Experimental plots were overlaid on commercial fields of broccoli (Brassica oleraceae L. var. Italica) and cauliflower (Brassica oleraceae L. var. botrytis L.) to evaluate legume species as interseedings in vegetable production. White clover (Trifolium repens L.), red clover (T. pratense L.), yellow sweet clover (Melilotus officinalis L.), hairy vetch (Vicia villosa Roth.) and crimson clover (T. incarnatum L.) were seeded 4-5 weeks after an early planting of broccoli and a late planting of cauliflower. Crop yields, forage biomass, weed biomass and percent fall ground cover were recorded. / A dry year coupled with difficulties in applying the treatments resulted in low forage biomass. Hairy vetch yielded the most within the early broccoli planting system. High rainfall the next year resulted in high biomass yields. Over the two year study, hairy vetch and crimson clover emerged as the two most productive species. Significant effects on fall weed biomass were observed with broccoli in the second year of the study, where interseeded plots reduced weed populations by at least 66%. No evidence was seen of weed suppression by interseedings prior to harvest. Crop yields were not affected by interseedings. All interseeded treatments provided the minimum of 30% ground cover required to help reduce erosion.

N-transfer from legume to grass when the two were intercropped in the field and the mechanisms of this transfer were studied. Studies involving either alfalfa-grasses or soybean-corn intercrops, were undertaken. Mycorrhizal and P-supplemented (to compensate for the lack of mycorrhizae) intercrops were compared. In these studies, the legume component of intercrops was labelled with $ sp{15}$N and any excess of the label was looked for in the associated grass plants. / There was no reversal of N transport at the legume-fungus interface. N-transfer from legume to grass must therefore, proceed via excretion of N by legume roots and subsequent uptake by grass roots. Soil microorganisms and proximity of plant root systems are important factors affecting N-transfer. Mycorrhizae enhance the transfer by increasing the absorptive efficiency of the receiver plants. / Mycorrhizal fungi enhance the competitiveness of the most mycotrophic component of the mixtures by either improving P uptake or the general nutrient balance of the plant. Mycorrhizal inoculation can decrease the level of P competition between corn and soybean by increasing the availability of P. / The observation that mycorrhizal plants differ in many regards from P-supplemented plants, emphasize the generally poor comprehension of the mycorrhizal symbiosis.

The effect of cover crops on N and weed dynamics was assessed within a pea (Pisum sativum L.) – cover crop – sweet corn (Zea mays L.) rotation. Cover crops of oat (Avena sativa L.), perennial rye (rye) (Secale cereale L.), oilseed radish (OSR) (Raphanus sativus L. var. oleoferus Metzg Stokes), and OSR plus perennial rye (OSR+rye) increased plant available N (PAN) over the cover crop growing season compared to the no cover control at the Bothwell site only. However, at neither site did cover crops result in increased PAN for the sweet corn, indicating that these cover crops will not reduce required N fertilizer applications. Also, cover crops posed neither an increased or decreased need for weed management during sweet corn production. However, OSR may be useful in pesticide reduced programs due to its potential ability to reduce fall herbicide applications, provided it does not set viable seed.

No description available.

Salinity problems seriously affect agricultural production by reducing crop yield and arable land. The evaluation of potato genotypes (Solanum spp.) for their salinity (NaCl) tolerance in conventional field trials is time consuming and labour intensive. The results are often confounded by many field and environmental variations. In vitro bioassays can overcome some of these difficulties by providing faster, more convenient and dependable methods for screening and selection of salt tolerant potato genotypes. The objective of this research was to develop in vitro bioassay methods for screening and selection of salt tolerant potato. Under in vitro NaCl stress conditions, seed germination, early seedling growth, and single-node cutting bioassays were used to evaluate salinity tolerance. The selected genotypes were further tested with three in vitro bioassays (single-node cuttings, root tip segments, and microtuberization). The rankings of potato cultivar salinity tolerance were similar in these bioassays. The single-node cutting bioassay was recommended because it was simpler to perform than the root tip segment and microtuberization bioassays and did not exclude certain genotypes as did the microtuberization bioassay. The in vitro bioassay rankings were compared with yield ranking in field lysimeters. In both the in vitro and in vivo saline stress experiments, cvs. Kennebec and Russet Burbank were more salt tolerant than Norland. The tubers and microtubers harvested from previous experiments were tested in the greenhouse to investigate salinity carry-over effect for seed tuber production. There was no apparent residual carry-over effect found. Microtuber yield increase in the presence of low NaCl concentration was induced primarily by specific ion (Na+), and not osmotic effects. This research clearly indicated that in vitro bioassays are relatively simple, rapid, convenient, repeatable, and agree with the field lysimeter results. They can be used to substitute for f

