Response of enzymes and storage proteins of maize endosperm to nitrogen supply.

To examine the effects of N nutrition upon endosperm development, maize (Zea mays) kernels were grown in vitro with either 0, 3.6, 7.1, 14.3, or 35.7 millimolar N. Kernels were harvested at 20 days after pollination for determination of enzyme activities and again at maturity for quantification of storage products and electrophoretic separation of zeins. Endosperm dry weight, starch, zein-N, and nonzein-N all increased in mature kernels as N supply increased from zero to 14.3 millimolar. The activities of sucrose synthase, aldolase, phosphoglucomutase, glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase, and acetolactate synthase increased from 1- to 2.5-fold with increasing N supply. Adenosine diphosphate-glucose pyrophosphorylase and both ATP- and PPi-dependent phosphofructokinases increased to lesser extents, while no significant response was detected for hexose kinases and glutamine synthetase. Nitrogen-induced changes in enzyme activities were often highly correlated with changes in final starch and/or zein-N contents. Separation of zeins indicated that these peptides were proportionately enhanced by N supply, with the exception of C-zein, which was relatively insensitive to N. These data indicate that at least a portion of the yield increase in maize produced by N fertilization is induced by a modification of kernel metabolism in response to N supply.

Nitrogen-induced changes in the growth and metabolism of developing maize kernels grown in vitro.

Cereal kernel growth and grain yield are functions of endosperm starch accumulation. The objective of this study was to examine how various metabolic factors in developing maize (Zea mays L.) endosperm influence starch deposition. Kernels were grown in vitro on medium with: (a) zero N (-N), (b) optimum N (+N), or (c) -N from 3 to 20 days after pollination followed by +N until maturity (+/-N) to produce different degrees of endosperm growth and to promote an enhancement of starch synthesis midway through development. At intervals, kernels were harvested and levels of enzyme activities and carbohydrate and N constituents examined. Endosperm starch and protein accumulation were decreased in -N compared to +N kernels, but relief of N starvation increased both constituents. With greater movement of N into +/-N kernels, endosperm sugar concentrations declined suggesting an inverse relationship between C and N transport. Unusually high concentrations of sugar in N stressed kernels did not appear to limit or enhance starch production. Rather, increased accumulation of starch in +/-N endosperm was correlated with significant increases in the enzymatic activities of sucrose synthase and PPi-linked phosphofructokinase, and to a lessor extent hexokinase. In addition, the occurrence of specific proteins of the albumin/globulin fraction either increased, decreased, or remained unchanged in relation to starch synthesis. These data suggest that lack of N limits starch deposition in maize endosperm primarily through an influence on synthesis of key proteins.

Overcoming drought-induced decreases in soybean leaf photosynthesis by measuring with CO2-enriched air.

Soybean [Glycine max (L.) Merr. cv. Williams 82 and A3127] plants were grown in the field under long-term soil moisture deficit and irrigation to determine the effects of severe drought stress on the photosynthetic capacity of soybean leaves. Afternoon leaf water potentials, stomatal conductances, intercellular CO2 concentrations and CO2-assimilation rates for the two soil moisture treatments were compared during the pod elongation and seed enlargement stages of crop development. Leaf CO2-assimilation rates were measured with either ambient (340 µl CO2 l(-1)) or CO2-enriched (1800 µl CO2 l(-1)) air. Although seed yield and leaf area per plant were decreased an average of 48 and 31%, respectively, as a result of drought stress, leaf water potentials were reduced only an average of 0.27 MPa during the sampling period. Afternoon leaf CO2-assimilation rates measured with ambient air were decreased an average of 56 and 49% by soil moisture deficit for Williams 82 and A3127, respectively. The reductions in leaf photosynthesis of both cultivars were associated with similar decreases in leaf stomatal conductance and with small increases in leaf intercellular CO2 concentration. When the CO2-enriched air was used, similar afternoon leaf CO2-assimilation rates were found between the soil moisture treatments at each stage of crop development. These results suggest that photosynthetic capacity of soybean leaves is not reduced by severe soil moisture deficit when a stress develops gradually under field conditions.

