Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

Wheat plants were grown in a controlled environment with daytemperatures of 18 ?C and with 500 µ Einsteins m^–28^–1 of photosynthetically active radiation for 16 h. Beforeanthesis and 2 to 3 weeks after, rates of net photosynthesiswere measured for leaves in 2 or 21% O₂ containing 350 vpm CO₂at 13, 18, 23, and 28 ?C and with 500 µEinsteins m^–2s^–1 of photosynthetically active radiation. Also, underthe same conditions of light intensity and temperature, therates of efflux of CO₂ into CO₂-free air were measured and,for mature flag leaves 3 to 4 weeks after anthesis, gross andnet photosynthesis from air containing 320 vpm ¹⁴CO₂ of specificactivity 39?7 nCi µmol^–1. When the O₂ concentration was decreased from 21 to 2% (v/v)the rate of net photosynthesis increased by 32 per cent at thelowest temperature and 54 per cent at the highest temperature.Efflux of CO₂ into CO₂-free air ranged from 38 per cent of netphotosynthesis at 13 ?C to 86 per cent at 28 ?C. