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 × 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 × 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 × Mo17. By 30 DAA, earless P3382 and B73 × 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.

     

Effect of Ear Removal on CO(2) Exchange and Activities of Ribulose Bisphosphate Carboxylase/Oxygenase and Phosphoenolpyruvate Carboxylase of Maize Hybrids and Inbred Lines

The effects of ear removal on gas exchange traits, chlorophyll, and leaf N profiles, and activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase were examined using four maize hybrids (B73 × Mo17, B73 × LH38, FS854, and CB59G × LH38) and four inbred lines (B73, Mo17, LH38, and CB59G) as experimental material. A diverse genotypic response to ear removal was observed which was generally typified by (a) greatly accelerated loss of chlorophyll, leaf N, enzyme activities, and CO₂ exchange relative to controls for B73, B73 × Mo17, and B73 × LH38, (b) intermediate rate of decline for leaf constituents for FS854, LH38, and Mo17, or (c) loss of leaf constituents at similar or slower rates than for control plants for CB59G and CB59G × LH38. For all genotypes which had accelerated senescence relative to controls, loss of CO₂ exchange activity was correlated with increased internal CO₂ concentrations. Thus, it was concluded that metabolic factors and not stomatal effects were responsible for loss of CO₂ exchange activity. Loss of chlorophyll, leaf N, and enzyme activities correlated well with loss of CO₂ exchange activity only for some of the genotypes. Accelerated leaf senescence in response to ear removal for the inbred line B73 and the hybrids B73 × Mo17 and B73 × LH38, as well as the apparent delayed leaf senescence for the inbred line CB59G and the hybrid CB59G × LH38 show that the contrasting responses to ear removal, rapid versus delayed senescence, can be transmitted as dominant traits to F₁ hybrids. The intermediate response by some genotypes, and the dominance of contrasting senescence traits, suggested a relatively complex inheritance for expression of the ear removal response.     

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

Field grown maize (Zea mays L. cv B73 × 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 × 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.

     

Interaction of carbon and nitrogen metabolism in the productivity of maize

Five maize (Zea mays L.) hybrids, FS854, B73 × Mo17, B84 × Mo17, B73 × 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.

     

Carbon dioxide exchange rates, ribulose bisphosphate carboxylase/oxygenase and phosphoenolpyruvate carboxylase activities, and kernel growth characteristics of maize

Four high-yield-potential maize hybrids (FS854, CB596 x LH38, B73 x LH38, and B73 x Mo17) and four inbred lines (LH38, CB59G, Mo17, and B73) were grown in the field to study traits associated with leaf area duration (LAD) and the relationship between LAD and kernel growth characters. Based on decline in chlorophyll, leaf N concentration, CO(2) exchange rate, and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase) activities, the hybrid B73 x Mo17 had a significantly shorter LAD than the other three hybrids. The shorter LAD was not due to maturity because B73 x Mo17 is in a maturity class similar to the other hybrids except CB59G x LH38, which is approximately 1 week earlier. At the time of grain maturity, leaves of B73 x Mo17 had lost all chlorophyll and CO(2) exchange and carboxylase activities. The other three hybrids, however, retained green leaves which still had 20% of the maximum CO(2) exchange rate. In addition, B73 x Mo17 remobilized leaf N more extensively. For all hybrids, declines in CO(2) exchange were closely correlated with declines in PEPCase activity, whereas the relationship between CO(2) exchange and Rubisco activity was weak. Responses of the inbred lines predicted, to some extent, physiological characteristics of the hybrids. CB59G and LH38 both had a longer LAD than either B73 or Mo17 as judged by decline in chlorophyll, leaf N, CO(2) exchange rate, and Rubisco and PEPCase activities. With the exception of B73 x LH38, kernel growth characteristics of the hybrids were related to LAD. Effective filling period (EFP) measured in days was 32.9, 31.5, 30.8, and 30.4 for FS854, CB59G x LH38, B73 x LH38, and B73 x Mo17, respectively. For FS854 and CB59G x LH38, the longer EFP was associated with a larger kernel weight. These data suggested that late season photoassimilate resulting from longer LAD could be utilized by the kernels of these two hybrids. For B73 x Mo17, the shorter LAD and EFP was associated with a kernel dry matter accumulation rate (10.1 milligrams per kernel per day) which was significantly higher than for the other three hybrids. Thus, the more rapid leaf senescence of B73 x Mo17 appeared to be coordinated with efficient leaf N remobilization and a relatively short grain-filling period characterized by rapid kernel dry matter accumulation.     