Whole plant senescence of sunflower following seedhead removal.

This study was undertaken to further clarify the relationship between seed development and monocarpic senescence of sunflower (Helianthus annuus L.). Field-grown plants with and without seedheads were evaluated for rate and duration of accumulation of dry weight, reduced N, and P by whole shoots, and for partitioning of these constituents within the individual plant parts. Concurrent with seedhead removal, [(15)N]nitrate was applied to the plants in a selected area of the experimental plot. Whole plants (above ground portions) were harvested seven times during the seed-filling period and analyzed for dry weight, reduced N, and P. Although seedhead removal depressed the rates of dry weight, reduced N, and P accumulation by whole shoots, it extended the duration of accumulation of these constituents, relative to headed control plants. As a result, the final whole shoot dry weight and N and P contents at seed maturity were similar for deheaded and headed plants. Seedhead removal also affected the partitioning of dry matter, reduced N, and P but the relative proportions varied as a function of constituent and growth stage. Analysis of (15)N present in whole shoots at physiological maturity showed that similar amounts of nitrate were absorbed during the postflowering period by headed and deheaded plants. These data indicate that the absence of seeds does not affect the total accumulation of dry matter, reduced N, or P, by sunflower plants, but does alter the rates of accumulation and partitioning of these constituents.

Growth and composition of maize kernels cultured in vitro with varying supplies of carbon and nitrogen.

This study employed in vitro seed culture to determine how C and N supply influence the growth (i.e. starch accumulation) and protein composition of maize (Zea mays L.) endosperm. Immature kernels were grown to maturity on liquid medium containing various concentrations of C (sucrose at 234 millimolar [low] and 468 millimolar [high]) and N (amino acid mixture ranging in N from 0 to 144 millimolar). Low C supply limited starch, but not N, accumulation in the endosperm. With high C, endosperm starch and protein content increased concomitantly as N supply increased from 0 to 13.4 millimolar. Endosperm growth was unaffected by additional N until concentrations exceeding approximately 72 millimolar reduced starch accumulation. A similar inhibition of starch deposition occurred with lower N concentrations when kernels were grown with low C. Endosperm total N content reached a point of saturation with approximately 36 millimolar N in the medium, regardless of C supply. Zein synthesis in the endosperm responded positively across all N levels, while glutelin content remained static and albumin/globulin proteins were reduced in amount when N supply was greater than 36 millimolar. A reciprocal, inverse relationship was observed in mature endosperm tissue between the concentrations of free amino acids and soluble sugars. Our data suggest that under N stress starch and protein accumulation in the endosperm are interdependent, at least in appearance, but are independent otherwise.

Increased induction of regenerable callus cultures from cultured kernels of the maize inbred FR27rhm.

Kernels of the maize inbred FR27rhm were cultured on various media to determine if the treatments would alter the frequency of formation of regenerable callus (induction frequency) by embryos excised from the kernels when they were placed on callus induction medium. The addition of 60 µM dicamba (3,6-dichloro-o-anisic acid) to the kernel culture medium resulted in an induction frequency of 27-38% compared to 0% for controls on standard kernel culture medium. Embryos excised from dicamba-treated kernels also showed in-ovule callus-like tissue proliferation. The increased induction frequency and the callus-like tissue proliferation could also be produced by injecting the ears of field grown FR27rhm plants, 3-d post pollination, with 1.08 µmoles of dicamba. The results indicate that treatment of the developing ear with dicamba, in vivo or the developing kernel in vitro, may be an effective means to increase the frequency of regenerable callus induction from recalcitrant maize genotypes, such as the B73 derivative FR27rhm.

Effect of head removal on leaf senescence of sunflower.

