摘要:

The effect of cold air advection on the estimates of daily thermal time with a base temperature of 8°C (DTT8), calculated using maximum and minimum screen temperatures (max./min. values), was examined. Screen temperature data for at least three maize-growing seasons (October—April) at Cedara, Dundee and Potchefstroom were obtained from thermohygrograph charts. Two-hourly values on the charts were used to calculate unbiased estimates of DTT8. The max./min. and two-hourly estimates of DTT8were closely correlated. Mean bias errors (MBE) associated with the max./min. DTT8, which were defined as being equal to the difference between mean max./min. DTT8and mean two-hourly DTT8, were positive for all seasons at the three sites. MBE in max./min. DTT8was positively related to the fraction of days experiencing cold air advection (FDCAA), withr2= 0.64. Since cold air advection in South Africa is most often due to the movement inland of cold marine air, proximity to the sea may be a strong indicator of MBE in max./min. DTT8. The Cedara and Dundee sites, which are relatively close to the coastline, experienced cold air advection more often than did Potchefstroom (mean FDCAA of 0.229, 0.225and 0.144, respectively) and had greater MBE in max./min. DTT8(mean MBE of 0.62, 0.70 and 0.37°C d, respectively). Using the max./min. and two-hourly DTT8, comparisons were made of predicted silking and maturity dates for dryland maize generated by the CERES-Maize V.2.10 crop model. The largest under-predictions of silking date (–4.9 days on average) and physiological maturity (–13.3 days on average) when using max./min. DTT8were generated for Dundee, followed by Cedara (−3.9 and−7.7) and then Potchefstroom (−1.2 and−2.2). The results indicate that caution must be exercised when using max./min. estimates of daily thermal time for predicting maize growth and development at sites where cold air advection occurs frequently. At such sites, estimates of thermal time should be based on frequently-sampled air temperatures.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634634

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

Construction details of an inexpensive, portable rain simulator are given. The dismountable components are easily transported on a light truck. The flat spray nozzles used produced rain with a>90% coefficient of uniformity over a 1-m2test plot. Drop sizes and drop size distributions were determined using a flour pellet method. The appropriate equation of motion for accelerating drops and drag coefficients were used to calculate the impact velocity of individual drops and the kinetic energy of rain produced by various nozzles. Rainfall intensity can easily be varied in the range 10–150 mm h−1and water consumption is low.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634635

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

Sorghum harvested for seed fromClaviceps africana-infectedfields becomes coated with sugary disease exudates in which the fungal spores are borne. The role of exudates in the inhibition of seed germination, seedling growth and predisposition to seedling diseases was investigated.In vitrostudies showed that low exudate concentrations could reduce germination and seedling growth and that cultivars differed in their response to exudates. In the field and greenhouse, seed-borne exudate concentrations were quantified on the basis of spores g−1seed. Concentrations of 6.47 x 106spores g−1seed artificially applied to healthy seed reduced germination and seedling emergence, and predisposed seedlings to mesocotyl discoloration and post-emergence damping off. The spore concentration on seed from a naturally-infected field was 55 x 106spores g−1seed. This could be reduced by seed washing but large volumes of water were required to reduce exudate concentrations to levels which would not affect germination and seedling development.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634636

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

Waterlogged conditions in hydromorphic soils during spring cause low temperatures which may restrict grapevine performance. Ridging is a soil preparation method that can improve internal drainage in the root zone. Temperature measurements showed that ridging tended to increase temperatures in the upper soil layers compared with unridged soil, but showed no significant difference in soil with ridges of varying width and height. During the later part of the growing season soil temperatures in the ridges exceeded 30°C. These high temperatures were, however, not maintained for long periods and may therefore not have had a negative effect on root activity. Soil temperature recorded during the entire growing season related well to soil water content. The soil temperature versus water content relationship improved with increasing soil depth. Temperature measurements showed that the diurnal rate of heat exchange was much higher in the upper layers of the ridged soil when compared with the unridged control. Accurate predictions of diurnal soil temperature cycles were made by using a sinusoidal function of time around the average daily soil temperature.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634637

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

The nitrification inhibitor dicyandiamide (cyanoguanidine, didin or DCD) reacts with 1-naphthol and diacetyl to form a red complex which measures spectrophotometrically at 540 nm. An automated analytical procedure was developed to determine DCD in soils extracted with 1.0 M KCI. The method was optimized and evaluated between 0.2 and 30 mg dm−3DCD-N. The calibration curve showed a high degree of linearity between 2 and 30 mg dm−3DCD-N. Accurate and reliable readings were achieved at a sampling frequency of 30 samples h−1. The transmittance between samples was less than 2.00%. A number of ions were tested for possible interference. Interference levels were reduced With the introduction of ethylene diamine tetra-acetic acid (EDTA) and mannitol.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634638