Carbohydrate, Nitrogen and Dry Matter Accumulation and Partitioning of Maize Hybrids Under Drought Stress

Maize (Zea mays L.) productivity under drought stress dependsto some extent upon a hybrid's capacity to produce and translocateassimilate to its developing kernels during the stress periodand/or after the stress is relieved. The objective of this studywas to evaluate differences in carbon and nitrogen accumulationand partitioning under drought stress among maize hybrids thatdiffer in yield potential and/or physiological metabolism duringreproductive development. The hybrids B73 x LH38, FS854, B73xMol7and US13 were subjected to drought stress from the 7th leafstage until pollination was completed, at which time the soilof the stressed plots was replenished with water. For d. wtand chemical constituent determinations, plants of each hybridwere harvested from the irrigated and drought stressed plotsat silking, mid-grain fill, and physiological maturity. Averagedover hybrids, vegetative biomass at silking was reduced 25%as a result of the drought stress treatment, with B73 x LH38and FS854 accumulating more total biomass during the later portionof grain fill than the other two hybrids under both soil moisturetreatments. At silking, the total non-structural carbohydratecontent of the hybrids' vegetative tissue was not changed asa result of drought stress, whereas their reduced nitrogen (N)contents were decreased by an average of 33%. B73 x LH38 andFS854 had greater grain carbohydrate and reduced N contentsunder irrigation and smaller decreases in those variables asa result of soil moisture deficit than did the other two hybrids.These results indicate that the greater drought tolerance ofB73 x LH38 and FS854 to stress imposed during vegetative andearly reproductive development resulted from their more activeN uptake and assimilation and sugar production during the laterportion of grain fill and from their more efficient partitioningof assimilate to the developing kernels. Zea mays L., maize, drought stress, nitrogen, carbohydrates, hybrids, partitioning     

Vacuolar/Extravacuolar Distribution of Aminopeptidases in Giant Alga Chara australis and Partial Purification of One Such Enzyme

Objective of research was (a) to evaluate the influence of pollination-prevention on various metabolic parameters of the two maize inbreds B73 and B14A and their F1, and (b) to gain information on the inheritance of leaf senescence, in response to pollination-prevention. The results show that the visual pattern of leaf senescence, in response to prevention of ear pollination, contrasts markedly between the two inbred lines. Relative to control plants, prevention of ear pollination, causes a premature senescence in B73 and B73 × B14A plants, while leaves of unpollinated B14A remain green and similar in appearance to pollinated controls. Furthermore, prevention of ear pollination induces a sizable reduction of dry matter accumulation of all above-ground material and changes in various metabolic parameters. An accumulation of sucrose in the leaves of unpollinated B73 and B73 × B14A plants is correlated with the development of premature senescence. Finally, the genetic analysis supports suggestions that a single dominant gene is responsible for the differences observed, in the visual pattern of leaf senescence, in response to prevention of ear pollination.     

Proline Oxidation in Corn Mitochondria : Involvement of NAD, Relationship to Ornithine Metabolism, and Sidedness on the Inner Membrane

Proline-dependent oxygen uptake in corn mitochondria (Zea mays L. B73 × Mo17 or Mo17 × B73) occurs through a proline dehydrogenase (pH optimum around 7.2) bound to the matrix side of the inner mitochondrial membrane. Sidedness was established by determining the sensitivity of substrate-dependent ferricyanide reduction to antimycin and FCCP (P-trifluoromethoxycarbonylcyanide phenylhydrazone). Proline dehydrogenase activity did not involve nicotinamide adenine dinucleotide reduction, and thus electrons and protons from proline enter the respiratory chain directly. Δ¹-Pyrroline-5-carboxylate (P5C) derived from proline was oxidized by a P5C dehydrogenase (pH optimum approximately 6.4). This enzyme was found to be similar to proline dehydrogenase in that it was bound to the matrix side of the inner membrane and fed electrons and protons directly into the respiratory chain.