Greenhouse and field studies examined the effect of flower or seedhead removal on leaf senescence and associated changes in sunflower (Helianthus annuus L.) plants. At intervals during seed development, selected leaves (leaves 6 through 8 from the top in the greenhouse and leaf 7 from the top in the field) were harvested and analyzed for chlorophyll, specific leaf weight, N, P, soluble protein, and electrophoretic gel profiles of soluble polypeptides. In both the greenhouse and the field, the leaves of headless plants retained or accumulated more N, P, soluble protein, and dry weight than leaves of plants with heads. Obviously, head removal affected the partitioning of these metabolites during seed development. None of the treatments resulted in the formation of new polypeptides (electrophoretic gel profiles). Comparisons of the rates and extent of loss of chlorophyll, soluble protein, and polypeptide bands (especially ribulose 1,5-bisphosphate carboxylase) from the leaves of headed and deheaded plants showed that head removal delayed the rate of development of leaf senescence for the greenhouse-grown but had much less effect on field-grown plants. These findings illustrate the variability in different parameters commonly associated with the leaf senescence processes of headed and deheaded sunflower plants grown under different environments.

Effect of foliar applications of urea on accelerated senescence of maize induced by ear removal.

Field grown maize (Zea mays L. cv B73 x Mo17) plants, with and without ears, were sprayed with urea solutions to determine whether foliar application of N could prevent or delay the accelerated loss of reduced N from the leaf and leaf senescence induced by ear removal. Urea sprays were applied at 7, 14, and 21 days after anthesis in three separate and equal applications that provided a total of 67 kilograms N per hectare or 1 gram N per plant. Treatments were arranged in a 2 x 2 factorial in a randomized complete block with five replicates. Appropriate plant and leaf samplings and assays were made.In response to spray treatments, net increases of reduced N were detected in the whole shoot and plant parts, especially the stalk of the earless plants and grain of the eared plants. There was no effect of urea spray treatment on the normal loss of N from the leaves or rate of senescence of the eared plants or on the accelerated loss of N from the leaves or rate of senescence induced by ear removal. Grain and stover yields were unaffected by the spray treatment.Apparently the plants were unable to utilize the urea N applied to the vegetation (primarily leaves) after anthesis to enhance or extend the accumulation of dry weight by either eared or earless plants.

Effect of nodulation on assimilate remobilization in soybean.

The objectives of this work were to determine the effect of nodulation on dry matter, reduced-N, and phosphorus accumulation and partitioning in above-ground vegetative parts and pods of field-grown soybean (Glycine max [L.] Merr. cv Harosoy).From comparison of nodulated and nonnodulated isolines, it was estimated that nodulated plants attained 81 and 71% of total-plant (above ground) N from uptake of soil N in 1981 and 1982, respectively. These data, along with visibly greener leaves of nodulated plants, led us to assume that nonnodulated plants were under a moderate N stress relative to nodulated plants. Nonnodulated plants accumulated less total-plant N and partitioned less dry matter and N to the pods, compared with nodulated plants. This occurred even though net photosynthesis, as estimated by rate and amount of dry matter accumulation, was the same for both nonnodulated and nodulated plants. Rate of dry matter and reduced-N accumulation in pods was less for nonnodulated than for nodulated plants while duration of podfill was similar for both isolines. From these data we concluded that moderate N stress affected partitioning of photosynthate rather than net photosynthesis, and that N played a role in translocation of photosynthate to the pods. Total plants (above-ground portion) and pods of both nodulated and nonnodulated plants accumulated similar amounts of phosphorus, which indicated that phosphorus and N accumulation were independent.Remobilization of nitrogen and phosphorus from vegetation to pods preceded dry matter remobilization. It appeared that either more nitrogen accumulation prior to podfill, or continued nitrogen assimilation during podfill would increase nitrogen and dry matter partitioning to pods, but that increasing photosynthesis without concomitantly increasing nitrogen input may not necessarily result in enhanced seed production.

Effects of Pod Removal on Metabolism and Senescence of Nodulating and Nonnodulating Soybean Isolines: II. Enzymes and Chlorophyll.

