Browsing by Author "Stephens, William"
Now showing 1 - 8 of 8
Results Per Page
Sort Options
Item Open Access Drought resistance of willow short rotation coppice genotypes(Cranfield University, 2004-12) Bonneau, Luc Joseph Gabriel; Stephens, WilliamThis thesis reports on an investigation of drought resistance of willow SRC genotypes. Experiments were conducted at Silsoe, Bedfordshire, in pots and field trials in 2002 and in lysimeters in 2003 to evaluate the range of water use efficiency (WUE) of 50 willows varieties (Salix sp.) and isolate morpho-physiological traits related to WUE and drought resistance. Within the genotype pool tested there was a wide range of responses. The results depict the morpho-physiology of an ideal candidate that plant breeding could produce for drier area of UK, which are summarised below. Its cuttings do not develop calluses when stored in darkness at +4°C. After planting, the candidate does not grow rapidly but has an early exponential phase of stem elongation, after a year of growth it has few stems per stool (< 5). Its long, narrow (Rl/w > 8) hairless leaves are characterised by small adaxial epidermal cells (AECS < 330µm2). The ideal candidate prioritises less biomass to its root system (root/shoot < 0.8) mainly in the top 0.2 m. When grown under optimum condition, the large leaf area has high stomatal conductance and leaf temperature. As water stress progresses, the leaf area decreases leaving little time for leaves to senesce and few yellow leaves remain on the stems. The stomatal conductance decreases slowly and the leaf temperature is almost unaffected. If water stress occurs before August the candidate is able to recover faster the initial physiological state and grow new leaves when re-watered. The results indicate that the best parents to produce such candidate are S. viminalis and S. schwerinii or their related hybrids. Water use (WU) of high yielding willow short rotation coppice hybrids is similar which indicates that the opportunity to reduce WU is limited and that productivity can be only improved by increasing WUE to produce above ground biomass and drought resistance. The current willow breeding programme has great chance to produce hybrids with high WUE however the production of a progeny population from high yielding hybrids that contrast widely in resistance to water stress is recommended. In theory, from such a population, valuable data on morpho-physiological traits related to drought resistance and high WUE can be collected and help genomics to develop quantitative trait loci to the condition that reference hybrids are grown along to quantify the level of water stress experienced by the planting.Item Open Access Effects of soil compaction and mechanical damage at harvest on growth and biomass production of short rotation coppice willow(Springer Science Business Media, 2004-06) Souch, C. A.; Martin, Peter J.; Stephens, William; Spoor, G.The effects of soil compaction and mechanical damage to stools at harvesting on the growth and biomass production of short rotation coppice (SRC) of willow (Salix viminalis L.) were monitored on clay loam (CL) and sandy loam (SL) soils. Moderate compaction, more typical of current harvesting situations did not reduce biomass yields significantly. Even heavy compaction only reduced stem biomass production by about 12% overall; effects were statistically significant only in the first year of the experiment on sandy loam. Heavy compaction increased soil strength and bulk density down to 0.4 m depth and reduced soil available water and root growth locally. Soil loosening treatments designed to alleviate the effects of heavy compaction did not markedly improve the growth of willow on compacted plots. Hence the focus fell on harvesting. Extensive mechanical damage to stools caused a 9% and 21% reduction in stem dry mass on the clay loam and sandy loam soils as a result of fewer stems being produced. The particularly severe effect on the sandy loam soil probably resulted from a combination of dry conditions in the year of treatment, root damage and soil compaction under stools and might have been aggravated by the young age of the plants (1 year) at the time of treatment.Item Open Access An evaluation of the responses of some tea clones to environment in Kenya(1995-02) Ng'etich, Wilson Kipkoech; Stephens, William; Othieno, Caleb O.An experiment set up at four sites in Kericho, Kenya was used to investigate Genotype x Environment interactions of four tea clones of commercial and scientific interest. Small differences in environment resulted in large variations in dry matter production and yield between sites and clones. The largest dry matter was from clone TNI 4/3 (291 