Ornithine-dependent oxygen uptake was measurable in corn mitochondria and resulted from an ornithine transaminase coupled with a P5C dehydrogenase. These enzymes existed as a complex bound to the matrix side of the inner membrane. P5C formed by ornithine transaminase was utilized directly by the associated P5C dehydrogenase and was not released into solution. Activity of this dehydrogenase involved the reduction of nicotinamide adenine dinucleotide.

     

High spatial resolution mass spectrometry imaging reveals the genetically programmed,developmental modification of the distribution of thylakoid membrane lipids among individual cells of maize leaf

Metabolism in plants is compartmentalized among different tissues, cells and subcellular organelles. Mass spectrometry imaging (MSI) with matrix‐assisted laser desorption ionization (MALDI) has recently advanced to allow for the visualization of metabolites at single‐cell resolution. Here we applied 5‐ and 10 μm high spatial resolution MALDI‐MSI to the asymmetric Kranz anatomy of Zea mays (maize) leaves to study the differential localization of two major anionic lipids in thylakoid membranes, sulfoquinovosyldiacylglycerols (SQDG) and phosphatidylglycerols (PG). The quantification and localization of SQDG and PG molecular species, among mesophyll (M) and bundle sheath (BS) cells, are compared across the leaf developmental gradient from four maize genotypes (the inbreds B73 and Mo17, and the reciprocal hybrids B73 × Mo17 and Mo17 × B73). SQDG species are uniformly distributed in both photosynthetic cell types, regardless of leaf development or genotype; however, PG shows photosynthetic cell‐specific differential localization depending on the genotype and the fatty acyl chain constituent. Overall, 16:1‐containing PGs primarily contribute to the thylakoid membranes of M cells, whereas BS chloroplasts are mostly composed of 16:0‐containing PGs. Furthermore, PG 32:0 shows genotype‐specific differences in cellular distribution, with preferential localization in BS cells for B73, but more uniform distribution between BS and M cells in Mo17. Maternal inheritance is exhibited within the hybrids, such that the localization of PG 32:0 in B73 × Mo17 is similar to the distribution in the B73 parental inbred, whereas that of Mo17 × B73 resembles the Mo17 parent. This study demonstrates the power of MALDI‐MSI to reveal unprecedented insights on metabolic outcomes in multicellular organisms at single‐cell resolution.     

Multivariate analysis of polypeptide synthesis in field-grown maize inbreds and hybrids

Summary A leaf disc method is described to permit the localized incorporation of ³⁵S-l-methionine into polypeptides synthesized in individual leaves of maize plants grown in the field. The method of incorporation employs minimal external manipulation of the intact leaf, is simple, repeatable, and may be used at any plant age after leaf emergence. Incorporation (cpm/g protein) in 12 leaves per plant was compared among three inbred (Oh43, W23, M14) and three F₁ hybrid (Oh43/M14, W23/M14, Oh43/W23) genotypes. The incorporation was 40% higher (hybrid versus inbred) in 9 of the 12 leaves studied. Samples from leaf 07 (7th leaf numbered from base of plant) for four inbreds (Oh43, M14, B73, Mol 7) and two pairs of reciprocal F₁ hybrids (Oh43/M14, M14/Oh43; B73/Mo17, Mo17/B73) were labelled in situ using the leaf disc method. Each cultivar was sampled at three different ages in each of 1985, 1986, and 1987. High-resolution, two-dimensional isoelectric focusing sodium-dodecyl-sulfate polyacrylamide gel electrophoresis and fluorography were used to display the polypeptides synthesized in the samples. Multivariate methods — Principal Coordinate Analysis, Cluster Analysis, and Standard Deviation Distance — were used to analyze variation and to identify trends in the variation for year, genotype, and age sampled. Our analyses disclose a hierarchy to polypeptide synthesis variation in maize leaves: differences in polypeptide synthesis are greater for year-to-year comparisons than differences due to sample age, which in turn are greater than differences for inbred versus hybrid comparisons.     

Ozone tolerant maize hybrids maintain Rubisco content and activity during long-term exposure in the field

Ozone pollution is a damaging air pollutant that reduces maize yields equivalently to nutrient deficiency, heat, and aridity stress. Therefore, understanding the physiological and biochemical responses of maize to ozone pollution and identifying traits predictive of ozone tolerance is important. In this study, we examined the physiological, biochemical and yield responses of six maize hybrids to elevated ozone in the field using Free Air Ozone Enrichment. Elevated ozone stress reduced photosynthetic capacity, in vivo and in vitro, decreasing Rubisco content, but not activation state. Contrary to our hypotheses, variation in maize hybrid responses to ozone was not associated with stomatal limitation or antioxidant pools in maize. Rather, tolerance to ozone stress in the hybrid B73 × Mo17 was correlated with maintenance of leaf N content. Sensitive lines showed greater ozone-induced senescence and loss of photosynthetic capacity compared to the tolerant line.     