The objectives of this work were to determine the effect of sink strength (presence or absence of pods) and nitrogen source (nodulating versus nonnodulating plants) on enzymic activities, chlorophyll concentration, and senescence of soybean (Glycine max [L.] Merr. cv Harosoy) isolines. A 2-year (1981-1982) field study was conducted.For both nodulated and nonnodulated plants, ribulose bisphosphate carboxylase (RuBPCase) activity of upper-canopy leaves was decreased by pod removal in both years, while chlorophyll concentration was decreased in 1981 only. Nonnodulated plants had lower RuBPCase activity in 1981 and lower chlorophyll concentration in both years compared with nodulated plants. In both years, and for all treatments, RuBPCase activity and chlorophyll began to decline at about the same time, but the rate of decline was less for depodded than for podded plants. Leaves in the middle and lower parts of the canopy had similar RuBPCase activity and chlorophyll concentration trends as upper-canopy leaves for all treatments.Profiles of nitrate reductase activity (NRA) were similar for all treatments in both 1981 and 1982. Acetylene reduction profiles were similar for nodulated-podded and nodulated-depodded plants. The peak and decline in NRA profiles preceded the peak and decline in acetylene reduction profiles. The rate of decline in acetylene reduction activity was less for depodded plants, especially in 1982, but activities reached zero by the final sampling time. Thus, nodule senescence was not prevented by pod removal.Based on seasonal profiles of RuBPCase activity, chlorophyll, NRA, and acetylene reduction activity, the initiation of senescence appeared to occur at the same approximate time for all treatments and, thus, did not depend on the presence or absence of pods or nodules. The hypothesis that nodules act as a nitrogen source and carbohydrate sink to delay senescence in the absence of pods was not correct.

Effects of pod removal on metabolism and senescence of nodulating and nonnodulating soybean isolines: I. Metabolic constituents.

Field studies were conducted in 1981 and 1982 to ascertain the effects of pod removal on senescence of nodulating and nonnodulating isolines of soybean (Glycine max [L.] Merr. cv Harosoy) plants. Specifically, the test hypothesis was that nodules act as a nitrogen source and a carbohydrate sink which would in turn prevent or delay senescence in the absence of pods. Senescence was judged by changes in metabolite levels, in dry matter accumulation, and by visual observation.For both nodulated and nonnodulated plants, pod removal had no effect on the magnitude or rate of dry matter and reduced-N accumulation by whole plants. Phosphorus accumulation was significantly less in both nodulated- and nonnodulated-depodded plants, compared with respective control plants with pods. These data suggested a role for pods in phosphorus uptake. Accumulation of dry matter, reduced N, and phosphorus ceased at approximately the same time for all treatments.Pod removal did affect partitioning of plant constitments, with leaves and stems of depodded plants serving as a major alternate sink for accumulation of dry matter, reduced N, phosphorus, and nonstructural carbohydrates (primarily starch). While depodded plants eventually lost a significant amount of leaves, leaf drop was delayed relative to plants with pods; and depodded plants still retained some green leaves at 2 weeks past grain maturity of control (podded) plants.The results indicated that senescence patterns of soybean plants were the same for nodulated and nonnodulated plants, and that pods did not control the initiation of senescence, but rather altered the partitioning of plant constituents and the visual manifestations of senescence.

Differential Senescence of Maize Hybrids following Ear Removal : I. Whole Plant.

Visual senescence symptoms and associated changes in constituent contents of three field-grown maize (Zea mays L.) hybrids (Pioneer brand 3382, B73 x Mo17, and Farm Service brand 854) were compared in response to ear removal. Whole plants were harvested at eight intervals during the grain-filling period, and analyzed for dry matter, total N and nitrate N, phosphorus, sugars, and starch.Upper leaves of earless P3382 and B73 x Mo17 showed reddish discoloration by 25 days after anthesis (DAA) and all leaves had lost most of their chlorophyll by 40 DAA. In striking contrast, leaves of earless FS854 plants remained green and similar in appearance to eared controls throughout the grain-filling period.For all hybrids, ear removal led to a decrease in dry weight, reduced N, total N, and phosphorus contents of the total plant, and an increase in carbohydrate content of the leaves and stalks, relative to respective controls. Although changes in carbohydrate and N contents, which previously had been associated with senescence, were observed for all earless hybrids, these changes were followed by accelerated senescence and early death only for P3382 and B73 x Mo17. By 30 DAA, earless P3382 and B73 x Mo17 plants ceased to accumulate dry weight, total N, and phosphorus, indicating a termination of major metabolic activities. In contrast, earless FS854 plants retained a portion of these metabolic activities until 58 DAA, indicating a role for roots in determining rate of senescence development. Thus, the course of senescence was more accurately reflected by measurements of metabolic activities than by measurements of metabolite contents at any given time. These results show that the ear per se does not dictate the rate or completion of the senescence process, and implicated an association between the continued accumulation of N and associated root activities with the delayed senescence pattern of the earless FS854 plants. It is evident that studies involving control of senescence among species must also consider genotypic influences within species.

