66 mg/g fresh weight. Percent leaf infection on plants at the same height ranged from 7.41 to 19.04%. A significant correlation (r =−0.61) was obtained between these two traits.DIMBOA concentrations also were determined in root, stem, and whorl tissues from five selected inbreds at various stages of growth. At 13‐cm plant height, samples from the inbreds contained uniformly high amounts of DIMBOA. The earliest acceptable separation of high and low DIMBOA lines was obtained using stem or whorl tissue from seedlings 35 to 40 cm in height. Generally those lines highest in DIMBOA at 35 to 40 cm plant height were correspondingly high in DIMBOA at the silking stage. Similarly, lines low in DIMBOA at 35 to 40‐cm plant height were low in DIMBOA at the silking stage.