S U M M A R YFour soyabean cultivars were grown with two N application rates (50 and 300 kg N/ha) in the field at Hiroshima University, Japan, from June to August 1988. Cell membrane stability (CMS) by the polyethylene glycol (PEG) test, leaf water relations and nutrient concentrations in cell sap and leaf tissues were measured when the plants were 50 days old, in the uppermost fully expanded leaves.Cell membrane stability was higher at the higher N rate, the increase over the lower rate being greater in the cultivars Lee+ and Lee -than in Tamahomare and T201. Leaf water potential was not affected by the higher rate of N application. Osmotic adjustment, which was independent of water stress, was observed with the higher rate of N and it was higher in Lee + and Lee -than in Tamahomare and T201. It is suggested that osmotic potential in leaf tissues may influence CMS measured by the PEG test. Solute concentrations in cell sap and leaf tissues were higher at the higher N rate. Sugar and K were the major contributors to osmotic potential.

Tomato [Solanum lycopersicum (formerly Lycopersicon esculentum) L. cv. Momotarou] plants were grown hydroponically inside the greenhouse of Hiroshima University, Japan. The adverse effects of potassium (K) deficiency stress on the source-sink relationship during the early reproductive period was examined by withdrawing K from the rooting medium for a period of 21 d. Fruits and stem were the major sink organs for the carbon assimilates from the source. A simple non-destructive micro-morphometric technique was used to measure growth of these organs. The effect of K deficiency was studied on the apparent photosynthesis (source activity), leaf area, partitioning (13)C, sugar concentration, K content, and fruit and stem diameters of the plant. Compared with the control, K deficiency treatment severely decreased biomass of all organs. The treatment also depressed leaf photosynthesis and transport of (13)C assimilates, but the impact of stress on these activities became evident only after fruit and stem diameter expansions were down-regulated. These results suggested that K deficiency diminished sink activity in tomato plants prior to its effect on the source activity because of a direct effect on the water status of the former. The lack of demand in growth led to the accumulation of sugars in leaves and concomitant fall in photosynthetic activity. Since accumulation of K and sugars in the fruit was not affected, low K levels of the growing medium might not have affected the fruit quality. The micro-morphometric technique can be used as a reliable tool for monitoring K deficiency during fruiting of tomato. K deficiency directly hindered assimilate partitioning, and the symptoms were considered more detrimental compared with P deficiency.

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Cell membrane stability (CMS) of maize (Zea mays L.) was measured by the polyethylene glycol (PEG) test and compared with the nutrio‐physiological characteristics to examine the relationship of plant processes involved in differential ion leakage. Two pot experiments were conducted in the glasshouse under three soil moisture levels (MPa 0 to −0.05, −0.3 to −0.9, and −1.2 to −1.5) using three cultivars (P‐3358, G‐4578, and FFR‐677) of maize. Leaf water potential, osmotic potential, stomatal resistance, cuticular resistance, excised‐leaf water retention capability, degree of leaf rolling, total plant weight, and total root length as well as the CMS by the PEG test were measured when the plants were 60 d old. Nutrient concentrations in leaf tissues, cell sap, and cell leachate were analysed. Cell membrane stability measured by the PEG test was well related to the other physiological measurements. It was highly or very highly correlated with leaf water potential (r = −0.869**), (significant at P = 0.01) osmotic potential (r = −0.946**), excised‐leaf water retention capability (r = 0.767**), K concentration in leaf tissues (r = 0.757*) (significant at P = 0.05) and cell sap (r = 0.690*), and the degree of leaf rolling. Cell membrane stability appears to be influenced by osmotic potential of leaf tissues. Sugar and K were the major osmotic contributors. In conclusion, we find the PEG test to be a useful measure of drought tolerance in maize.

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