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

Potatoes,Solanum tuberosum(L), are cultivated in South Africa under a variety of different climates and seasons. The wide environmental variation encountered and the confirmed occurrence of genotype-environmental interaction necessitates the use of criteria, other than averages, in the evaluation and selection of new potato cultivars. Yield data from cultivar trials countrywide, were used to evaluate the use of the AMMI model against the existing Finlay-Wilkinson regression technique. The AMMI model was found to be suitable for determining the reaction of cultivars/lines in an environment. The usefulness of the technique improves with a graphical presentation of the results to display the stability of a genotype in different environments.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634639

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

A diallel set of hybrids involving eleven maize inbred lines was used to determine the genetic basis of resistance toBusseola fusca.Larval mass gain over a period of three weeks after the artificial infestation of plants was closely correlated with the levels of damage caused by leaf feeding. Estimates of the combining ability indicated that general combining ability effects were the most important source of genetic variance for resistance. Antibiosis was largely additive and 35% heritable. Moderate levels of resistance were observed in some locally developed inbred lines. Significant differences in the combining ability for yield in crosses between local elite inbreds and the resistant Mississippi inbreds Mp706 and Mp707 seemed to be related to variation in prolificacy.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634640

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

A glasshouse study was conducted to determine the allelopathic potential of sweetpotato (Ipomoea batatas(L.) Lam.) cultivars Bosbok, Brandal and Koedoe. Nine indicator plants, three weeds: yellow nutsedge (Cyperus esculentusL.), purple nutsedge (Cyperus rotundusL.) and common pigweed (Amaranthus hybridusL.), and seven crop species: carrot (Daucus carotaL. cv. Kaapse Mark), tomato (Lycopersicon esculentumMill. cv. Heinz 1370), cucumber (Cucumis sativusL. cv. Special Rust Resistant), radish (Raphanus sativusL. cv. White Icicle), onion (Allium cepaL. cv. Pyramid), lettuce (Lactuca sativaL. cv. Great Lakes).and oats (Avena sativaL. cv. SWK 001) were grown in soil from field plots previously cropped with these sweetpotato cultivars. The three cultivars were grown on separate plots. Soil from adjacent uncropped and weed-free fallow plots served as a control. Yellow nutsedge plants grown from tubers in soil from sweetpotato cv. Brondal field plots accumulated significantly less dry matter than plants grown in soil from cv. Bosbok, cv. Koedoe or fallow plots. It is suggested that substances inhibitory to yellow nutsedge growth were present in soil subsequent to growing Brondal sweetpotato. Compared with growth in soil from fallow plots, growth of the other test species was not inhibited in soil previously cropped with sweetpotato. In fact, the growth of carrot, cucumber, lettuce, oats, onion and tomato plants tended to be stimulated in some soils.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634641

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

Montmorillonite clay type soils have been implicated in cases where the excessive persistence of atrazine caused damage to sensitive follow-up crops. A study was conducted to evaluate the influence of temperature and soil water content on the persistence of atrazine in a clay soil (total clay 55%, 80% montmorillonite, 0.7% organic C, CEC = 48 me 100 g−1, pH = 7.3) and a loamy sand soil (total clay 7%, 0.34% organic C, CEC = 0.8 me 100g−1pH = 5.4). The latter soil represented soils in which atrazine carry-over is not expected to occur. A factorial experiment (2×2×3×3) comprising the two soil types; atrazine rates of 1 and 2 mg kg−1; soil water contents of about 0 (air-dry), the amount at field capacity and 2×field capacity; and temperature regimes (day/night) of 30/16°C, 30/8°C and 16/8°C, was conducted in growth cabinets. Samples of soil incubated in the dark were analysed for atrazine on day 0 (herbicide applied), day 30 and day 60 by means of high performance liquid chromatography (HPLC). By day 30, distinct but not significant degradation of atrazine had occurred in the light soil only. Subsequent determinations on day 60 after atrazine application, however, revealed that soil water at the field capacity level significantly increased degradation in both soils, with a difference in magnitude such that the half-life of atrazine in the loamy sand was reached after about 60 days, while at the same stage 70–75% of the applied atrazine remained in the clay soil. The increase in soil water from field capacity to 2×field capacity had little or no effect on persistence. Virtually no degradation of atrazine occurred in air-dry soil. The lowest temperature regime significantly reduced the rate of degradation in the light soil only. It is suggested that the higher pH and higher adsorptive capacity of the clay soil caused atrazine to persist longer in that soil than in the loamy sand soil.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634642

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor

摘要:

The changes in protein profiles of six cotton cultivars subjected to osmotic stress were examined. Osmotic stress was applied by incubating leaf segments in mannitol. The efficiency of protein synthesis decreased with increasing concentrations of mannitol. Drought-related protein synthesis during osmotic stress was found in the cultivars Lido, Delta Pine, Acala 1517–70 and OR3, while drought-related proteins were not observed in the cultivars Selati and Letaba. It would: appear that drought-related protein synthesis was cultivar specific.

ISSN:0257-1862    

DOI:10.1080/02571862.1993.10634643

出版商:Taylor&Francis Group    

年代:1993

数据来源: Taylor