H. A. van de Venter

The effect of factors associated with fire on seed germination of Erica sessiliflora and E. hebecalyx (Ericaceae)

Elevated temperature is an obvious, but not necessarily the only, germination cue associated with fire. The possibility that gases released during a burn may also constitute germination cues, has hitherto been overlooked. Stimulation of seed germination of Erica hebecalyx Beth. by heat pretreatments as well as by exposure of both dry and imbibing seeds to ethylene and ammonia, suggest that fire under natural conditions will have a direct stimulatory effect on seed germination of this species, possibly by means of multiple cues. In contrast, seed germination of Erica sessiliflora L.f. was not stimulated by any of these treatments.

Stimulation of seedling root growth by coal-derived sodium humate

Coal-derived sodium humate was found to stimulate primary root growth of seedlings of cantaloupe (Cucumis melo L.) at 1000 mg L−1, lettuce (Lactuca sativa L.) and onion (Allium cepa L.) at 500 and 1000 mg L−1, as well as hypocotyl growth of cantaloupe at 1000 mg L−1. Growth enhancement was not due to release of nutrient elements by the product, and, in the case of lettuce and onion, was not due to increased availability or uptake of mineral elements. Growth stimulation of cantaloupe, however, was dependent on the provision of nutrient solution. Growth stimulation of onion roots under axenic conditions indicated an effect of the humate per se rather than a response mediated via microbial breakdown products.

Qualitative estimation of atrazine, hydroxyatrazine and 6-methoxy-2-benzoxazinone (MBOA) in maize plants

A rapid and simple method for the extraction and quantification of the herbicide atrazine, the atrazine hydroxylating agent in maize plants DIMBOA, and the hydroxylated product hydroxy-atrazine, is described. These substances were quantified by means of HPLC techniques, employing a RP-18 column and a mobile phase of methanol:water (40:60), buffered at pH 7,4. This method was developed to determine the efficiency of the atrazine hydroxylation detoxifying mechanism in maize plants cultivated under different environmental stress conditions.

Influence of sub-optimal imbibition temperatures on seed vigour and respiration in maize (Zea mays L.)

Germination and coleoptile growth of nine seed lots of maize (representing seven cultivars) were studied at 25°C (constant), 13°C (constant) and also at 7°C (7 days) followed by 25°C. Relative germination response to sub-optimal temperatures reflected relative differences in germination rate at 25°C. Exposure to 7°C before incubation at 25°C had a marked stimulatory effect on germination of cold- resistant cultivars at the higher temperature. Low temperature appeared to have a differential effect on germination rate and coleoptile growth. Differences in the magnitude of respiration rate did not relate to differences in low temperature susceptibility but kernels of cold-resistant cultivars failed to exhibit a typical respiratory lag phase. S. Afr. J. Plant Soil 1985, 2: 203–206

Ultrastructural changes in the leaves of Zea mays L. plants treated with atrazine

Veinal chlorosis was induced in the lower leaves of young maize plants following treatment with atrazine (2-chloro-4-ethylamino-6- isopropylamino-s-triazine). An ultrastructural investigation indicated that mesophyll chloroplasts, consisting of grana, thylakoids and stroma showed severe disintegration. The seemingly unaffected bundle sheath chloroplasts with stroma, thylakoids and sometimes a few rudimentary grana which normally only show PS I activity, appeared to transform to mesophyll-type chloroplasts with formation of small granai stacks. The mesophyll chloroplasts became more spherical and various ultrastructural changes became visible. The thylakoid system was destroyed, the granai compartments became swollen and the double-layered membrane broke down, leading to disintegration of the chloroplast. Atrazine had little effect on the bundle sheath chloroplasts except in a few cases where the chloroplast membrane exhibited swellings. At rates of 6 ppm atrazine, or prolonged treatment with 3 ppm atrazin...

