Early Leaf Senescence Research Articles

Isolation and characterization of an Arabidopsis mutant, fireworks (fiw), which exhibits premature cessation of inflorescence growth and early leaf senescence.

To examine the mechanism underlying the reproductive development in monocarpic plants, we screened for mutants that exhibit premature cessation of inflorescence growth in Arabidopsis. We identified a novel mutant line that exhibited earlier cessation of flower formation and inflorescence stem elongation. This mutant also exhibited accelerated rosette leaf senescence after the cessation of the inflorescence growth. We designated the mutant fireworks (fiw) because flowers and siliques were clustered at the top of the fiw inflorescence. The fiw mutation was a single, recessive mutation and mapped on the lower part of chromosome 4. The fiw phenotype was not observable during vegetative growth, but the inflorescence growth was arrested more than 7 d earlier than the wild type (WT). Microscopic observation revealed that the fiw apical meristem was structurally preserved. The premature arrest of growth was observed not only in the primary inflorescence but also in the lateral inflorescence, which is consistent with the global proliferative arrest observed later in WT. Regardless of such dramatic phenotypic features, the fiw plants bore normal flowers and set fully matured siliques.

Salinity increases CO2 assimilation but reduces growth in field-grown, irrigated rice.

Salinity is a major yield-reducing factor in coastal and arid, irrigated rice production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf area (SLA), sodium concentration and leaf CO2 exchange rates (CER) at photosynthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Senegal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 under irrigation with fresh or saline water (flood water electrical conductivity = 3.5 mS cm-1). Relative leaf chlorophyll content (SPAD method) and root, stem, leaf blade and panicle dry weight were measured at weekly intervals throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (topmost) and second leaf. Salinity reduced yields to nearly zero and dry-matter accumulation by 90% for the susceptible cultivar in the HDS, but increased leaf chlorophyll content and CER at PAR- saturation. The increase in CER, which was also observed in the other cultivars and seasons, was explained by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf senescence and chlorophyll degradation. The growth reductions were attributed to loss of assimilates (mechanisms unknown) that must have occurred after export from the sites of assimilation. The apparent, recurrent losses of assimilates, which were between 8% and 49% according to simulation with the crop model for potential yields in irrigated rice, ORYZA S, might be partly due to root decomposition and exudation. Possibly more importantly, energy-consuming processes, such as osmoregulation, interception of sodium and potassium from the transpiration stream in leaf sheaths and their subsequent storage, drained the assimilate supply.

Effect of snow removal on leaf water potential, soil moisture, leaf and soil nutrient status and leaf peroxidase activity of sugar maple

Effect of removal of snow cover in winter was investigated in an 80-year-old sugar maple (Acer saccharum Marsh.) stand in southern Quebec. We hypothesized that winter soil frost would induce some of the decline symptoms observed in sugar maple stands in southern Quebec in the early 1980's. Snow was continuously removed from around trees for a one week (partial removal) or for a four-month period (complete removal) during the 1990–1991 winter. Foliage and soils were sampled periodically during the summer of 1991. The complete snow removal treated trees showed decreased leaf water potential and increased peroxidase activity over most of the growing season. Foliar Ca was reduced in both snow removal treatments early in the growing season while foliar N was reduced in the complete snow removal trees late in the growing season. Soil NO 3 − and K+ were elevated in both snow removal treatments at various times throughout the growing season. Prolonged soil frost in a sugar maple stand can induce lower leaf water potential, higher leaf peroxidase activity and early leaf senescence during the following growing season. Soil frost may have reduced nutrient uptake without affecting significantly the leaf nutrient status.

Interactions of Crop Level and Late Season Water Stress on Growth and Physiology of Field-Grown Concord Grapevines

The effects and interactions of late season soil water deficit and crop level on the growth, yield, fruit quality, and physiology of four-year-old Concord own-rooted grapevines in a loam soil in New York were examined. Soil moisture stress was induced in half of the vines by rain-shielding beginning after bloom, while natural and supplemental drip irrigation were used to maintain high water status of the other half. Within each stress plot crop levels were adjusted to give unthinned vines with a mean of 130 clusters/kg pruning weight versus 67 clusters/kg pruning weight giving about 18 and 9 t/ha, respectively. Thinning of the crop significantly decreased cluster number and yield, but increased shoot growth, juice °Brix, and pruning weights as expected. Since significant soil water stress did not developed until about 10 to 20 days after veraison, the stress only significantly reduced net photosynthesis (Pn) and leaf conductance (g_l) after veraison at leaf and pre-dawn soil water potential below -1.1 and -0.25 MPa. A significant water stress x crop level interaction occurred at harvest for juice °Brix and total soluble solids (TSS) per berry, indicating that reduction of sugar accumulation due to water stress was aggravated in heavier cropping vines. No significant effects of crop level on leaf gas exchange or vine water use (<i>i.e.</i>, soil water reduction in shielded plots) were observed. The reduction of leaf function and earlier leaf senescence in the unthinned stressed vines were not severe enough to cause carry-over effects the following season.

