The cowpea, (Vigna unguiculata (L.) Walp.), is considered a drought‐tolerant legume. The N fixation process, however, is sensitive to water stress. This study was conducted to describe the development of nodulation and N fixation of field‐grown Californian cowpeas as influenced by stage of growth and drought. California No. 5 blackeyes were evaluated for nodulation and N fixation (as measured by acetylene reduction assay) utilizing four strains of Rhizobium sp. applied as seed inoculum. Two water regimes were utilized in this study: 1) the well‐watered regime was irrigated when the soil matric potential reached ‐0.6 bars at a depth of 20 cm, 2) the droughted regime received sufficient irrigation to allow good seed emergence, followed by a 42‐day period free of rain or irrigation. Maximum nodulation of 175 mg/plant was observed at flowering in the well‐watered plots versus 15 mg/plant in the droughted plants. Following this development stage, there was no significant difference in nodulation between water regimes. Maximum total nitrogenase activity was 22.5 and 18.8 µmoles of ethylene/plant/hour, respectively at flowering and pod‐fill in the well‐watered plants. The dry regime by contrast achieved a maximum nitrogenase activity of 5.3 and 4.5 µmoles ethylene/plant/hour at pod‐fill and pod yellowing stages of maturity, respectively. Consequently, there was a significant reduction in total nitrogenase activity (mmoles of ethylene/plant/growing season) averaged over four Rhizobium strains from 14.5 in the well‐watered to 3.7 in the droughted regime. This decrease in N fixation capacity did not decrease seed yields of droughted plants below those of well‐watered plants.
The freezing‐point method, using a small probe‐type thermistor as the sensing element, was used to study certain factors believed to lower the freezing‐point depression of soil moisture. The factors investigated were aggregate size, particle size, and undercooling temperature. The materials used were homogeneous and consisted of Palouse silt loam obtained from the A1 horizon, synthetic soil, and quartz sand. The Palouse silt loam was treated with soil conditioner, formed into aggregates, and separated into groups of various sizes. The freezing‐point depression measured on the various groups of aggregates increased with decreasing size of aggregates. This same trend was obtained for different‐sized sand separates. With successive freezing and thawing, the freezing‐point depression decreased for Palouse silt loam aggregates but was relatively unchanged for sand separates. The synthetic soil was made from ground quartz sand and illite clay. Part of the synthetic soil was treated with various rates of soil conditioner and formed into aggregates. The freezing‐point depression, made on aggregates of 1 to 2 mm. diameter, increased with increasing rate of soil conditioner. Different undercooling temperatures had little effect in changing the freezing‐point depression for Palouse silt loam or sands. However, the freezing‐point depression was increased with increasing undercooling temperatures for nonaggregated synthetic acid‐soil.
Mesquite (Prosopis spp) is a nitrogen-fixing and drought-resistant leguminous tree which exists in many of the semi-arid regions of the world but little has been done to develop its potential for wood, nitrogen fixation or pod yield. We report field and greenhouse cultural practices for mesquite and the results of a field screening trial for biomass production of 32 accessions, representing North and South American and African germplasm. Non-destructive biomass estimates, derived from height and stem diameter measurements, had a 180-fold range; the preponderance of slow-growing accessions were from the rangelands of southwestern USA.Prosopis (mesquite) is an extremely diverse leguminous genus of 44 species of shrubs and trees of semi-arid regions, primarily in Chile, Argentina, Peru, Mexico and southwestern United States. A native and an introduced species occur in west Africa, and two native and several introduced species occur in the Caucasus-Indian region. Forms are known that are useful for soil conservation and as sources of fodder, and the genus may be valuable for timber, fuel, nitrogen fixation and the accumulation of leaf litter (Felker, 1979).We have assembled 400 single tree selections (accessions) of 13 species of Prosopis. This paper reports the results of two studies on the growth of 39 of these accessions (Table 1) in respect of height and stem diameter, and describes the silvicultural procedures and methods of controlling weeds and insects. Methods of inoculation with Rhizobium will appear elsewhere (Felker and Clark, in press). Most of the material was supplied by Professor O. T. Solbrig of Harvard University, and was obtained during work on the International Biological Program (IBP) in North and South America. The North American mesquite supplied by Solbrig was collected by Simpson along an east-west transect from Texas to Arizona. Other accessions were supplied by the USDA Northern and the Desert Nursery, Riverside, California. METHODS Silvicultural proceduresSeeds were dissected from the pods with a utility knife and pruning shears, scarified by nicking them on the blunt end (opposite the radicle) and surface
Simple methods for estimating standing biomass in a stand of tree legumes containhtg the genera Prosopis, Cercidium, Olneya, Leucaena, and Parkinsonia are reported. Fresh and dry biomass were related to height and stem diameter measurements for 212 leguminous trees ranging in biomass from 0.04 to 17.8 kg using linear regression. The dry matter content of the above-ground biomass of these genera ranged from 40 to 56% and the stem dry matter percentage ranged from 70 to 96%. The best functional form of the model was log10 dry weight (kg) = 2.55 log basal diameter (cm)-1.25, which had an rJ of 0.956 for 212 samples. The leguminous trees and shrubs such as mesquite (Prosopis), palo Verde (Parkinsonia aculeata and Cercidium jloridium) and ironwood (Olneya tesota) occur on at least 30 million hectares in southwestern United States (Parker and Martin 1952) and may constitute a significant biofuels resource for southwestern United States. Easy methods for estimating this biomass from diameter and height measurements would facilitate biomass estimation in experimental plots designed to screen for biomass productivity and in regional surveys attempting to quantitate the biofuels resource potential. Whisenant and Burzlaff (1978) have reported highly significant regression equations relating stem area at ground level, stem area 60 cm above the ground, and canopy area with the fresh standing biomass of native mesquite (Prosopis) stands in Texas. Whittaker and Marks (1978) suggested that parabolic cone volume, which is the product of area and height measurements, is the preferred variable for estimating biomass with linear regressions. Whittaker and Marks (1975) clearly point out the utility of converting height and trunk diameter measurements and biomass determinations into log-log form to allow the regression to find the preferred power, e.g., linear, quadratic, or cubic, for the desired expressions. We have examined linear regressions similar to that reported by Whisenant and Burzlaff (1978) and parabolic cone volume expressions and log-log regressions suggested by Whittaker and Marks (1975) in deliberately established mesquite biomass plantation in southern California. Coefficients for converting fresh weight to dry weight and for partitioning dry matter into leaf and woody tissue have also been examined. Methods Part of a 9-month-old tree legume biomass varietal trial in the California Imperial Valley was harvested for thesedeterminations.
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