P. Hadley

Growth and yield of winter wheat (Triticum aestivum) crops in response to CO2 and temperature

Crops of winter wheat (Triticum aestivum L. cv. Hereward) were grown within temperature gradient tunnels at a range of temperatures at either c. 350 or 700 μmol mol -1 CO 2 in 1991/92 and 1992/93 at Reading, UK. At terminal spikelet stage, leaf area was 45 % greater at elevated CO 2 in the first year due to more tillers, and was 30 % greater in the second year due to larger leaf areas on the primary tillers. At harvest maturity, total crop biomass was negatively related to mean seasonal temperature within each year and CO 2 treatment, due principally to shorter crop durations at the warmer temperatures. Biomass was 6-31% greater at elevated compared with normal CO 2 and was also affected by a positive interaction between temperature and CO 2 in the first year only. Seed yield per unit area was greater at cooler temperatures and at elevated CO 2 concentrations. A 7-44 % greater seed dry weight at elevated CO 2 in the first year was due to more ears per unit area and heavier grains. In the following year, mean seed dry weight was increased by > 72 % at elevated CO 2 , because grain numbers per ear did not decline with an increase in temperature at elevated CO 2 . Grain numbers were reduced by temperatures > 31°C immediately before anthesis at normal atmospheric CO 2 in 1992/93, and at both CO 2 concentrations in 1991/92. To quantify the impact of future climates of elevated CO 2 concentrations and warmer temperatures on wheat yields, consideration of both interactions between CO 2 and mean seasonal temperature, and possible effects of instantaneous temperatures on yield components at different CO 2 concentrations are required. Nevertheless, the results obtained suggest that the benefits to winter wheat grain yield from CO 2 doubling are offset by an increase in mean seasonal temperature of only 1.0 °C to 1.8 °C in the UK.

Effects of elevated growth temperature and carbon dioxide levels on some physicochemical properties of wheat starch

Abstract Crops of winter wheat (cv. Hereward) were grown in the field under double-skinned polyethylene tunnels in two consecutive seasons (1991–92 and 1992–93). Air containing ambient (350 ppm) or elevated (700 ppm) concentrations of carbon dioxide was circulated through the tunnels, and temperature gradients, typically from 1°C below ambient to 4–7°C above ambient, were maintained within each tunnel. Despite a shorter crop duration and warmer temperatures in the first season, most grain and starch properties showed a similar response to temperature between seasons. Thousand grain weight and grain starch content declined with increase in temperature (from 55±5 mg to 18±2 mg, and from 31±3 mg to 7±2 mg, respectively), the latter reflecting both decreases in granule sizes and fewer amyloplasts per endosperm. Contents of total amylose and lipid-free amylose increased with temperature (from 26±1% to 31±1%, and from 21±1% to 25±1%, respectively), but the contents of lipid-complexed amylose (5·2±1·5%) and lysophospholipids (0·9±0·2%) varied independently of temperature. Starch gelatinisation temperatures ranged from 57·5 to 64·5°C in the first season, and from 58·0 to 61·9°C in the second season, increasing with increase in temperature in both seasons, the data for the two seasons providing almost separate clusters. Gelatinisation enthalpy was constant in the first season (12·6±1 J/g amylopectin) and in the second season (15·5±0·5 J/g amylopectin) with no effect of temperature. The differences in carbon dioxide concentration had no consistent effects on the parameters measured, but small effects were discernible on thousand grain weight, starch content and lipid-free amylose content. There were also effects in certain treatment combinations, specifically at warmer temperatures in the first season and at cooler temperatures in the second season, on thousand grain weight, non-starch solids and lipid-complexed amylose contents.

Low relative humidity triggers RNA-directed de novo DNA methylation and suppression of genes controlling stomatal development

Environmental cues influence the development of stomata on the leaf epidermis, and allow plants to exert plasticity in leaf stomatal abundance in response to the prevailing growing conditions. It is reported that Arabidopsis thaliana ‘Landsberg erecta’ plants grown under low relative humidity have a reduced stomatal index and that two genes in the stomatal development pathway, SPEECHLESS and FAMA, become de novo cytosine methylated and transcriptionally repressed. These environmentally-induced epigenetic responses were abolished in mutants lacking the capacity for de novo DNA methylation, for the maintenance of CG methylation, and in mutants for the production of short-interfering non-coding RNAs (siRNAs) in the RNA-directed DNA methylation pathway. Induction of methylation was quantitatively related to the induction of local siRNAs under low relative humidity. Our results indicate the involvement of both transcriptional and post-transcriptional gene suppression at these loci in response to environmental stress. Thus, in a physiologically important pathway, a targeted epigenetic response to a specific environmental stress is reported and several of its molecular, mechanistic components are described, providing a tractable platform for future epigenetics experiments. Our findings suggest epigenetic regulation of stomatal development that allows for anatomical and phenotypic plasticity, and may help to explain at least some of the plant’s resilience to fluctuating relative humidity.