Differential Senescence of Maize Hybrids following Ear Removal : II. Selected Leaf.

In conjunction with a study of the effects of ear removal on the senescence of whole maize (Zea mays L.) plants, visual symptoms and associated changes in constituent contents and activities of a selected leaf (first leaf above the ear) were determined. Leaves were sampled from field-grown eared and earless Pioneer brand 3382, B73 x Mo17, and Farm Services brand 854 maize hybrids at nine times during the grainfilling period.VISUAL SYMPTOMS INDICATED THE FOLLOWING SEQUENCE AND RATE OF SENESCENCE: earless B73 x Mo17 > earless P3382 >> eared B73 x Mo17 >> eared P3382 eared FS854. All earless hybrids showed increases in leaf dry weight and sugar content; however, the increases were transitory for P3382 and B73 x Mo17, but continuous throughout the grain-filling period for FS854, indicative of continued photosynthetic activity of the latter. All earless hybrids exhibited similar and transitory starch accumulation patterns. Thus, FS854 was an exception to the concept that carbohydrate accumulation accelerates leaf senescence. Ear removal resulted in accelerated losses of reduced N, phosphoenolpyruvate and ribulose bisphosphate carboxylases, phosphorus, chlorophyll, nitrate reductase activity, and moisture for P3382 and B73 x Mo17 plants. In contrast, the loss of all components (except phosphorus) was similar for the selected leaf of earless and eared FS854.Although the loss of nitrate reductase activity, reduced N, and carboxylating enzymes accurately reflected the development of senescence of the selected leaf, the rate of net loss of reduced N and carboxylating enzymes appeared to be regulated. We deduced that the rate of flux of N into the leaf was a factor in regulating the differing rates of senescence observed for the six treatments; however, we cannot rule out the possibility of concurrent influence of growth regulators or other metabolites.

Metabolism of carbon and nitrogen by soybean seedlings in response to vegetative apex removal.

Short-term (31-hour diurnal) growth-chamber studies were conducted to determine the effects of removing the vegetative apex (meristem and developing trifoliolate leaves) on net photosynthesis (changes in plant dry weight), on distribution of metabolites among plant parts, and on nitrate metabolism and reduced-N accumulation by soybean [Glycine max (L.) Merr.] seedlings. Roots and stems served as alternate sinks for dry matter accumulation in the absence of the vegetative apex. Sugar concentration in roots increased (42%) within 4 hours of vegetative apex removal, and remained higher than for the controls during the 31-hour experimental period. Nitrate assimilation (nitrate reductase activity and total accumulation of reduced-N) was also enhanced in response to vegetative apex removal. Although dry matter accumulation was similar between treated and control plants (113 versus 116 milligrams per plant) over the 31-hour sampling period, more nitrate (1.31 versus 0.79 milligrams per plant) and more reduced-N (3.96 versus 3.45 milligrams per plant) accumulated in treated plants during the same interval. It was concluded that vegetative apex removal had little effect on overall net photosynthesis of soybean seedlings during the 31-hour treatment period, but did alter partitioning of photosynthate and enhanced uptake, transport, and reduction of nitrate. Implications are that uptake and metabolism of nitrate by soybeans may be limited by flux of carbohydrate to the roots, although hormonal effects due to vegetative apex removal cannot be ruled out.