Leaf Nitrate Reductase, d-Ribulose-1,5-bisphosphate Carboxylase, and Root Nodule Development of Genetic Male-Sterile and Fertile Soybean Isolines

The objectives of this study were to determine the effect of pod and seed development on leaf chlorophyll concentration, and on activities of leaf ribulose bisphosphate carboxylase, leaf nitrate reductase, and root nodule acetylene reduction in field-grown soybean (Glycine max [L.] Merr.). Two genetic male-sterile lines and their fertile counterparts (Williams and Clark 63) were compared in both 1978 and 1979. Two additional lines (Wells × Beeson and Wells × Corsoy) were compared in 1979.

The expression of male-sterile character was nearly complete as very little outcrossing due to insect pollinators was observed. Male-sterile plants showed a delayed late season decline in leaf chlorophyll content and ribulose bisphosphate carboxylase activity when compared with fertile plants. A slight delay in the loss of in vivo leaf nitrate reductase activity was also observed for male-sterile plants. Root nodule fresh weight and acetylene reduction activity declined slightly more rapidly for fertile lines than for male-sterile lines in both years with differences significant on the last two to three sampling dates as leaf loss occurred in the control plants.

Seed development was found to increase slightly, the rate of decline of metabolic activity in fertile lines compared with that of male-sterile lines. However, pod development was not an a priori requirement for leaf and root nodule senescence. Male-sterile plants also lost photosynthetic and nitrogen metabolic competence, but at a slower rate. These results support the concept that pod and seed development does not signal monocarpic senescence per se but rather affects the rate at which senescence occurs after flowering.

     

Submitochondrial location and electron transport characteristics of enzymes involved in proline oxidation

Isolated corn mitochondria (Zea mays cv. B73 × Mo17) were fractionated and the fragments were separated on a 20-45% (weight/weight) continuous sucrose gradient. Soluble enzymes remained at the top of the gradient overlapping with the outer membranes, while inner membrane vesicles and intact inner membranes were distributed farther down the gradient. Proline oxidase and Δ¹-pyrroline-5-carboxylic acid dehydrogenase activities were associated only with the inner mitochondrial membrane. Glutamate dehydrogenase was confirmed as a matrix enzyme.     

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.     

IAA-Induced Growth Responses of Decapitated Corn Seedlings: Indications of Two Apparent Adaptations with a Possible Role in Gravitropism

The vertical growth responses of corn seedlings (Zea mays L. Mo17 × B73) were determined over an 8-hour period. When seedlings were decapitated 3 millimeters from the coleoptile's tip and supplied with indole-3-acetic acid (IAA) in 1.5% agar blocks, the response was dependent both on time and IAA concentration. The dose-response curves changed in shape and magnitude depending on the total time of IAA application. High concentrations (>3.2 × 10⁻⁶ molar) initially produced high relative growth rates that decreased back to the intact rate (0.03 millimeter per hour per millimeter) after 3 hours. Low concentrations (<1.0 × 10⁻⁶ molar), or agar blocks without IAA, resulted in a rapid decrease from the intact rate to a level that stabilized at 0.01 millimeter per hour per millimeter until the growth rate began to recover after 3 to 4 hours. Intermediate concentrations produced responses similar to that of the intact organ, though some features of these responses were unique.

The coleoptile curvature in response to gravity depended upon whether the coleoptiles were intact, decapitated, or decapitated and supplied with IAA. Coleoptiles decapitated and not supplied wth IAA showed little or no curvature for 3 hours after decapitation. By this time an adaptation, evoked by the low IAA level, had developed and the coleoptiles began to curve steadily. When 1.0 or 3.2 × 10⁻⁶ molar IAA was supplied, curvature was initiated within the first 30 minutes and reached a maximum rate before decreasing and stopping after 3 to 4 hours. The sequence of events in response to these concentrations was similar to the intact sequence but the curvature rate was reduced to one-third to one-half. A model for the autotropic response involving an auxin concentration-dependent, growth-modulating mechanism capable of two modes of adaptation is described.

