Cleon W. Ross

The growth physics and water relations of red-light-induced germination in lettuce seeds : III. Changes in the osmotic and pressure potential in the embryonic axes of red-and far-red-treated seeds.

Irradiation of Grand Rapids lettuce (Lactuca sativa L.) seeds with red light (R) results in an increase in the growth response in the embryonic axes which enables them to overcome the mechanical restriction of the endosperm or the osmotic restriction of an external solution. When the growth rates of axes from R-and far-red(FR)-treated seeds are equalized with polyethylene-glycol-4000 solutions, a water potential difference of 3.4 bars between the axes is obtained. Measurements of the osmotic potential indicate a 1.0–2.0-bar difference between the treatments, depending upon incubation time. Although the difference in water potential remained constant during the growth of the axes, the osmotic constituents of the axes of R-treated seeds were diluted to a greater extent than those of the axes of FR-treated seeds. The length-to-volume ratio of the growing axes of R-treated seeds increased at a greater rate than that of the axes of FR-treated seeds. These data indicate that changes in the cell-wall properties may account for the remainder of the observed water potential difference. We propose that the phytochrome-mediated growth increase in these embryonic axes is an integrated function of the cells: increased wall loosening may be inextricably linked to simultaneous increases in the osmotic constitutents. The driving force for growth may not involve solely a change in the osmotic potential or the pressure potential, with passive readjustment of the other parameter, but the activation of a cellular component that links changes in the wall properties with the accumulation of osmotic constituents.

Growth regulators and flowering

I t seems clear tha t transformation of the vegetative bud of eocklebur (Xanthium pennsylvanicum WALL.) to the reproductive condition occurs in response to a hormone which is synthesized in the leaf starting about 8 hours after the beginning of darkness (e.g. SALISBURY 1961, 1963). The present paper describes the results of one indirect approach used to gain insight into the physiology and biochemistry of flowering hormone synthesis, translocation, and action at the bud. Antimetabolites which are known to inhibit specific metabolic steps in other systems are applied. I f flowering is inhibited, the suspected step is implicated in a preliminary way in the flowering process. The participation of this suspected step can be further tested by a t tempts to reverse the effects of the antimetabolite by simultaneous application of a suspected corresponding metabolite or the product of the blocked step. The time of effectiveness of the antimetabolite can also be determined. A related approach using conventional growth regulators has shown tha t measurement of the critical 8 hour 20 minute dark period (critical night) is metabolically different from the subsequent synthesis of flowering hormone, and tha t ATP synthesis is an essential par t of hormone synthesis (SALISBURY 1957). Thus cobaltous ion inhibits flowering only when applied during time measurement, while 2,4-dinitrophenol inhibits during hormone synthesis. Auxins inhibit during the translocation period, and 2,2-dichloropropionic acid, 2,4-diehlorophenoxyacetie acid, and maleic hydrazide inhibit development of the floral bud.

The growth physics and water relations of red-light-induced germination in lettuce seeds : IV. Biochemical changes in the embryonic axes of red-and far-red-treated seeds.

Using lettuce (Lactuca sativa L., cv. Grand Rapids) embryos in osmotica, we have demonstrated that when the growth rates of the embryonic axes of seeds treated with red (R) or far-red (FR) light are equalized, the axes of R-treated seeds develop a 3.4-bar decrease in water potential (paper No. III).As axial growth begins, reserve protein and phytin decrease rapidly, concomitant with increases in reducing sugars, α-amino nitrogen, and inorganic and esterified soluble phosphates. However, no differences between the axes of R-and FR-treated seeds are found with respect to the changes in these compounds, indicating that these changes arise as a result of growth and are not under immediate phytochrome control. Little change in the total lipid content is found in either treatment. The axes of FR-treated seeds hydrolyze endogenous sucrose at a greater rate thant those of R-treated seeds. Axes of R-treated seeds accumulate K+ and Na+ to a greater extent than those of FR-treated seeds. When potassium salts are added to the incubation medium, R induces increased K+ uptake by the axis and greater medium acidification by the axis. Malate and other organic acids and acidic amino acids increase at equal rates in both treatments, indicating that inorganic anions may also be taken up to balance the ionic charges. The results are compatible with the assumption that changes in the osmotic and pressure potentials of the embryonic axes of R-treated seeds are the result of a phytochrome-stimulated proton pump which, in whole dormant seeds, would initiate water-potential changes allowing the embryos to overcome the mechanical restraint of the surrounding seed layers, resulting in germination.

Photodormant lettuce seeds: Phytochrome-induced protein and lipid degradation.

SummaryPhotodormant lettuce (Lactuca sativa L.) seeds were incubated in mannitol to prevent germination. Under these conditions, red light induced an extensive degradation of protein and a moderate one of lipid in the cortical cells of the radicle 24 h after treatment. It is suggested that the previously reported phytochrome-caused lowering of the water potential in light-treated embryos, which enables these embryos to overcome the resistance of the surrounding layers of the seed, is caused by lowering of the osmotic potential due to degradation of storage compounds in the radicle.