Limited water supplies are available to satisfy the increasing demands of crop production. It is therefore very important to conserve the water, which comes as rainfall, and water, which is used in irrigation. A proper irrigation water management system requires accurate, simple, automated, non-destructive method to schedule irrigations. Utilization of infrared thermometry to assess plant water stress provides a rapid, nondestructive, reliable estimate of plant water status which would be amenable to larger scale applications and would over-reach some of the sampling problems associated with point measurements. Several indices have been developed to time irrigation. The most useful is the crop water stress index (CWSI), which normalizes canopy to aIr temperature differential measurements, to atmospheric water vapour pressure deficit. A field experiment was conducted at Cedara, KwaZulu-Natal, South Africa, to determine the non-water-stressed baselines, and CWSI of cereal rye (Secale cereale L.) from 22 July to 26 September 2002, and aImual (Italian) ryegrass (Lolium multiflorum Lam.) from October 8 to December 4, 2002, when the crops completely covered the soil. An accurate measurement of canopy to air temperature differential is crucial for the determination of CWSI using the empirical (Idso et al., 1981) and theoretical (Jackson et al., 1981) methods. Calibrations of infrared thermometers, a Vaisala CS500 air temperature and relative humidity sensor and thermocouples were performed, and the reliability of the measured weather data were analysed. The Everest and Apogee infrared thermometers require correction for temperatures less than 15 QC and greater than 35 QC. Although the calibration relationships were highly linearly significant the slopes and intercepts should be corrected for greater accuracy. Since the slopes of the thermocouples and Vaisala CS500 air temperature sensor were statistically different from 1, multipliers were used to correct the readings. The relative humidity sensor needs to be calibrated for RH values less than 25 % and greater than 75 %. The integrity of weather data showed that solar irradiance, net irradiance, wind speed and vapour pressure deficit were measured accurately. Calculated soil heat flux was underestimated and the calculated surface temperature was underestimated for most of the experimental period compared to measured canopy temperature. The CWSI was determined using the empirical and theoretical methods. An investigation was made to determine if the CWSI could be used to schedule irrigation in cereal rye and annual rye grass to prevent water stress. Both the empirical and theoretical methods require an estimate or measurement of the canopy to air temperature differential, the non-waterstressed baseline, and the non-transpiring canopy to air temperature differential. The upper (stressed) and lower (non- stressed) baselines were calculated to quantify and monitor crop water stress for cereal rye and annual ryegrass. The non-water-stressed baselines were described by the linear equations Te - Ta = 2.0404 - 2.0424 * VPD for cereal rye and Te - Ta = 2.7377 - 1.2524 * VP D for annual ryegrass. The theoretical CWSI was greater than the empirical CWSI for most of the experimental days for both cereal rye and annual ryegrass. Variability of empirical (CWSI)E and theoretical (CWSI)T values followed soil water content as would be expected. The CWSI values responded predictably to rainfall and irrigation. CWSI values of 0.24 for cereal rye and 0.29 for annual ryegrass were found from this study, which can be used for timing irrigations to alleviate water stress and avoid excess irrigation water. The non-water-stressed baseline can also be used alone if the aim of the irrigator is to obtain maximum yields. However the non-water-stressed baseline determined using the empirical method cannot be applied to another location and is only valid for clear sky conditions. And the non-water-stressed baseline determined using theoretical method requires computation of aerodynamic resistance and canopy resistances, as the knowledge of canopy resistance, however the values it can assume throughout the day is still scarce. The baseline was then determined using a new method by Alves and Pereira (2000), which overcomes these problems. This method evaluated the infrared surface temperature as a wet bulb temperature for cereal rye and annual ryegrass. From this study, it is concluded that the infrared surface temperature of fully irrigated cereal rye and annual ryegrass can be regarded as a surface wet bulb temperature. The value of infrared surface temperature can be computed from measured or estimated values of net irradiance, aerodynamic resistance and air temperature. The non-water-stressed baseline is a useful concept that can effectively guide the irrigator to obtain maximum yields and to schedule irrigation. Surface temperature can be used to monitor the crop water status at any time of the day even on cloudy days, which may greatly ease the task of the irrigator. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003