     

Species Origin of Genomic Factors in Nicotiana nudicaulis Watson Controlling Hybrid Lethality in Interspecific Hybrids between N. nudicaulis Watson and N. tabacum L

Hybrid lethality is expressed at 28°C in the cross Nicotiana nudicaulis×N. tabacum. The S subgenome of N. tabacum has been identified as controlling this hybrid lethality. To clarify the responsible genomic factor(s) of N. nudicaulis, we crossed N. trigonophylla (paternal progenitor of N. nudicaulis) with N. tabacum, because hybrids between N. sylvestris (maternal progenitor of N. nudicaulis) and N. tabacum are viable when grown in a greenhouse. In the cross N. trigonophylla×N. tabacum, approximately 50% of hybrids were vitrified, 20% were viable, and 20% were nonviable at 28°C. To reveal which subgenome of N. tabacum was responsible for these phenotypes, we crossed N. trigonophylla with two progenitors of N. tabacum, N. sylvestris (SS) and N. tomentosiformis (TT). In the cross N. sylvestris×N. trigonophylla, we confirmed that over half of hybrids of N. sylvestris×N. trigonophylla were vitrified, and none of the hybrids of N. trigonophylla×N. tomentosiformis were. The results imply that the S subgenome, encoding a gene or genes inducing hybrid lethality in the cross between N. nudicaulis and N. tabacum, has one or more genomic factors that induce vitrification. Furthermore, in vitrified hybrids of N. trigonophylla×N. tabacum and N. sylvestris×N. trigonophylla, we found that nuclear fragmentation, which progresses during expression of hybrid lethality, was accompanied by vitrification. This observation suggests that vitrification has a relationship to hybrid lethality. Based on these results, we speculate that when N. nudicaulis was formed approximately 5 million years ago, several causative genomic factors determining phenotypes of hybrid seedlings were inherited from N. trigonophylla. Subsequently, genome downsizing and various recombination-based processes took place. Some of the causative genomic factors were lost and some became genomic factor(s) controlling hybrid lethality in extant N. nudicaulis.     

Concentrations and resorption patterns of 13 nutrients in different plant functional types in the karst region of south-western China

Background and Aims

Elucidating the stoichiometry and resorption patterns of multiple nutrients is an essential requirement for a holistic understanding of plant nutrition and biogeochemical cycling. However, most studies have focused on nitrogen (N) and phosphorus (P), and largely ignored other nutrients. The current study aimed to determine relationships between resorption patterns and leaf nutrient status for 13 nutrient elements in a karst vegetation region.

Methods

Plant and soil samples were collected from four vegetation types in the karst region of south-western China and divided into eight plant functional types. Samples of newly expanded and recently senesced leaves were analysed to determine concentrations of boron (B), calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), molybdenum (Mo), N, sodium (Na), P, sulphur (S) and zinc (Zn).

Key Results

Nutrient concentrations of the karst plants were lower than those normally found in other regions of China and the rest of the world, and plant growth was mainly limited by P. Overall, four nutrients revealed resorption [N (resorption efficiency 34·6 %), P (48·4 %), K (63·2 %) and Mg (13·2 %)], seven nutrients [B (–16·1 %), Ca (–44·0 %), Cu (–14·5 %), Fe (–205·5 %), Mn (–72·5 %), Mo (–35·6 %) and Zn (–184·3 %)] showed accumulation in senesced leaves and two nutrients (Na and S) showed no resorption or accumulation. Resorption efficiencies of K and Mg and accumulation of B, Ca, Fe and Mn differed among plant functional types, and this strongly affected litter quality. Resorption efficiencies of N, P and K and accumulation of Ca and Zn increased with decreasing concentrations of these nutrients in green leaves. The N:P, N:K and N:Mg ratios in green leaves predicted resorption proficiency for N, K and Mg, respectively.

Conclusions

The results emphasize the fact that nutrient resorption patterns strongly depend on element and plant functional type, which provides new insights into plant nutrient use strategies and nutrient cycling in karst ecosystems.     