Interaction of carbon and nitrogen metabolism in the productivity of maize.

Five maize (Zea mays L.) hybrids, FS854, B73 x Mo17, B84 x Mo17, B73 x B77, and P3382, grown under field conditions, were sampled at intervals during the grain-filling period. Plants were subdivided into stalks (including sheaths), leaves, and kernels. These parts were assayed for dry weight, reduced nitrogen, and extractable nonstructural carbohydrates. The duration and rates of net nitrate reduction and photosynthesis were approximated by the changes over time in the accumulation of reduced nitrogen and dry weight by the plant (total, above ground), respectively.Data on the accumulation of reduced nitrogen and dry weight by the plant show that decreases in nitrate reduction preceded (in time and extent for four of the hybrids and in extent for FS854) decreases or cessation of photosynthesis. FS854 continued to accumulate reduced nitrogen and dry matter throughout the grain-filling period.The patterns of change in stalk carbohydrate and reduced nitrogen during the early stages of ear development show the stalk serves as a storage reservoir and that these reserves were remobilized during the final stages of grain development. The marked increase and maintenance of dry weight and carbohydrate content of stalks until 34 days after anthesis, shows the capacity of the leaves to produce photosynthate through the first half of the grain-filling period exceeds the needs of the ear and/or the transport system. In contrast, stalk nitrogen content shows a slight increase up to 12 days after anthesis and decreases continually thereafter. Leaf nitrogen was lost continuously throughout grain development. The potential capacity of the plant to supply newly reduced nitrogen was inadequate to support initiation and early development of the kernels without remobilization of vegetative nitrogen. Of the two hybrids having delayed leaf senescence, FS854 with its initially higher concentration and content of reduced nitrogen in the stalk, initiated and developed a bigger ear than P3382, which had lower levels of stalk nitrogen.Three of the five hybrids had ;near linear' rates of accumulation of kernel dry weight, whereas none of the hybrids had linear rates of gain in kernel nitrogen. All hybrids had maximum or near maximum rates of gain of kernel nitrogen between 26 and 34 days after anthesis and a marked reduction (41-52%) of rates in the following sampling interval. These decreases are concurrent with decreases in rates of nitrate reduction (nitrogen accumulation) by the whole plant for four of the hybrids and with decreases in remobilization of nitrogen from the vegetation of FS854. Data for the ratio of rates of accumulation of dry weight/reduced nitrogen by the kernels versus time after anthesis, show that the accumulation of dry weight and reduced nitrogen are independent of each other. The variations in the ratio values appear best related to variations in the availability of nitrogen from the vegetation.

The effects of ear removal on senescence and metabolism of maize.

Ears were removed from field grown maize (Zea mays L.) to determine the effects on senescence and metabolism and to clarify conflicting literature reports pertaining to these effects. Ears were removed at three days after anthesis and comparisons were made of changes in metabolism between eared and earless plants until grain of the eared plants matured as judged by black layer formation.The initial visual symptom following ear removal was the development of reddish colored leaves. As judged by leaf yellowing, the removal of ears not only initiated an earlier onset but enhanced the rate of senescence. With this exception, the visual patterns of senescence were similar for earless and eared plants. Other characteristics associated with ear removal were: (a) marked decrease in dry weight and reduced N accumulation by the whole plant; (b) progressive, parallel decreases in leaf reduced N, nitrate reductase activity, and chlorophyll; (c) increases in carbohydrate content of both the leaf and stalk and of reduced N in the stalk. These changes indicate that ear removal reduced photosynthesis and nitrate reduction by approximately equal proportions and that the stalk serves as an alternate sink for both carbohydrate and nitrogen.The remobilization of nitrogen from the leaf was not dependent on the presence of an ear. A logical reason for the more rapid loss of nitrogen from the leaf of the earless plants appears to be the cessation of nitrate uptake and/or flux of nitrate to the leaves.From these results and from related experiments we tentatively conclude that the loss of nitrogen from the leaf is a major cause of death of the intact maize plant.