This research was carried out to identify salt tolerant potato genotypes in vitro among 131 tetraploid potato cultivars (Solanum tuberosum), 9 diploid simple hybrid clones (4 clones of S. chacoense $ times$ S. tuberosum, 4 clones of S. phureja/S. stenotomum $ times$ S. tuberosum, and 1 clone of S. tuberosum $ times$ S. tuberosum), 1 primitive cultivated diploid S. phureja/S. stenotomum accession, 12 tetraploid complex hybrids, and 13 diploid S. chacoense accessions. Four levels of NaCl (0, 40, 80, and 120 mM) were used. The cultivars, and the simple and complex hybrids were tested for salt tolerance at the vegetative stage in the nodal cutting bioassay. The thirteen S. chacoense accessions were tested for salt tolerance at the germination and early seedling growth stage, in a seedling bioassay. Eleven of these S. chacoense accessions were further tested at the vegetative stage, in the nodal cutting bioassay. There was a progressive decline in the morphological parameters measured, with increased salt levels, in the nodal cutting bioassay. The parameters were used collectively in ranking the different genotypes, averaged over three NaCl levels (40, 80, and 120 mM). Twenty potato cultivars, two clones of the simple hybrid S. chacoense $ times$ S. tuberosum, and one complex hybrid were all considered salt tolerant at the vegetative stage. Ranking of seven S. chacoense accessions was similar between early seedling growth and later vegetative stage. Two of these accessions were promising as sources of salt tolerance.

Development of biomass energy plantations is one approach to mitigate and adapt to climate change and the energy challenges related to it; however, climate change will affect the climate conditions and in turn the selection of crops and trees suitable for renewable energy sources. In Estonia, electricity is mainly based on oil shale but since their integration in the European Union they are required to increase the share of energy from renewable sources. In this study, the possible changes of suitable species are assessed by examining the current and the future prospects and potentials with biomass energy derived from energy plantations in Estonia, taking climate change into consideration. The biomass energy potentials for the species that are climate suitable in current and future time are manually estimated, using a case study approach when determining the yields. The study result suggests that biomass energy from crops and trees have great development possibilities and that climate is not a key limitation for the selection of suitable species; in addition, the energy crops and trees appear to suit the future climate conditions better than the current. The results indicate that the established national target of 25% of energy from renewable sources in gross final consumption of energy by 2020 could be achieved to a large extent by putting energy plantations into practice.

The use of high N fertilizer in sweet corn (Zea mays L.) and wheat (Triticum aestivium L.) production often results in leaching losses and contamination of ground water. Cover crops planted after harvest of sweet corn and wheat may reduce residual soil NO$ sp- sb3$-N levels by crop N uptake and subsequently minimize NO$ sp- sb3$-N content in gravitational water. Field experiments were conducted on a Ste. Rosalie heavy clay (Humic Gleysol) and a St. Bernard sandy clay loam (Melanic Brunisol) to determine the contribution of the six cover crops to nutrient uptake, subsequent N release, leaching losses, denitrification rates and soil properties such as aggregate stability, organic matter. The cover crops were red clover (Trifolium pratense L.), crimson clover (Trifolium incarnatum L.), forage radish (Raphanus sativus L.), canola (Brassica rapa L.), barley (Hordeum vulgare L.), annual rye grass (Lolium multiflorum L.). Three replicates were used in a split plot arrangement of a randomized complete block experiment. Sweet corn and wheat were grown at three fertilizer N rates, 0-75-150 kg N ha$ sp{-1}$ for sweet corn and 0-45-90 kg N ha$ sp{-1}$ for wheat. Cover crop of forage radish, canola and barley were more effective at absorption or soil N than rye grass and clover species at all sample times. Levels of soluble N in the soil were reduced with cover crop in the off-season. Cover crop plots had higher NO$ sp- sb3$-N levels than control plots in the spring, indicating net mineralization and nitrification. Gravitational water NO$ sp- sb3$-N contents were higher in the control plots relative to cover crop plots at both sites. Therefore growing cover crops after harvest of sweet corn and wheat can reduce residual NO$ sb3$-N level in the soil and thus restrict ground water contamination with fertilizer N.

No description available.