Screening for DIMBOA (benzoxazinone) concentration among South African inbred maize lines and sorghum cultivars

Twenty-seven popular South African inbred maize lines, four commercial maize cultivars, two sweet corn cultivars and two grain sorghum cultivars were screened by means of a rapid, semi-quantitative method to determine DIMBOA concentration. Significant differences were found between inbred lines. Sweet corn cultivars exhibited a relatively high concentration of DIMBOA, while grain sorghum contained a very low concentration. Although differences in concentration between plants cultivated from different seed sizes were found, no definite trend could be established. S. Air. J. Plant Soil 1984, 1: 99–102

Seed development and maturation in edible dry bean (Phaseolus vulgaris L.) cv. Teebus

A study of seed development and maturation in edible dry bean cv. Teebus was conducted on field-grown plants. Mass maturity (maximum seed dry mass) occured 43 days after flowering (DAF) when seed moisture content was 52%. Harvest maturity (at a seed moisture content of 12.7%) was reached 15 days after mass maturity (58 DAF). Fresh seeds produced 90.5% normal seedlings at 30 DAF (13 days before mass maturity), when they had acquired only 48% of their maximum dry mass. Only 3% of rapidly dried seeds produced normal seedlings at this stage. Maximum seed quality (as determined by normal germination and conductivity of steep water), as well as maximum desiccation tolerance, were attained 5 days after mass maturity when seed moisture content was 34%. Respiration initially increased sharply, followed by a relatively constant high level for several days, after which it diminished to near zero levels when moisture content was 16.8% (5 days before harvest maturity). ATP content followed much the same pattern, but t...

Differential effect of temperature on mitrochondrial activity in shoots from freshly harvested and moderately aged kernels of maize (Zea mays L.)

This paper reports on a study of mitochondrial activity in etiolated shoots of freshly harvested and moderately aged kernels of maize. Activity was investigated after incubation at a favourable temperature (25°C), sub-optimal temperature (13°C) and after a heat shock (46°C for 2h). Although impaired mitochondrial activity in shoots from moderately aged maize kernels was not detected at 25°C, deficiencies became evident under low temperature stress (13°C). State 3 oxygen uptake, cyanide-insensitive oxygen uptake and cytochrome oxidase activity were lower in mitochondria from these shoots at 13°C than in mitochondria from shoots of freshly harvested kernels at this temperature. After a heat shock, cyanide-insensitive oxygen uptake was higher, and cytochrome oxidase activity lower, in shoots of aged kernels than in shoots of fresh kernels. No significant differences in ADP: O ratio or succinate dehydrogenase activity occurred between mitochondria from shoots of the two seed lots in any of the temperature treatments.

A comparison of seed vigour tests for maize (Zea mays L.)

A number of seed vigour tests were conducted on two groups of maize seed lots. One group consisted of six seed lots of different cultivars while the other group consisted of six sub-lots of one cultivar, artificially aged for different periods. Seed vigour tests included in the study were, a low temperature germination test (incubation in rolled paper towels at 13°C), the soil cold test, the accelerated ageing test and an osmotic stress test. The results of the vigour tests were correlated with the emergence of the seed lots in four field trials representing different sets of environmental conditions. The low temperature germination test was found to have the greatest overall predictive value for field emergence. The advantages and disadvantages of the low temperature germination test compared with the soil cold test, which has gained wide acceptance as a standard seed vigour test for maize, are discussed.

THE EFFECT OF HEAT TREATMENT OF WATERMELON SEED ON GERMINATION, HYPOCOTYL EMERGENCE AND ABSCISIC ACID CONTENT

Seeds of watermelon (Citrullus lanatus) with a moisture content of 7.2% were heat treated at 60 °C for 15 or 20 h, and their performances compared with that of untreated seeds. Radicle emergence of treated seeds was only slightly lower than that of the control when germinated at 25 °C. To ascertain the effect of the treatments on hypocotyl growth (emergence) per se, radicle emergence was synchronized in all treatments and emergence from deep plantings in sand determined. Small differences in emergence were observed when seeds were germinated at 25 °C, but seed treatment resulted in large reductions in emergence at 15 °C. This demonstrates that the hypocotyls developing from heat-treated, germinated seeds are less able to withstand low germination temperatures, which could be an important factor in emergence failure. Levels of free abscisic acid (ABA) were higher at 15 °C than at 25 °C, with hypocotyls of treated seeds containing more free ABA and its breakdown products than the control at the lower temperature.