Abstract
In sugar-beet plants, leaf growth continues even when a leaf area index is reached which is in optimum for light absorbance. Plants develop new leaves until the end of the vegetation period at the expense of sucrose accumulation into tap roots. Additinonally, considerably dry matter losses occur as the number of dead leaves increases.There are good evdence that mild drought stress inhibits leaf growth while tap root growth continues. This modified growth behaviour under mild drought stress conditions results in a broader root to shoot ratio, i. e. greater harvest index but severe drought stress inhibits both shoot and root growth decreasing the root to shoot ratio.Regulation mechanisms and adaptation processes of sugar-beet plants to drought stress are not well understood yet. The objective of this investigation is to determine drought stress paramaters in pot experiments coresponding with various harvest indices of sugar-beet plants. The knowledge of the relation between water supply, growth behaviour and drought stress is a prerequesite for further sudies of the physiological mechanisms of adaption processes in sugar beet plants, e. g. hormonal responses to water shortage.Our results show that that the hightest leaf growth coresponds with the highest water content of the growth substrate (60% of maximum waterholding capacity — max. WHC). Lowering the water content of the substrate resulted in a continous decrease only in leaf growth. The maximum root growth was observed at substrate humidity of about 42...48 % of max. WHC. In consequence, mild drought stress did not result in a decrease in dry matter yield of sugar-beet plants but in a significant shift of the shoot to root-ratio from 0.83 to 0.62. Severe drought stress inhibited shoot and root growth and decresed the total dry matter yield by 10%. Under conditons of severe drought stress (water content of the substrate was about 20...30% max. WHC), the harvest index was not significantly different from that of adequate watered sugar-beet plants.
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