Six varieties of oat {Avena sativa L. cv. Stormogui II, Risto, Sol II, Selma, Sang and Pendek, arranged according to decreasing drought resistance) were cultivated under controlled conditions and exposed to water stress on 4 consecutive days. Seven-day-old seedlings were stressed by cooling the roots for 3 h to 1.0°C. Dudng this treatment the leaf water potential decreased from -7 to -12 bars. Cuticular transpiration rate, total amount of epicuticular wax and amounts of some wax components (primary alcohols, alkanes, fatty acids) were determined.Unstressed seedlings of the most drought resistant variety (Stormogui II) showed the highest cuticular transpiration rate. After stress treatment the cuticular transpiration rate was most strongly reduced in this variety and at the same time it showed the largest increase in amount of epicuticular wax of the tested varieties. In Pendek and Sang, showing the least increase in epicuticular wax, the cuticular transpiration rate was only 5% lower after stress treatment.In all varieties the primary alcohol content of the epicuticular wax was slightly higher in stressed seedlings than in controls. Further, in Stormogui and Risto the content of the predominant alkanes was much lower in stressed seedlings than in controls. On the contrary, in Pendek the stressed seedlings showed a higher alkane content. In Stormogui II, Risto and Sol II the total amount of fatty acids was higher in stressed seedlings than in controls while the opposite was true in Sang.The relation between the epicuticular wax (amount and composition) and the cuticular transpiration rate is discussed as well as the possibility of using the tested parameters in a screening test for drought resistance.CURT BENGTSON, STIG LARSSON AND CONNY LIUENBERG Physiol. Plant. 44. 1978

Young wheat plants (Triticum aestivum L. cv. Weibulls Starke II) were exposed to water stress for 1, 2 or 3 hours by cooling the roots. The plants were subjected to a constant water stress during the stress periods. By this treatment the leaf water potential was lowered from −6.5 to −11.5 bars. Leaf water potential, transpiration rate and abscisic acid content were determined during the stress periods and during the recovery.The water stressed plants showed an after‐effect on transpiration rate lasting for between 10 and 24 hours depending on the duration of the stress. The amount of water stress in the stressed plants compared with the controls is defined as the difference in leaf water potential between the controls and the stressed plants during the stress period integrated over time. The amount of after‐affect on transpiration is analogously defined as the difference in transpiration rate between the controls and the stressed plants during the recovery period integrated over time. There was a linear relationship between the amount of water stress and the amount of after‐effect on transpiration of the leaves.The abscisic acid content of the leaves increased between 3.0 and 4.5 times the original content depending on the duration of the stress. However, during the recovery the abscisic acid content reattained the pre‐stress level within 3 hours for all three stress periods. There was thus no direct relationship between the after‐effect and the abscisic acid content of the leaf.

The influence of phytohormones on chlorophyll and carotenoid formation during the greening of irradiated dark grown wheat leaves (Triticum aestivum L. cv. Starke II Weibull) was studied. Leaves were floated on solutions of abscisic acid, gibberellic acid and kinetin for 24 h. The chlorophyll and carotenoid contents were determined during a subsequent period of 48 h of continuous irradiation. Leaves treated with abscisic acid showed a longer lag phase and a lower rate of accumulation of chlorophyll as compared to the control than did leaves treated with gibberellic acid and kinetin. The carotenoid content was low both in leaves treated with abscisic acid and in those treated with gibberellic acid. Treatment with abscisic acid lowered the protochlorophyllide regeneration after a saturating light flash while gibberellic acid as well as kinetin had no effect. The influence of ABA was partly dependent on an increase of the wounded part of the cut leaf segments. The accumulation of protochlorophyllide in leaves treated with δ‐aminolevulinic acid was not affected by the different hormonal treatments. These results suggest that the main effect of abscisic acid is probably outside the chloroplast, i.e. on the formation or transport of δ‐aminolevulinic acid.

Cut seedlings of wheat plants {Triticum aestivum L, cv, Starke II Weibull) between 6 and 7 days old were water stressed in darkness by exposing them to air of 35% relative humidity 2,5 to 20 h. This treatment resulted in a water potential of -11 bars in the leaves after 20 h. The leaves were then rewatered and irradiated. The chlorophyll formation that took place in fully turgid leaves during the greening was markedly decreased in the case of the water-stress pretreatment, and especially the lag phase was prolonged. The longer the stress pretreatment the more evident was the subsequent effect on chlorophyll formation. However, no linear relationship was found between the amount of stress and the chlorophyll content. Protoehlorophyllide regeneration from endogenously formed ^aminolevulinic acid was markedly decreased even after the shortest water-stress period. However, protoehlorophyllide accumulation from exogenously supplied ^-aminolevulinic acid was only slightly decreased following the water-stress pretreatment, Eurthermore, the ratio of protochlorophyllide^jo to protochlorophyllide^^s was slightly reduced by the same conditions.During the stress period both abscisic acid and proline were accumulated in the leaves. The content of abscisic acid increased up to six times the normal level during water stress lasting for 20 h. The increase of proline was about three-fold for similar treatment. After rewatering the leaves the levels of both abscisic acid and proline rapidly declined and reached, 10 h later, the levels found in unstressed seedlings.The increase in abscisic acid during water stress associated with impaired chlorophyll metabolism suggested that the after-effect of water stress might be linked to chlorophyll metabolism through abscisic acid or some of its metabolites. The changes in proline content open the possibility that this substance could function as a reserve substance for the formation of chlorophyll after the discontinuation of the stress. 206CURT BENGTSON ET AL.

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