Chilling, Ethephon, and Photoperiod Affect Cormel Production of Brodiaea

Stratification by chilling is the primary factor controlling germination of brodiaea (Triteleia laxa Benth.) seeds. A minimum of 8 weeks of low temperature (3C) was required for maximum germination. Soaking seeds in a solution of 1000 mg ethepho/liter for 20 h reduced the number of weeks at low temperature required for germination. Cormel size was affected by the photoperiod during seedling growth. Long days induced early leaf senescence, and cormels harvested from seedlings grown under long days weighed only half as much as those grown under short days. Chemical name used: 2-chloroethyl phosphoric acid (ethephon).

Correlation between arrested secondary plasmodesmal development and onset of accelerated leaf senescence in yeast acid invertase transgenic tobacco plants

Mature leaves of a transgenic tobacco plant (Nicotiana tabacum L. var. Samsun, line A41-10) that constitutively express a yeast-derived acid invertase gene develop symptoms which are characterized by the presence of greenish-yellow and green sectors in the same leaf, and onset of early leaf senescence. Previous studies indicated that invertase activity was two- to threefold higher in the greenish-yellow sectors than in the green sectors. Our structural analyses revealed that development of secondary plasmodesmata, via modification of existing primary plasmodesmata, between mesophyll cells was inhibited severely in the greenish-yellow sectors, but only marginally in the green sectors. In contrast, the structure and function of primary plasmodesmata in the same symptomatic sectors remained unaltered as determined by structural and dye coupling studies. It is hypothesized that secondary plasmodesmata differ from primary plasmodesmata in having special abilities to traffic information molecules to coordinate leaf development and physiological function(s). Arrest of secondary plasmodesmal development by high invertase activity in the transgenic tobacco leaf may have prevented this type of trafficking and hence resulted in early leaf senescence. The results also indicate that the yeast acid invertase-expressing tobacco may provide an effective experimental system for the molecular characterization of cellular mechanisms that regulate the development, function, and possible turnover of secondary plasmodesmata.

Mortality of Microrhopala vittata (Coleoptera: Chrysomelidae) in Outbreak and Nonoutbreak Sites

Population density of Microrhopala vittata (F.) (Coleoptera: Chrysomelidae), a leaf-miner of goldenrod ( Solidago ), is low in most old fields, but populations occasionally reach densities at which they severely damage their host plant. A survey of 21 old fields revealed that the beetle was present at all sites. To determine whether development was retarded or mortality was elevated in sites with fewer beetles, growth and mortality were experimentally assessed in three low-density sites and three high-density sites. A major source of mortality was early leaf senescence in sites with taller plants, and in these sites larvae grew more slowly. This form of mortality was not present in all three low-density sites, however, and thus was not sufficient to explain differences in beetle density. Only eulophid parasitism of larvae was more prevalent in the low-density sites. Unexplained mortality of first instars was greater in high-density sites. However, overall egg-to-pupa mortality was not correlated with initial beetle density. The combined effect of several mortality agents may be responsible for maintaining M. vittata populations at low densities most of the time. A release from these agents that would allow populations to irrupt may be short-lived and thus difficult to detect.

Factors influencing the transplantation success of micropropagated narcissus bulbils

SummaryThe transplantation phase of narcissus micropropagation was studied using bulbils from bulbing shoots sub-cultured to hormone-free bulbing medium. Transplantation success was enhanced by the incorporation of activated charcoal in the medium, culturing the shoots at a constant 20°C and subjecting bulbils to a period of cold storage at 5°C to overcome dormancy prior to planting out. These measures gave overall figures for transplantation success in a large-scale evaluation experiment of 47.8-81.2% depending on cultivar. Greatest success was achieved using bulbils weighing more than 0.2 g. However, the overall proportion of such bulbils was only 0.19 to 0.20. Early leaf senescence, which limits bulbil growth in culture, appeared to be linked with initial bulbing shoot weight. The beneficial effects on transplantation of using full-strength mineral salts over halfstrength salts and of using one particular clone of cv. St. Keverne, were probably mediated via effects on final bulbil weight. Elevated concentrations of sucrose in the bulbing medium gave no benefit even though final bulbil weight was increased. High sucrose levels gave earlier shoot senescence and increased dormancy. Culture of bulbils in darkness did not effect final bulbil weight but was extremely detrimental to transplantation success. Most unsuccessfully transplanted bulbils showed a foliar growth phase suggesting a premature re-entry into dormancy. With few exceptions, bulbils judged to have been successfully transplanted grew on without problem in their second year.

