Wayne J. McIlrath

Yellow-Stripe Phenotype in Maize. I. Effects of ys1 Locus on Uptake and Utilization of Iron

1. The yellow-stripe maize mutant (ys1) resembles maize plants grown on iron-deficient or low-iron media. Homozygous recessive ys1/ys1 plants have been shown to respond to various iron sources differently than do plants carrying at least one dominant allele at this locus. Normally green homozygous dominant (+/+) and heterozygous (+/ys1) plants are visually indistinguishable and appear to be members of a single population distinct from ys1/ys1. This investigation was planned to delimit the site of the lesion and to characterize processes associated with this locus. 2. Foliar tissues of plants of genotypes +/+, +/ys1, or ys1/ys1 metabolize either Fe++ or Fe+++ iron. 3. Plants with the ys1/ys1 genotype are visually indistinguishable from +/+ plants when grown in nutrient solutions containing 2 p.p.m. FeSO4 or Fe-HEDTA (iron chelate of trisodium N-hydroxy-ethylethylenediaminetriacetic acid) but display the yellow-stripe phenotype when supplied with iron as Fe-EDTA (ferric chelate of ethylenediaminetetraacetic acid) or certain other iron chelates. 4. Derooted ys1/ys1 plants utilize Fe-EDTA efficiently in complete nutrient solution, whereas intact ys1/ys1 plants remain yellow-striped. Thus it appears that the lesion for iron uptake is localized in the root tips of these plants. 5. Plants of genotypes ys1/ys1 and +/+ respond to iron supplied by the split root-system technique in the same manner as when nutrients are combined in a single solution. 6. Yeast extract enhanced greening of new leaves of ys1/ys1 plants grown in sterile culture with Fe-EDTA in a complete nutrient agar medium; casein hydrolysate, corn steep water, and partitions of the yeast extract were ineffective. Yeast extract did not affect utilization of Fe2O3. 7. Plants of genotype ys1/ys1 greened effectively on a phosphorus-deficient (-P) solution on all forms of soluble iron tested; Fe2O3 when supplied without phosphorus was utilized in sand cultures. 8. Low levels of organic or inorganic phosphate allowed iron utilization by intact plants; increased levels of phosphate affected ys1/ys1 plants more than those of the +/+ genotype. 9. Plants of the ys1/ys1 genotype interplanted in vessels of nutrient solution with +/+ maize, Lycopersicon, or Xanthium greened completely; plants in control vessels containing only the ys1/ys1 genotype remained yellow-striped. Furthermore, plants of the ys1/ys1 phenotype were able to take up iron, supplied as Fe-EDTA, from nutrient solutions in which +/+ plants had grown. This response was observed regardless of whether the nutrient solutions were supplemented with additional phosphate or not. Various of these data suggest that a natural iron chelator may be formed by +/+ maize and other species of plants but not by ys1/ys1 plants grown at levels of phosphorus normal for +/+ maize.

Accumulation of scopoletin glucoside in boron-deficient tobacco leaves

Abstract Isopropyl alcohol-water extracts of tobacco leaves yielded a blue-fluorescent compound that was identified as the glucoside of scopoletin (6-met hoxy-7-hydroxycoumarin). Leaves of boron-deficient plants produced a twenty-fold increase of this material over leaves of normal plants. The possible role of boron in lignin biosynthesis is pointed out on the basis that certain phenolic compounds that may serve as lignin precursors accumulate in boron-deficient plants.

Effects of Boron on Growth and Lignification in Sunflower Tissue and Organ Cultures

The influence of boron on the growth of and lignin formation in stem callus and root cultures of sunflower, Helianthus annuus L., was investigated. A significantly lower rate of growth in both types of tissue was observed to result from a deficiency of boron in the nutrient medium. This effect was noted during the third 30-day transfer period with callus cultures, while it appeared in the second 10-day transfer period with roots. Boron deficiency resulted in a lower degree of lignification of both callus and roots. The total lignin content of boron-deficient tissues was significantly less than that of those receiving adequate boron. Although boron deficiency resulted in changes in catalase activity in the tissues, the trends were not consistent. The peroxidase activity of boron-deficient tissues was consistently and significantly lower than that of tissues receiving boron in the nutrient medium. It is proposed that the lower peroxidase activity may account in part for the lower degree of lignification of boron-deficient tissues.

Mobility of Boron in Several Dicotyledonous Species

The translocation of dialyzable (soluble) boron from mature leaves of plants deprived of an external supply of this element was determined for several dicotyledonous species including bean, chard, cocklebur, cotton, cucumber, sunflower, tobacco, tomato, and turnip. Although a significantly lower dialyzable-boron concentration was found in all species showing deficiency symptoms except cocklebur and cucumber, this reduction was caused primarily by an increased fresh weight of the leaves and not by a change in absolute quantities of boron. The total dialyzable boron per leaf was significantly lower only in cotton and turnip. It was concluded that cotton and turnip were the only species showing a high degree of boron mobility under the experimental conditions used in this investigation.