ha'1) but it had low yield (2.11 ha'1/ 1). By contrast, the largest yield was from clone S15/10 (3.81 h a 'V 1), which gave the smallest dry matter ( I S th a 1). These contrasting responses in dry matter and yield were analysed in terms of the interception of solar radiation, radiation use efficiency, partition of dry matter and components of yield. The differences in clonal dry matter production were attributed to site to site variability in solar radiation and temperature which affected the rates of ground cover development and the solar radiation interception. The differences in yield were attributed mainly to differences in air temperature and saturation deficits of the air between the sites which affected the shoot replacement cycles through rates of shoot extension and development. Differences in measured and derived environmental variables between the sites were found. Incident solar radiation differed between the sites by up to 2.4M Jm 2 d 1. Differences in mean air and soil temperatures between the extreme sites were 2.5 °C and 3 °C respectfully. A lapse rate of 6.1 “Ckm*1 for mean air temperature was found. Annual rainfall amounts also differed between the sites by up to 700 mm. There were no differences in wind speeds, but there were large differences were in saturation deficits (0.5 kPa) and évapotranspiration (1 mm d*1). During the course of the experiment light interception ranged from less than 5% at planting to more than 80% in April 1994. Radiation use efficiencies for dry matter ranged from 0.30 to 0.45 g MT1 in the clones studied and appeared conservative with respect to temperature across the sites. Clone TNI4/3 had the lowest base temperature for shoot extension (6.1 °C), while clone BB35 had the highest (10.4 °C). The base temperatures for shoot extension were 1.3 to 2.9 °C higher than those for development, hence the differences in shoot lengths between sites. Shoot replacement cycle ranged from 57 days to 110 days between the sites and was the main determinant of yield. Abstract Silsoe College WK Ng’etich PhD. 1995 CranfisM,m The dry matter content of shoots varied between sites and seasons and ranged from 18% to 33%. Mean shoot dry mass was inversely proportional to temperature, ranging from 6 to 12% difference between extreme sites. Loss in yield due to water stress during the dry seasons ranged from 14 to 20% across the sites. Yield losses due to ‘hut sites’ ranged from 10 to 50%, while that from hail damage accounted for up to 20% of annual yields. These results are discussed in the light of other work in Kericho and southern Tanzania. The thesis concludes by considering the implication of the results for the tea industry.Item Open Access The impact of climatic variability over the period 1961- 1990 on the soil water balance of upland soils in the North East Arid Zone of Nigeria(Cranfield University, 1991-12) Hess, Tim M.; Stephens, WilliamOver the period 1961 – 90 the North East Arid Zone of Nigeria experienced a decline in annual rainfall totals and increased aridity which placed increasing pressure on rain fed, millet-based farming systems. The changes in seasonal rainfall total and distribution have been examined and it has been shown that the rate of decline has been consistent across the region. The decline has been dominated by reduction in the number of rain days during the middle of the rainy season and there is no evidence of a significant change in the length of the growing season. Over the same time period, there has been a small, but significant, increase in mean air temperature which has resulted in a small increase in potential evapotranspiration. Other climatic parameters (vapour pressure, solar radiation and wind speed) appear to have remained stable, although the paucity and dubious quality of much of the historical meteorological data make rigorous statistical analysis difficult. A water balance model (BALANCE) developed by the author, was calibrated for a millet crop grown on a typical sandy loam soil in Maiduguri (Nigeria). The model was necessarily parsimonious, but was shown to perform well when calibrated against observed soil water content. However, the empirical nature and high sensitivity of key parameters relating to bare soil evaporation and drainage mean that it is difficult to parameterise the model by laboratory, or independent field measurements. Applying the calibrated model to daily rainfall and average evapotranspiration data from Nguru (Nigeria) for the period 1961 – 93 showed that, with the exception of extreme drought years, the increased aridity would have had little impact on the viability of traditional millet and millet-cowpea intercropping systems prior to the early 