Water relations of two almond cultivars under anomalous rainfall in non-irrigated culture

SummaryPlants of two cultivars of almond (Amygdalus communis L.), Ramillete and Garrigues, were grown in the field under non-irrigated culture. Concurrent measurements of leaf water potential (a|>,), leaf conductance (g,), and environmental conditions (vapour pressure deficit (VPD), and photosynthetically active radiation (PAR)) were made throughout one growing season. Leaf water potential of ‘Garrigues’ showed a closer dependence on VPD than did ‘Ramillete’. Daily dynamics of op, and g, differed in the two cultivars, oj), being lower in ‘Garrigues’ than ‘Ramillete’ during spring and summer. Earlier leaf senescence in ‘Ramillete’ induced larger leaf water deficits in late season (October and November) in this cultivar. The g, values were lower for ‘Ramillete’ than in ‘Garrigues’. The relationships between leaf water potential and leaf conductance revealed a larger hysteresis in ‘Ramillete’ during summer. Frequent high rainfall before May induced in ‘Ramillete’ a lower leaf conductance, in the May sample. The traditional use of the two cultivars together in almond plantations is questioned.

Radiation utilization efficiency and the growth of soybeans exposed to ozone: a comparative analysis

Effects of ozone (O 3) on field-planted soybeans ( Glycine max (L.) Merr. ‘Davis’) were examined using a simple crop-growth model that resolves the rate of increase in crop dry weight into the total amount of radiation intercepted by a crop canopy and the efficiency of conversion of this energy into dry matter (ϵ, termed utilization efficiency). The objectives were : (1) to examine the effects of O 3 on light interception and ϵ, and compare these results with a similar study by Unsworth et al. to determine whether the effects of O 3 are consistent from year to year, and (2) to determine whether ϵ is more sensitive to O 3 during reproductive than vegetative growth and whether allocation of aboveground biomass to seed is affected by O 3. Five O 3 concentrations (seasonal means of 30, 52, 57, 76 and 99 ppb) were maintained in open-top chambers over the growing season. All treatments were irrigated. Leaf, stem and pod weights and leaf area were measured weekly throughout the season. Total incident solar radiation and total radiation transmission through the canopy were measured with tube solarimeters and electronic integrating devices. Increased O 3 concentrations were accompanied by depressed leaf area expansion and earlier leaf senescence, but did not affect total light interception over the entire season. Utilization efficiency (ϵ) was reduced in high O 3 treatments and was the most important factor in the reduction of yield; ϵ for the whole season declined from 0.89 g MJ −1 in the 30-ppb treatment to a low of 0.68 g MJ −1 in the 99-ppb treatment. Efficiencies were higher during reproductive than vegetative growth, but were not more sensitive to O 3. Seed yield decreased from 450 g m −2 in the 30-ppb treatment to 320 g m −2 in the 99-ppb treatment. Fractional allocation of aboveground dry weight to seed was not affected by O 3. Reduced ϵ values with increasing O 3 concentrations were consistent with an earlier study by Unsworth et al. however, ϵ values for the vegetative phase of growth in this study were lower at all O 3 concentrations. Lower ϵ values were attributed to canopy light saturation because of higher incident radiation and water stress caused by a shallow rooting zone during a hot, dry growing season.

The effects of sodium sulphate and sodium chloride on growth, morphology, photosynthesis, and water use efficiency of Chenopodium rubrum

The effects of sodium sulphate and sodium chloride salinity on the anatomy, water relations, and photosynthesis of Chenopodium rubrum L. were compared. Low concentrations of either salt stimulated growth, but higher concentrations resulted in large decreases in dry weight and leaf area. Leaf succulence and the number of layers of palisade cells were increased, but these effects were more pronounced with NaCl than with Na2SO4. Stomatal density was reduced at low to moderate salinities, but then increased again at high salinity. Stomatal size was reduced at all salinities. Increasing salinity had no great effect on photosynthetic rates except with older plants grown at the highest level of Na2SO4. Stomatal conductance decreased at all salinities. This reduced transpiration and led to increased intrinsic water use efficiency. Total tissue stable carbon isotope ratios also indicated that water use efficiency was improved. Chenopodium rubrum adjusted osmotically by accumulating electrolytes from the nutrient solution and by synthesizing glycinebetaine. Plants in NaCl limited osmotic adjustment more than those growing in Na2SO4. Despite this, Na2SO4 was more damaging than NaCl and caused earlier leaf senescence at high concentrations.