Photoperiodic, Induction of Senescence in Xanthium Plants

Xanthium plants placed under photoinductive conditions were accelerated in their senescence whether or not the plants had a capacity to produce flowers and fruits. Thus, the role of the mobilization influence of these organs on the senescence of certain monocarpic plants can be questioned.

Dehydration of Seeds in Intact Tomato Fruits

Although the moisture content of the placenta and pericarp of tomato fruits older than 15 days never dropped below about 94 percent, the seeds dehydrated to approximately 50 percent moisture as these organs developed and matured. There was a loss in the total amount, as well as the percentage, of water per seed. Part of this dehydration was mediated by the seeds themselves and part appeared to be due to other portions of the fruit.

Boron Nutrition and Lignification in Sunflower and Tobacco Stems

Sunflower plants (Helianthus annuus L.) decapitated above the third node and tobacco plants (Nicotiana tabacum L.) decapitated above the tenth node were grown in plus- and minus-boron solutions in an attempt to determine whether boron plays a role in tissue lignification. After 18 days the stems of sunflower and after 82 days those of tobacco were harvested and analyzed for lignin. The percentage lignin in the stems of the minus-boron sunflower plants was higher, although there was no significant difference between the two series in total quantity of lignin. The percentage and total quantity of lignin in the seventh and eighth internodes of tobacco stems were lowest in the minus-boron plants; in the ninth internode these values were highest in the minus-boron plants. The present results seem to indicate that boron has some function in lignin production, but it was not established whether this is a direct role. Some of the factors which have resulted in the apparent discrepancies in the literature with respect to the effect of boron on lignification are discussed.

Absorption and Translocation of Boron by Sunflower Plants

The absorption and translocation of boron by sunflower plants (Helianthus annuus L.) grown under controlled-environment conditions were investigated. Exudates in a 43-hr period from the cut stem ends of the root systems of detopped plants contained the same amount of boron whether the roots were immersed in nutrient solutions containing 500 μg or 1,000 μg of boron per liter. Plants given a total of 60 μg of boron in the substrate, but in varied amounts and times of application, showed no differences among the treatments in the accumulation of boron by their mature leaves. Two of the four series with multiple application, however, showed delays in the onset of severe symptoms of boron deficiency as compared with the series with an initial single 60-μg application. Significant lateral transport of boron occurred at the stem tip and in leaf blades when boron was supplied to only half of a split root system at 500 or 1,000 μg per root-half. Slight, though not significant, lateral transport occurred across the lower levels of the stems in these series. Lateral transport of boron at the lower internodes was demonstrated when boron was supplied to only one-half of the root system in higher quantities (5,000 or 10,000 μg per root half). Little or no lateral transport in leaves or stems was observed with boron application of 50 or 100 μg per root-half. Although mature leaves sprayed with 10-ppm boron solution (as boric acid) showed an increase in dialyzable boron content, this soluble boron was not translocated from the leaves. No differences in growth were evident between plants receiving dialyzable boron extracted from leaf tissues and those receiving boric acid in the nutrient solutions. Non-dialyzable boron in the non-particulate fraction of leaf homogenate, however, was not utilized by plants when it was added to the nutrient solution. Reversing the transpiration stream of a leaf by placing it in a dilute minus-boron nutrient solution and withholding a water supply from other organs of the plant did not result in a movement of boron from the leaf to the apical meristem. Some of the boron in the leaf, however, was readily leached into the nutrient solution. Covering the top surface of a leaf with aluminum foil to limit photosynthesis did not result in a change in its boron content. A nitrogen-deficient nutrient solution adversely affected the capacity of plants to accumulate boron. From the results of the experiments described, it is concluded that one of the prime factors controlling the distribution of boron in sunflower plants is the extent and location of the metabolic requirement for this element. It is further proposed that immobility of boron in the leaves of this species results from some type of translocation block in the vascular tissue and not from a fixation of boron in a soluble but non-utilizable form.

INFLUENCE OF PHOTOPERIOD ON WATER UPTAKE OF XANTHIUM

1. The water absorption of plants of Xanthium pensylvanicum Wallr. grown in solution culture and subjected to photoinductive and non-photoinductive photoperiods was investigated. 2. Plants grown under greenhouse conditions and given continuous photoinduction exhibited peaks in absorption, in comparison with non-induced plants, coincident with the appearance of macroscopic flowers and with anthesis. 3. Plants given continuous photoinduction in the control room had a single period of accelerated water uptake coincident with meiosis. Anthesis in this case occurred after the rate of absorption had declined. 4. Aqueous solutions of indoleacetic acid in concentrations of 0.01-1000 p.p.m. sprayed on the leaves of non-induced plants did not promote water uptake. Leaf applications of 10 and 100 p.p.m. 2,4-dichloroanisol resulted in a slight stimulation of absorption. 5. Water absorption by debudded plants under photoinductive daylengths was similar to that of intact photoinduced plants. From this it was concluded that the onset of stages in the reproductive process normally correlated with accelerated water uptake is a merely parallel occurrence and not a causative factor.