Yield and partitioning in crops of contrasting cultivars of winter wheat in response to CO2 and temperature in field studies using temperature gradient tunnels

Diverse cultivars of winter wheat ( Triticum aestivum L.) were grown in the field in 1993/94 and 1994/95 at Reading UK in temperature gradient tunnels at normal atmospheric ( c . 370) or elevated CO 2 concentration ( c . 700 μmol CO 2 mol −1 air). In 1993/94, grain yield of cv. Avalon was insensitive to mean temperature (between 8·8 and 10·9°C), while elevated CO 2 increased yield by 1·3 t ha −1 (12·6%). In all other cultivars, warming reduced grain yield and CO 2 increased grain yield. In 1993/94, in cvs Galahad and Mercia the effects of CO 2 and temperature on yield were additive. However, for cv. Hereward in both years and for cv. Soissons in 1994/95, there were negative interactions between the effects of CO 2 and temperature on yield: the maximum benefit of doubling CO 2 to grain yield, 4·5 and 2·7 t ha −1 (65 and 29%) respectively, occurred at cooler temperatures; there was no benefit from doubling CO 2 (i.e. 0%) once the temperature had increased above the seasonal mean by 2·2–2·6°C in cv. Hereward and by 1·3°C in cv. Soissons. The beneficial effect of doubling CO 2 on grain yield in cvs Galahad, Hereward, Mercia and Soissons was negated by an increase in mean seasonal temperature of only 0·7–2·0°C. Warming decreased root dry mass at anthesis in 1994/95 while it increased at elevated CO 2 (49 and 186%, coolest and warmest regime, respectively). Carbon partitioned to roots declined progressively with warming, while at elevated CO 2 there was an average of 56% increase in allocation to roots. The relative impacts of both CO 2 and temperature were greater on root dry mass than on either grain yield or total above-ground biomass, while the effects on grain and biomass yield varied considerably between cultivars, suggesting that the impact of rising CO 2 and temperature are likely to be dependent on cultivar.

Sensitivity of Chickpeas ( Cicer arietinum) to Hot Temperatures during the Reporductive Period

Plants of two genotypes of chickpea ( Cicer arietinum ), classified as early or late-maturing in the field, and relying either on dinitrogen fixation by nodules or on nitrate-N, were grown in various simulated tropical environments in growth cabinets. Plants were transferred between cabinets at various times so that they experienced either warm (30°C) or hot (35°C) days (both in combination with a typical night temperature of 10°C) for different durations of reproductive growth, after growing in average (30°C day - 10°C night) or warmer than average (30° - 18°C) temperatures for the first 28 days from sowing and then average temperatures until transferred into the hot regime. Diurnal vapour pressure deficits were adjusted so that plants experienced a constant atmospheric relative himidity (70%) in all thermal regimes. The greater the proportion of the reproductive period spent in hot days the smaller the seed yields produced; plants transferred at 50% flowering were almost barren. The implications of these data for breeding chickpeas well adapted to hot environments are discussed.

Varying responses of insect herbivores to altered plant chemistry under organic and conventional treatments

The hypothesis that plants supplied with organic fertilizers are better defended against insect herbivores than those supplied with synthetic fertilizers was tested over two field seasons. Organic and synthetic fertilizer treatments at two nitrogen concentrations were supplied to Brassica plants, and their effects on the abundance of herbivore species and plant chemistry were assessed. The organic treatments also differed in fertilizer type: a green manure was used for the low-nitrogen treatment, while the high-nitrogen treatment contained green and animal manures. Two aphid species showed different responses to fertilizers: the Brassica specialist Brevicoryne brassicae was more abundant on organically fertilized plants, while the generalist Myzus persicae had higher populations on synthetically fertilized plants. The diamondback moth Plutella xylostella (a crucifer specialist) was more abundant on synthetically fertilized plants and preferred to oviposit on these plants. Glucosinolate concentrations were up to three times greater on plants grown in the organic treatments, while foliar nitrogen was maximized on plants under the higher of the synthetic fertilizer treatments. The varying response of herbivore species to these strong differences in plant chemistry demonstrates that hypotheses on defence in organically grown crops have over-simplified the response of phytophagous insects.