Availability of reduced N and carbohydrates for ear development of maize.

Changes in dry weights, reduced N, nitrate, and nitrate reductase activity of various plant parts of the above ground vegetation (stover) and ears of field grown maize were measured at intervals between anthesis and grain maturity. Nonstructural carbohydrate contents were also measured in some instances. Changes in dry weight and reduced N content were used to approximate net in situ photosynthetic and nitrate assimilation activities and to determine whether the availability of photosynthate or reduced N was limiting grain production.Of the five hybrids studied, all showed extensive remobilization (loss) of reduced N from the stover during grain development. This loss of stover N was initiated by 18 to 21 days after anthesis. Most of this loss of N (about 70%) was from the leaves. In contrast, three of the five hybrids had more vegetative dry weight at grain maturity than at anthesis, while the loss of stover dry weight by the other two hybrids was negligible. By 42 days after anthesis when the bulk of the ear weight had been acquired, the average gain in stover dry weight for the five hybrids was 12% while the loss of stover reduced N was 28%. Where measured, the increase in stover dry weight was largely due to deposition of carbohydrates in the stalk. These results show that the photosynthetic capacity was adequate while nitrate reduction capacity was inadequate for ear demands. The changes in the rate of accumulation of dry weight and reduced N by the ear indicated that the rate of supply of reduced N to the ear could have limited ear development for one of the five hybrids. The dry weight and carbohydrate (where measured) accumulation in the vegetation during the first 42 days after anthesis infers that the rate of supply of photosynthate to the ear was probably not a limiting factor for any of the five hybrids.The maximum remobilization of stover N during grain development was 1.8 g N plant(-1) for the genotypes examined, while the amount of reduced N accumulated by the grain varied from 1 to 5 g plant(-1). The amount of newly reduced N (nitrate reduced after anthesis) provided from 48 to 72% of the total N accumulated by the ear. The relative amounts of newly reduced N and remobilized N vary with genotype and environment. With respect to insuring high productivity, it was concluded that there is more flexibility in the system (genotype and environment) for increasing the supply of newly reduced N than in enhancing the remobilization of vegetative N.

Grain Protein Accumulation and the Relationship between Leaf Nitrate Reductase and Protease Activities during Grain Development in Maize (Zea mays L.): I. VARIATION BETWEEN GENOTYPES.

Four maize hybrids, two with high and two with low levels of postanthesis nitrate reductase activity were grown under field conditions. The characteristic enzyme patterns had been established in previous work. Nitrate reductase and proteases were measured in three representative leaves (ear leaf, fourth leaf above and fourth leaf below the ear) at intervals throughout the period of grain development. Concurrent with enzyme sampling, other plants were harvested and subdivided into top, middle and lower leaves, husks, stalks, and ear. Dry weights, nitrate, and reduced N were determined on all plant parts for each sampling. These data established the rate of N accumulation by the grain and depletion from the vegetative material and provide some insight into the relation between newly reduced and remobilized N and accumulation of grain N. Other plants were harvested at maturity for yield and harvest indices for dry weight and N.Nitrate reductase activity was higher in comparable leaves from the high than from the low nitrate reductase genotypes throughout the grain development period. There was no mathematical correlation between nitrate reductase activity and nitrate content of the leaves or stalks, however the high nitrate reductase genotypes maintained a higher amount of nitrate per plant (largely in the stalk) during the later stages of grain development. From the patterns of plant nitrate content it was deduced that the low nitrate reductase genotypes terminated nitrate absorption sooner than the high nitrate reductase types. Proteolytic activities (casein as substrate at pH 5.5 and 7.5) were higher and increased earlier in the low than in the high nitrate reductase genotypes. The "low nitrate reductase-high protease" genotypes had a higher percentage of grain N, and higher harvest index for N than did the "high nitrate reductase-low protease" genotypes. These results permit the tentative conclusions that: (a) redistribution of vegetative N accounted for more of the grain N in the low than in the high nitrate reductase genotypes; and (b) leaf protease activities are more closely related to the accumulation of grain N than leaf nitrate reductase activity.