1980s. However, after that date, predicted seasonal millet transpiration, and hence predicted yields, have declined, and long duration cowpea intercrops, which were traditionally matured on residual soil moisture after the millet harvest have had insufficient water. Whilst the BALANCE model has been useful in examining the impact of climatic variability on agro-hydrology, it is not a crop physiological model and the interaction between soil water and crop development is poorly represented. The model cannot, therefore be applied with confidence to investigate the potential yield benefits of physical or agronomic interventions to alleviate the impacts of aridity. Although more complex models exist to do this, they require detailed parameterisation of the crop physiology, which was not possible within the scope of this study.Item Open Access Responses of composite tea plants to drought and irrigation in the Southern Highlands of Tanzania.(2002-01) Mizambwa, Firmin Cyprian; Stephens, WilliamThe responses of composite tea plants to drought and irrigation were studied in a line-source experiment in the Southern Highlands of Tanzania. Two scion clones (SI5/10 and K35) were grafted onto each of three rootstock clones (6/8, SFS150 and PC81). The scions were also grafted onto each other (i.e. SI5/10 on K35 and vice versa), and cuttings of ungrafted clones SI5/10 and K35 were prepared at the same times as the controls. In February 1996, the plants were field planted at a spacing of 1.2 x 0.8 m. Six differential irrigation treatments ranging from rainfed conditions (no irrigation) to full irrigation were imposed during the dry seasons, though from planting up to the end of August 1998, the experiment was uniformly irrigated to ensure successful crop establishment. The experiment was irrigated whenever the potential soil water deficit in the fully irrigated plots reached 40 - 50 mm. Between December 1996 and May 1997 shoots above 0.5 m in length were removed or “tipped” to produce a level plucking surface. Routine harvesting started on 16 June 1997, and continued at intervals of 13 to 17 days during the warm season (September to May) and 22 to 29 days in the cool season (June to August). The choice of the scion influenced both the response to water stress and the yield response to drought of the composite plants. Similarly, the soil water availability appeared to modify the effects of rootstock on the yield of the scion. Although not reaching statistical significance, the use of composite plants led to an overall mean annual yield increase of 9% (range between combinations 4 to 21%) in the unirrigated plots, but less than 1% (range -6 to 6%) in the well watered plots when clone S I5/10 was used as a scion. By contrast, using clone K35 as a scion, the overall mean annual yield increase whether irrigated or unirrigated was less than 3% (range -12 to 11%). In the driest plots, rootstocks 6/8 and SFS150 consistently increased the yields of scion clones S I5/10 (by 21%) and K35 (by 11%) respectively. During the dry season, some composite plants survived the drought better than the ungrafted plants. Assessing the effect of rootstocks on the composition of harvested shoots (i.e. shoots of different sizes/stages and the proportion of the total weight that were composed of coarse and broken leaf) revealed that none of the rootstocks had a significant effect on the composition of harvested shoots. Likewise, the rootstocks did not affect the bush morphology as assessed by ground cover development, shoot population density, stem diameter and number of branches. The use of composite plants in tea production by both smallholders and commercial companies is technically feasible, but the benefits to be realised depend on the rootstock/scion combinations used. The extra cost of producing plants by grafting when large-scale tea growers opt to use composites rather than the conventional plants was estimated to be only US $ 100 ha⁻¹. Due to low labour costs, the corresponding cost for the smallholders was US $ 47 ha⁻¹. The payback period when composite plants were used in tea production was the same as that of the conventional plants, but this period was shorter for smallholders than the commercial companies. The payback period was in year 1 (i.e. the first season of harvesting) for the small-scale growers compared with year 2 or 3 (depending on the prices of made tea) for the commercial companies. Assessing the probability of getting a yield increase necessary to make the use of composites worthwhile revealed that for some combinations, there were higher chances of getting such yield increase. For example, the probability of getting a yield increase of 200 kg of made tea ha⁻¹ in one year was 81% when scion clone SI5/10 was grafted onto rootstock 6/8. The corresponding probability was 74% when clone K35 was grafted onto rootstock SFS150. These results and their implications for the tea industry are discussed. The way forward commercially as well as the future research to increase our knowledge for efficient use of composite plants in tea production is proposed.Item Open Access Review of the effects of energy crops on hydrology.