A comparison of organic and inorganic nitrogen fertilizers: Their nitrate-N and ammonium-N release characteristics and effects on the growth response of lettuce (Lactuca sativa L. cv. Fortune)

The response of pot grown lettuce to inorganic (ammonium nitrate) and organic (dried blood and Protox) N fertilizers was determined at two temperature regimes (15°C day/10°C night and 20°C day/15°C night) and related to the NH4−N and NO3−N release characteristics of each material. The N release characteristics of the organic materials matched the N requirements of lettuce more closely than the inorganic fertilizer. However, was rapidly released from the protein based materials such that composts were depleted of available fertilizer N at the same time irrespective of the form supplied. The warmer temperature regimes resulted in a more rapid depletion of the fertilizers due to biological immobilization such that N recoveries in shoots, roots and leachates were reduced. Approximately 20% of the N present in Protox (a material derived from activated sewage sludge, processed to reduce the heavy metal content to minimal levels) appeared to be resistant to microbial degradation and was unavailable to the plants. Therefore, the growth response of lettuce was slightly reduced with Protox compared to the other materials at similar rates of incorporation. The organic materials did not contribute NO3−N to the plant and small NO3−N concentrations in petioles were derived from the water used for irrigation. However, NO3−N levels in plants receiving inorganic ammonium nitrate were initially high but progressively declined as the fertilizer NO3−N became depleted.

A model of the effects of temperature on the growth and development of cauliflower (Brassica oleracea L. botrytis)

Abstract A model is described which predicts the time to curd initiation, and the duration or rate of curd growth of cauliflowers. The model was calibrated with data collected from commercial crops grown during the 1989, 1990 and 1991 seasons. The reciprocal of the time from planting to curd initiation, for cultivar ‘Revito’, was linearly related to effective temperature determined with an optimum of 14°C. Curd initiation occurred after the accumulation of 296 degree-days above a base temperature of 2.8°C. Curd growth of three cultivars, ‘Jubro’, ‘Revito’ and ‘White Fox’ was described by a simple model which incorporated two terms; one describing a linear ontogenetic decline in potential relative curd growth rate (RGR) with thermal time and the second which described the instantaneous effects of temperature on RGR. The optimum temperatures for curd growth of cultivars ‘Jubro’, ‘Revito’ and ‘White Fox’ were 16°C, 25°C and 21°C, respectively. The model accounted for 37.4%, 40.4% and 95.8% of the variance in curd growth of the varieties ‘Jubro’, ‘Revito’ and ‘White Fox’, respectively. The curd initiation and curd growth components of the model accurately described several independent data sets. The model can be used for both crop scheduling and for predictions of crop maturity.

Effects of supplemental nitrate and thermal regime on the nitrogen nutrition of chickpea (Cicer arietinum L.): II. Symbiotic development and nitrogen assimilation

SummaryNodulated chickpea plants were grown in pots in a glasshouse programmed to simulate either hot (32.5°C day/18°C night) or warm (25°C/18°C) thermal regimes characteristic of those experienced by crops grown in different seasons or locations in the semi-arid tropics. The plants were irrigated with nutrient solution either devoid of inorganic nitrogen or containing 0.71, 1.43 or 2.86 mM nitrate. Hot days delayed nodulation, decreased specific dinitrogen fixation (acetylene reduction) activity and hastened nodule and host plant senescence compared with these symbiotic attributes in warm days. The effects of nitrate on acetylene reduction activities plant−1 were mediated primarily through changes in the respective rates of nodule establishment, growth and senescence. Nitrate at 0.71 or 1.43 mM stimulated early nodulation and nodule growth but at 1.43 and 2.86 mM it hastened nodule senescence compared with plants totally dependent on symbiotic dinitrogen fixation, particularly in the hot regime. Larger concentrations of nitrate decreased not only symbiotic N2 assimilation during seed filling but also the total N assimilated during the same period. Plants given 2.86 mM nitrate, in either the warm or hot regime, assimilated substantially larger amounts of total N than those dependent on nodules, and then mostlybefore the seed filling period. However, nodule-dependent plants assimilated about 50% of their total Nduring the seed filling stage and the partitioning of N to fruits, stems and roots in mature plants was similar irrespective of thermal regime or nitrogen nutrition. The implications of these data for the nitrogen nutrition of chickpea crops are discussed.

Transgenerational, dynamic methylation of stomata genes in response to low relative humidity

Transgenerational inheritance of abiotic stress-induced epigenetic modifications in plants has potential adaptive significance and might condition the offspring to improve the response to the same stress, but this is at least partly dependent on the potency, penetrance and persistence of the transmitted epigenetic marks. We examined transgenerational inheritance of low Relative Humidity-induced DNA methylation for two gene loci in the stomatal developmental pathway in Arabidopsis thaliana and the abundance of associated short-interfering RNAs (siRNAs). Heritability of low humidity-induced methylation was more predictable and penetrative at one locus (SPEECHLESS, entropy ≤ 0.02; χ2 < 0.001) than the other (FAMA, entropy ≤ 0.17; χ2 ns). Methylation at SPEECHLESS correlated positively with the continued presence of local siRNAs (r2 = 0.87; p = 0.013) which, however, could be disrupted globally in the progeny under repeated stress. Transgenerational methylation and a parental low humidity-induced stomatal phenotype were heritable, but this was reversed in the progeny under repeated treatment in a previously unsuspected manner.