Carbon dioxide exchange rates in soybeans during podfilling as a function of potassium supply

Photosynthetic and dark respiration rates of single leaflets and whole plant canopies were measured during podfilling of soybean plants that were grown under low and high K regimes. Dark respiration rates of detached seed from these plants were also determined during the latter part of seed development. The study was carried out to test the hypothesis that low oil/protein ratios of seed from K-deficient plants resulted from the reduction in carbon availability within the plant, as a result of lower carbon assimilation rates and higher rates of respiratory carbon loss. Photosynthetic rates of upper canopy leaflets during early podfilling were depressed under K deficiency but this effect did not occur with whole plant canopies. In fact, towards the latter part of the podfilling period canopy photosynthetic rates were higher in K-deficient plants as nitrogen was exported earlier from the leaves in high-K plants, resulting in earlier leaf senescence in these plants. The level of K supply had no consistent effect on dark respiration rates of single leaflets or plant canopies, and had no effect on CO2 evolution rates from detached, developing seed. The findings do not substantiate the hypothesis that reduced photosynthetic efficiency or enhanced respiratory carbon loss are responsible for lower oil/protein ratios in seed from K-deficient soybean plants.

Temperature and irradiance effects on yield in Cowpea ( Vigna unguiculata)

Cowpea ( Vigna unguiculata (L.) seed yield is very sensitive to environment despite the reputation the species has for hardiness. The responses of the variety TVu4552 to differing temperatures after flowering and to irradiance were examined in controlled environment glasshouses. Seasonal variation in irradiance was positively correlated with pod number, while seed number per pod and mean seed weight remained constant. Carbon dioxide assimilation rate (ifA) of recently expanded leaves of plants grown at 21 16 °C day/night temperature regime was half that of plants grown at 27 22 ° C and 33 28 ° C , although leaves of plants grown at these warmer regimes had instantaneous A's that were unchanged in the range 20–28°C. Grain yield was influenced by the temperature after flowering in two ways: with high temperature there was a decrease in pod number; with temperatures below 21°C there was a decrease in seed number and size. Associated with decrease in pod number at high temperatures there was earlier leaf senescence. Although early senescence would limit assimilate supply and pod development at high temperatures, at a constant intermediate temperature ( 27 22 ° C ) there was no seasonal variation in the timing of senescence despite a doubling of radiation from winter to summer. The failure to set seed at low temperatures was a response localized within the flower and the developing pod. Histochemical staining of isolated pollen and of the stigma indicated that at low temperature (18°C) pollen was viable and that pollination occurred. Early embryo development was apparently restricted at low temperature and this response was evident over a period of up to 5 days after pollination.

Effects of pre- and post-bloom temperature regimes on development of Lilium longiflorum Thunb.

Glasshouse grown ‘Ace’ and ‘Nellie White’ Easter lily plants were subjected to different temperature regimes to determine temperature requirements during pre- and post-bloom development. Rate of leaf- and flower-bud development and stem elongation on the primary (mother) axis were directly proportional to the range of temperatures used (6–24°C), and were equally effective in predicting crop development. Scale initiation on the secondary (daughter) axis during pre-bloom phases was proportional to growing temperature, reaching maximum activity at 18°C in ‘Ace’ and at 12°C in ‘Nellie White’. The shift from scale to leaf initiation and development following anthesis was favored by 12 rather than 18°C with significant reductions in leaf initiation in both cultivars at 24°C. No difference in secondary meristem diameter occurred with temperature during pre-bloom, but large dome size was associated with 12°C or lower during the post-bloom phase. Primary scale weight increase (filling), reached a maximum 50 days following anthesis, and was greatest at 18°C. Secondary scale filling reached a maximum 80 days after anthesis at both 18 and 24°C. The secondary axis became increasingly responsive to sprout-inducing temperatures with increasing age and development. Fifty days after anthesis, 12 and 18°C were equally effective in sprouting ‘Ace’ bulbs, while 12°C was more effective with ‘Nellie White’. Early leaf senescence, associated with high (24°C) temperature, did not favor increased bulb size, daughter leaf primordia count and meristem diameter, or sprouting.

Increasing productivity. Crop physiology and nutrition

In the context of physiology and nutrition, many of the treatments which affect crop yields do so by influencing either the total photosynthesis per unit area of land or the partition of assimilates within the plant or both. Examples are given to illustrate the inter-relationships of nutrition, crop physiology, leaf growth and yields in cereals, grasses, potatoes and sugar beet which represent four very different models of crop growth. In each case limitations to yield are discussed but the main emphasis is on those points where there seems to be promise for future practical application to give improved yields, for example by changing models, the use of growth regulators, the time of supply and quantity of plant nutrients especially nitrogen and the timing of husbandry operations which these changes will permit and demand. With cereals the main aim should be to extend the interval between anthesis and time of ripening, with potatoes to break the apparent linkage between early tuber initiation and early leaf senescence to give a longer period of tuber bulking, with sugar beet to advance leaf growth earlier in the season and to control the partition of assimilates between leaves and storage roots and with grassland to replace inferior species with better ones.