Alexandra Poljakoff-Mayber

Essential Oils as Allelochemicals and Their Potential Use as Bioherbicides

Essential oils were extracted from 32 aromatic plants and evaluated for allelopathic properties. The major components of the oils were identified by gas chromatography and GC-MS. Extracts from Origanum syriacum, Micromeria fruticosa, and Cymbopogon citratus were selected for further study. The germination of several species, including wheat, was strongly inhibited by essential oils when applied at 20–80 ppm. Essential oils mixed with the top 0.5 cm of soil inhibited germination of wheat and Amaranthus seeds. This effect depended on the type of soil. The possible use of essential oils as herbicides is discussed.

A Rapidly Equilibrating Thermocouple Contact Thermometer for Measurement of Leaf‐Surface Temperatures

The iron—constantan thermocouple contact thermometer here described is mounted on spring clamp and can be quickly attached to the leaf; a large number of determinations can be made in a short time. The thermocouple junction equilibrates very rapidly (1.5 sec), has a low heat capacity, and is spring loaded. The performance of the contact thermometer compares well with that of a thermocouple inserted within the leaf mesohyll and with measurements made with an infrared radiation thermometer. See full-text article at JSTOR

The effect of chloride and sulphate types of salinity on growth and on osmotic adaptation of pea seedlings

SummaryGermination of pea seeds sown in saline substrate was delayed. The higher the salinity level the longer was the delay. From root protrusion and until the seventh day after sowing the rate of growth of the seedlings was practically unaffected by salinity. From that time on the effect of salinity became more and more apparent. An increase in growth rate of shoots was observed on exposure to −3 to −4 atm of either NaCl or Na2SO4; a slight increase in growth of roots exposed to −3 atm NaCl was apparent. Otherwise increasing levels of NaCl salinity resulted in a reduced growth rate. The effects of sulphate salinity were practically non-significant. Pea plants grown in Na2SO4-salinized media up to 14 days showed a complete osmotic adjustment. This was not the case with plants grown in NaCl-salinized media. Sodium accumulated in the roots of plants exposed to both types of salinity but it was hardly transported into the shoots. Kcontent dropped considerably on exposure to salinity. Chloride accumulated in both roots and shoots. In roots its concentration exceeded that of the external medium and was higher than the concentration of Na + K. The relevance of these results for the explanation of the effect of salinity on growth of the pea seedlings is discussed.

The Effect of Salt Species and Concentration on Photosynthesis and Growth of Pea Plants (Pisum sativum L. cv. Alaska)

Osmotic adjustment (Δπ), stomatal resistance (rs), net photosynthesis (PN) and three growth parameters (fresh weight-FW; dry weight-DW and length-L) were measured in pea plants grown in different concentrations (85–390 mosmol) of either NaCl , KCl , Na2SO4 or K2SO4. Complete osmotic adjustment was not attained in any of the treatments, and Δπ decreased with increasing external salinity, indicating a possible reduction in turgor. FW and L of the plants decreased accordingly.

GERMINATION OF PEA SEEDS EXPOSED TO SALINE STRESS

ABSTRACT Salinity up to 200 mM NaCl has very little effect on the germination of Alaska peas as expressed by root protrusion. However, it strongly affects the emergence of the epicotyl. This effect is expressed by slower increases of fresh weight and of the water potential of the embryonic axis and results in delayed emergence. The imbibition of the cotyledons is apparently not affected. Parallel to inhibition of epicotyl emergence, salinity induces accumulation in the cotyledons of ABA and proline, although the rate of increase in content of these two substances is not identical. It is proposed that on exposure of pea seeds to salinity ABA accumulates in the cotyledons, is transported to the embryonic epicotyl and inhibits water uptake. When proline accumulates, water uptake is resumed and epicotyl emergence occurs.