Plant factors controlling seed set in maize : the influence of silk, pollen, and ear-leaf water status and tassel heat treatment at pollination

In maize (Zea mays L.) large decreases in kernel number result when low water potentials (Ψ_w) and high temperatures occur during pollination. To gain insight into the basis for the decreased seed set, silk, pollen, and ear-leaf Ψ_w, the capability for silk osmotic adjustment, and pollen appearance were measured to determine their relationship to seed set. A multiple-eared or prolific (high carbohydrate availability to the pistillate inflorescence) hybrid (B73 × FR25), a heat sensitive hybrid (WF9 × A632), and a commercial hybrid (B73 × Mo17) were studied. A cross-pollination experiment, with pollination limited by pollen amount, was conducted to determine the impact on seed set of water and heat stressing the tassel and water stressing the ear. At low Ψ_w, silk Ψ_w and seed set were decreased whereas pollen Ψ_w, appearance, and viability were unaffected. High temperature resulted in a 2 megapascal decrease in pollen Ψ_w, visually damaged pollen being shed, decreased pollen viability, and, in two of the hybrids, substantially decreased pollen shed. Prolificacy did not result in increased silk solute accumulation but did result in superior seed production by the pistillate inflorescence at low Ψ_w. The magnitude of the decrease in silk solute potential was small (0.2 megapascal) and similar for all genotypes. One hybrid maintained a relatively high silk turgor but this hybrid also decreased the most in seed production when the pistillate inflorescence was water deficient. These results indicated an adverse effect of high temperature on pollen development, a positive relationship between seed production and silk water status, and no advantage to high silk turgor after silk emergence in maintaining seed production. Additionally, there was no evidence of variation in silk solute regulation capability among hybrids which varied in prolificacy, a trait important in drought tolerance, but the seed production of the pistillate inflorescence of the prolific hybrid was least affected by water deficit.     

Precise mapping of quantitative trait loci for resistance to southern leaf blight, caused by Cochliobolus heterostrophus race O, and flowering time using advanced intercross maize lines

The intermated B73 × Mo17 (IBM) population, an advanced intercross recombinant inbred line population derived from a cross between the maize lines B73 (susceptible) and Mo17 (resistant), was evaluated in four environments for resistance to southern leaf blight (SLB) disease caused by Cochliobolus heterostrophus race O. Two environments were artificially inoculated, while two were not inoculated and consequently had substantially lower disease pressure. Four common SLB resistance quantitative trait loci (QTL) were identified in all environments, two in bin 3.04 and one each in bins 1.10 and 8.02/3. There was no significant correlation between disease resistance and days to anthesis. A direct comparison was made between SLB QTL detected in two populations, independently derived from the same parental cross: the IBM advanced intercross population and a conventional recombinant inbred line population. Several QTL for SLB resistance were detected in both populations, with the IBM providing between 5 and, in one case, 50 times greater mapping resolution.     

Changes in autumn senescence in northern hemisphere deciduous trees: a meta-analysis of autumn phenology studies

Background and Aims Many individual studies have shown that the timing of leaf senescence in boreal and temperate deciduous forests in the northern hemisphere is influenced by rising temperatures, but there is limited consensus on the magnitude, direction and spatial extent of this relationship.Methods A meta-analysis was conducted of published studies from the peer-reviewed literature that reported autumn senescence dates for deciduous trees in the northern hemisphere, encompassing 64 publications with observations ranging from 1931 to 2010.Key Results Among the meteorological measurements examined, October temperatures were the strongest predictors of date of senescence, followed by cooling degree-days, latitude, photoperiod and, lastly, total monthly precipitation, although the strength of the relationships differed between high- and low-latitude sites. Autumn leaf senescence has been significantly more delayed at low (25° to 49°N) than high (50° to 70°N) latitudes across the northern hemisphere, with senescence across high-latitude sites more sensitive to the effects of photoperiod and low-latitude sites more sensitive to the effects of temperature. Delays in leaf senescence over time were stronger in North America compared with Europe and Asia.Conclusions The results indicate that leaf senescence has been delayed over time and in response to temperature, although low-latitude sites show significantly stronger delays in senescence over time than high-latitude sites. While temperature alone may be a reasonable predictor of the date of leaf senescence when examining a broad suite of sites, it is important to consider that temperature-induced changes in senescence at high-latitude sites are likely to be constrained by the influence of photoperiod. Ecosystem-level differences in the mechanisms that control the timing of leaf senescence may affect both plant community interactions and ecosystem carbon storage as global temperatures increase over the next century.