(2001-02-15T00:00:00Z) Stephens, William; Hess, Tim M.; Knox, Jerry W.1) MAFF has recently launched a scheme to support the expansion of the area of energy crops grown in England in order to meet the nation’s Kyoto targets for the reduction in CO2 emissions. This has highlighted concerns about the possible effects of willow short rotation coppice (SRC) and Miscanthus on the hydrology of catchments in which they are grown. High levels of water use by these crops could result in less surface runoff, decreased aquifer recharge and hence reduced stream flow (Chapter 1Item Open Access Willow growth in response to nutrients and moisture on a clay landfill cap soil. I. Growth and biomass production(Elsevier, 2005) Martin, Peter J.; Stephens, WilliamThis paper describes studies into the effect of soil factors and water stress on the growth and biomass production of willow (Salix viminalis L.) on a clay landfill cap soil. Individual plants were grown in lysimeters containing Oxford clay and using different soil amendments, bulk densities and watering regimes. Three years from planting, stem biomass production in well-watered plants was least (0.28 kg plant-1) with high bulk density soil (1480 kg m-3) and no nutritional amendment but was increased 10-fold (2.53 kg plant-1) by reducing soil bulk density (1200 kg m-3) and adding thermally dried sewage sludge and fertilisers (N, P and K). This was low, however, compared with production on a sandy loam soil (6.23 kg plant-1) with the same amendments and a similar bulk density. These large treatment differences were also reflected in number of stems plant-1, stem basal area plant-1 and plant leaf area. Plants growing on the Oxford clay had higher root:stem ratios than those on the sandy loam. Water stress reduced stem biomass production by 26 - 37% and caused higher root:stem ratios. Foliar and soil analyses and pot trials indicated that the effects of the soil amendments could be attributed to the addition of N and P which are low in Oxford clay. The research suggests that reasonable biomass production from willow SRC on Oxford clay landfill caps will be dependent on the application of nutritional amendment to the soil at these sites.Item Open Access Willow growth in response to nutrients and moisture on a clay landfill cap soil. II: Water use(Elsevier, 2005) Martin, Peter J.; Stephens, WilliamThis paper describes studies into the effects of soil factors and water stress on water use by willow (Salix viminalis L.) on a clay landfill cap soil and a sandy loam. Individual plants were grown in lysimeters containing these soils under different watering regimes and with different soil amendment treatments. Stemflow and throughfall were measured to determine rainfall entering the lysimeters and evapotranspiration (ET) calculated from a water balance. With plentiful water, seasonal ET increased annually in most treatments, reflecting increases in plant leaf area and dry matter production. For the most vigorous plants, in the sandy loam treatment, it increased from about 360 l plant-1 in the establishment year to almost 1200 l plant-1 in the third year. Seasonal ET was highly correlated with leaf area duration. Nutritional amendment of Oxford clay resulted in plants with larger leaf area and higher dry matter production and seasonal ET than in the unamended treatment. Water stress reduced seasonal ET by up to 41%, as a result of defoliation and stomatal closure. In unstressed plants, in the sandy loam treatment, daily ET rates per unit leaf area reached a maximum of about 1.5 l m-2 d-1 in July. Without nutritional amendment, water use efficiency (WUE) calculated from plant ET and root, stump and stem dry weight, was low for the unamended Oxford clay treatment (1.4 g kg-1) but was similar in the amended clay (5.0 g kg-1) and sandy loam (4.9 g kg-1) treatments. The study has shown the dependence of biomass production by willow on Oxford clay on both nutritional amendment and water availability. Although both nutrients and water could be supplied at these sites by growing SRC within a leachate management system, there are several practical issues like the narrow window of opportunity for mechanised operations and concerns about long-term sustainability which still need to be addressed.