Influence of 6-azauracil on pyrimidine metabolism of cocklebur leaf discs

Abstract 1. 1. The influence of 6-azauracil on metabolism of 14 C-labeled orotic acid and uracil was studied in leaf discs of the cocklebur plant, Xanthium pennsylvanicum Wall. 2. 2. Conversion of orotic acid to ribonucleic acid was inhibited by azauracil. Considerable evidence indicates an interference with decarboxylation of orotidylic acid: CO 2 release from [ carboxy - 14 C]orotic acid was reduced by an amount approximately equal to the inhibition of ribonucleic acid synthesis from [2- 14 C]orotic acid. Under these conditions orotic acid accumulated in the cells. Synthesis of ribonucleic acid from labeled uracil was not reduced, provided corrections were made for inhibition of uracil absorption into the leaf discs by azauracil. 3. 3. 12 metabolites of uracil were detected in hot ethanol (80%) extracts of the tissues, including the catabolic products dihydrouracil and β-ureidopropionic acid. Certain uridine nucleotides were also present. Azauracil inhibited conversion of uracil into a nucleotide tentatively identified as uridine-diphosphate glucose. 4. 4. Orotidylic acid and orotidine were not present in the leaves in amounts detectable by the techniques used.

The influence of plant growth regulators on the growth of the embryonic axes of red- and far-red-treated lettuce seeds

The influence of several plant growth regulators on the growth of the embryonic axes from red- and far-red-(R- and FR-)treated lettuce (Lactuca sativa L., cv. Grand Rapids) seeds was examined; as shown previously, the water potential of the axes from R-treated seeds has been lowered by 3.5–5.6 bars compared to that in axes from FR-treated ones. Kinetin and abscisic acid (ABA), when included in the incubation medium, reduced the elongation of the axes whereas fusicoccin stimulated it; however, these effects were the same in axes of both R- and FR-treated seeds. In contrast, elongation of axes from FR-treated seeds was stimulated by gibberellic acid (GA3, but elongation of axes from R-treated ones was not affected by this hormone. This latter result indicates that gibberellins may be involved in the phytochrome-mediated growth responses in lettuce axes.When the root caps of the embryos were removed prior to light treatment, R was still able to induce a water-potential decrease in the embryonic axes, indicating that at least a portion of the active Pfr resides in the axis and not the root cap.

Influence of cycloheximide (actidione) upon pyrimidine nucleotide metabolism and RNA synthesis in cocklebur leaf discs

Abstract Cycloheximide inhibited the absorption of labeled orotic acid, uridine and cytidine and the catabolism of these compounds to CO 2 in cocklebur ( Xanthium strumarium l .) leaf discs. It markedly inhibited the conversion of orotic acid and uridine to RNA cytidylic acid, but had less influence upon their conversions to RNA uridylic acid. However, when labeled cytidine was fed, the antibiotic had little influence upon its conversion to RNA cytidylic acid, but did block its conversion to uridylic acid. These results suggest that cycloheximide somehow interferes with the interconversion of UTP and CTP. Investigations of soluble nucleotides supported this interpretation. It is suggested that the present results and some described by others may arise from an inhibition of glutamine synthesis or of reactions in which glutamine participates. Cycloheximide also reduced by half or more the amount of 14 C in uracil after the leaf discs metabolized either labeled cytidine or uridine, but increased the amount of 14 C in UMP. The decreased uracil formation might account for the inhibited catabolism of orotic acid, uridine and cytidine.

Metabolism of 6-azauracil and its incorporation into RNA in the cocklebur

Abstract Leaves or shoot tips of the cocklebur, Xanthium pennsylvanicum Wall., catabolized 6-azauracil and converted it to the nucleotide level and into RNA. Both catabolic and anabolic reactions were weak when compared to orotic acid, presumably a normal intermediate in pyrimidine nucleotide synthesis in plants.

Molecular weight estimations of some pyrimidine-metabolizing enzymes from pea cotyledons by gel filtration

Abstract The molecular weights of 4 pyrimidine-metabolizing enzymes from imbibed pea cotyledons were estimated by filtration on a Sephadex G-150 column to be as follows: orotidine-5′-phosphate pyrophosphorylase and orotidine-5′-phosphate decarboxylase, 62,000; uridine nucleosidase, 55,000; cytidine deaminase, 47,000.

Enhanced activity of a Ca-calmodulin-dependent isozyme of NAD kinase caused by gibberellic acid in photodormant lettuce seeds

Summary We tested the hypothesis that GA3, like red light (Zhang et al., 1994), enhances activity of a Ca-calmodulin- dependent isozyme of NAD kinase and causes lower levels of NAD+ and higher levels of NADP+ in half-seeds of Grand Rapids lettuce (Lactuca sativa L.). Half-seeds containing hypocotyl/radicle sections were imbibed for 1 h in H2O or 560 μM GA3, treated with far-red light, then kept in darkness for various times up to 17 h. NAD kinase activity from crude extracts was measured, and activities of a partially purified Ca-calmodulin-independent and a Ca-calmodulin-dependent isozyme were determined. In all experiments kinase activity increased with time after treatment. GA3 enhanced activity in crude extracts and of the Ca-calmodulin-dependent isozyme 30 min after irradiation, but it had no effect on the Ca-calmodulin-independent isozyme at any time period. GA3 caused increased amounts of NADP+ and NADPH and decreased amounts of NAD+, all effects significant 30 min after irradiation. Although these results might be explained partially by GA3-enhanced formation of Ca-calmodulin in vivo, it appears that GA3 additionally or alternatively induced formation of the Ca-calmodulin-dependent isozyme of NAD kinase. These effects of GA3 are similar to those of red light, and they support the hypothesis that Pfr somehow promotes synthesis of a gibberellin as it overcomes photodormancy of the seeds.