The contribution of glycolate as a precursor of oxalate in newly expanding spinach leaves was compared with that of L-ascorbic acid (ASA). Detached spinach leaves were fed with [2-14C]glycolate and [1-1~C]ASA. Using the values of the rate of photorespiratory glycolate synthesis and the incorporation of glycolate-~C into oxalate, the rate of oxalate biosynthesis via glycolate amounted to 34 ~g g-1 fr wt h -1 under light conditions. When the values of incorporation of ASA-14C into oxalate and the turnover rate of ASA were used, the rate of oxalate biosynthesis via ASA in light and darkness, amounted to 1.6 and 2.9 #g g-1 fr wt h -1, respectively. Glycolate was found to be more efficient as a precursor of oxalate than ASA in newly expanding spinach leaves.Key Words: glycolate, L-ascorbic acid, oxalate biosynthetic rate, photorespiration, spinach.Spinach is an edible plant species belonging to the family Chenopodiaceae. Oxalate accumulates in spinach leaves and a high-oxalate diet can increase the risk of formation of calcium oxalate stones in kidneys (Hodgkinson 1978) and may also affect calcium absorption (Liebman and Doane 1989). Plants which produce calcium oxalate are known to induce a painful sensation in the mouth when eaten (Perera et al. 1990). Oxalate may, therefore, contribute to the harsh taste of spinach and it is thus necessary to produce spinach with a low oxalate content. Many attempts have been made to reduce the level of oxalate in spinach leaves with regard to nitrogen and inorganic ion availability as well as other growth conditions (Watanabe et al. 1987;lwanami 1989;Zhang et al. 1990). However, oxalate biosynthesis has not been well documented. We therefore investigated the biosynthesis of oxalate to obtain information about the possibility of reducing the production of oxalate in spinach. Glyoxylate, glycolate, and ASA are the precursors of oxalate as indicates by feeding experiments in many plants (Chang and Beevers 1968;Seal and Sen 1970;Yang and Loewus 1975;Williams et al. 1979;Chang and Huang 1981). The major part of glyoxylate may be produced from glycolate in the peroxisomes in green spinach leaves. The presence of a barrier to glyoxylate transportation in peroxisomal membranes, and compartmentation have been reported (Liang and Huang 1983;Heupel et al. 1991). As external application of glyoxylate for metabolic studies is not suitable, we compared the contribution of glycolate Abbreviations: ASA, k-ascorbic acid; fr wt, flesh weight; mol wt, molecular weight. or ASA as a precursor of oxalate biosynthesis in spinach leaves. MATERIALS AND METHODSUsing the values of the rate of photorespiratory glycolate synthesis and the incorporation of glycolate-14C into oxalate, the rate of oxalate biosynthesis via glycolate was calculated as follows:Rate of glycolate synthesisX incorporation of glycolate-14C into oxalate X tool wt of oxalate/mol wt of glycolate, (formula 1) and the following formula was used to calculate oxalate biosynthesis via ASA using the values of ASA-~4C incorporation into oxalate...
Two different kinds of pits in InGaN/GaN MQWs were observed. They are generated by In atoms in the InGaN quantum well layers migrating in opposite directions, which may imply different mechanisms for the pit formation.InGaN multi-quantum wells (MQWs) are used widely in opto-electric devices, such as blue and green light emitting diodes (LEDs) and ultraviolet laser diodes (LDs). But high quality materials are difficult to grow because of the large lattice mismatch between the epilayer and the substrate, especially the high-density native defects, dislocations and domain boundaries. These defects have given rise to detailed investigations, including the effect on the electrical and optical properties. Recently, a kind of pit defect with hexahedral cone morphology was found in InGaN/GaN MQWs and GaN epilayers [1 to 3]. In this research, we focused on the hexahedral cone defects and their effects on the segregation of In and the optical properties by cathodoluminescence (CL) spectra, CL images and high-resolution scanning electron microscopy (HRSEM).The samples used in this research were grown on sapphire substrates with (0001) orientation by low pressure (0.1 atm) MOCVD. High purity H 2 and N 2 were used as carrier gases. The main procedures of the material growth were the following. Firstly, the substrate was heated to 1150 C under the stream of H 2 for 10 min. Then, the temperature was decreased to 550 C for growing a 20 nm GaN buffer layer. After this, the growth temperature was elevated to 1060 C for growing n-type GaN layers with Si doping and then InGaN/GaN MQW layers with three periods were grown on the GaN layer at a growth temperature of 760 C. Finally, the p-type Al 0.1 Ga 0.9 N and GaN top layers were grown with Mg doping at growth temperature of 1060 C. The TMGa, TMIn, NH 3 , Cp 2 Mg and SiH 4 (10 ppm in H 2 ) were used as Ga, In, N, Mg and Si sources, respectively. The typical growth rate is about 1.2 to 1.3 mm/h. From the relationship of the position of satellites and the thickness of strained quantum wells and barriers, the period D = d w + d b of the MQWs was about 7.7 nm. According to the nominal thickness of GaN and In 0.18 Ga 0.82 N monolayers (half of c-axis length) [4], the Z. J. Yang et al.: Effect of Pits in InGaN/GaN Multi-Quantum Wells 81 1 )
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