Effect of bicarbonate on root growth and accumulation of organic acids in Zn-inefficient and Zn-efficient rice cultivars (Oryza sativa L.)

Bicarbonate has been regarded as a major factor for inducing Zn deficiency in lowland rice, but the mechanisms responsible for this effect are not yet fully understood. The objective of the present study was to test whether early effects of bicarbonate (HCO₃ ⁻)are inhibition of root growth due to the accumulation of organic acids induced by HCO₃ ⁻. Solution culture experiments were conducted using two rice cultivars differing in susceptibility to Zn deficiency, and four bicarbonate concentrations (0, 5, 10, 20 mM). Bicarbonate (5–20 mM) strongly inhibited root growth of the Zn-inefficient cultivar within 4 days of treatments. In contrast, root growth of the Zn-efficient cultivar was slightly stimulated with bicarbonate at 5–10 mM and not affected at 20 mM. The inhibitory effect of bicarbonate on root growth in the Zn-inefficient cultivar was mainly that of impairment of new root initiation rather than suppression of elongation of individual roots. Bicarbonate (5–20 mM) increased the concentrations of malate, succinate and citrate in the roots of both cultivars, but to a greater extent for the Zn-inefficient than for the Zn-efficient cultivars. The results suggest that the impairment of root growth was likely to be the initial action of bicarbonate in inducing Zn deficiency in lowland rice, and the inhibitory effect of bicarbonate on root growth of the Zn-inefficient cultivar might result from high accumulation and an insufficient compartmentation of organic acids in the root cells.     

Adaptation to sub- and supraoptimal temperatures of inbred maize lines differing in origin with regard to seedling development and photosynthetic traits

Six inbred lines of maize ( Zea mays L.) from cool temperate regions (C) and from warm regions (W) were grown at 14, 22, 30 and 38°C up to the same physiological age, the full expansion of the third leaf. Generally, plants developed smaller shoot dry weights and leaf areas at extreme temperatures. The shoot:root ratio was lowest at 22°C and highest at 30°C. Most lines had a minimum for specific leaf dry weight at 30°C, but W lines had a second lower minimum at 14°C. Phosphofructokinase activity scarcely reacted to temperature between 22° and 38°C; at 14°C one C line and all W lines had rather low activities. Generally, the chlorophyll content increased steeply from 14 to 22°C and decreased somewhat from 30 to 38°C. In C lines the carotenoid level decreased from 14 to 38°C. No uniform temperature response was found for PEP carboxylase activity, but the highest activity was mostly attained at 38°C. RuBP carboxylase activity increased considerably from 14 to 22°C and remained comparatively constant at higher temperatures. The highest activity of NADP malate dehydrogenase was found at 22°C, with a decrease up to 38°C and with second lowest values at 14°C. C lines possessed larger leaf areas, shoot dry weights and higher shoot:root ratios than W lines at 14 and 22°C, and higher specific leaf dry weights over the whole temperature range. The genotypic pattern of shoot dry weight at 14°C corresponded reasonably well with that of phosphofructokinase activity. A better adaptation of C lines to suboptimal temperatures was mostly clearly indicated for photosynthetic traits which have a well proven relationship with the chloroplast membranes: chlorophyll, carotenoids and RuBP carboxylase. The least distinct effects of origin were observed at 38°C; a tendency prevailed for a better performance of C lines with regard to phosphofructokinase, carotenoids, RuBP carboxylase and NADP malate dehydrogenase.     

Effect of thyroidectomy on testicular enzymes of the pyruvate/malate cycle involved in lipogenesis

The specific activities of testicular enzymes of the pyruvate/malate cycle involved in lipogenesis after thyroidectomy and thyroxine replacement were studied in prepubertal, pubertal and adult rats. Thyroidectomy induced testicular ATP citrate-lyase, malate dehydrogenase and malic enzyme activities and inhibited isocitrate dehydrogenase (NADP⁺) activity. Thyroxine treatment on thyroidectomized animals reverted all enzyme activities to normal. The result suggests that thyroid hormones have a differential effects on testicular enzymes of the pyruvate/malate cycle involved in lipogenesis.     

Properties and primary structure of a thermostable l-malate dehydrogenase from Archaeoglobus fulgidus

A thermostable l-malate dehydrogenase from the hyperthermophilic sulfate-reducing archaeon Archaeoglobus fulgidus was isolated and characterized, and its gene was cloned and sequenced. The enzyme is a homodimer with a molecular mass of 70 kDa and catalyzes preferentially the reduction of oxaloacetic acid with NADH. A. fulgidus l-malate dehydrogenase was stable for 5 h at 90° C, and the half-life at 101° C was 80 min. Thus, A. fulgidus l-malate dehydrogenase is the most thermostable l-malate dehydrogenase characterized to date. Addition of K₂HPO₄ (1 M) increased the thermal stability by 40%. The primary structure shows a high similarity to l-lactate dehydrogenase from Thermotoga maritima and gram-positive bacteria, and to l-malate dehydrogenase from the archaeon Haloarcula marismortui and other l-lactate-dehydrogenase-like l-malate dehydrogenases. Received: 20 November 1997 / Accepted: 28 February 1997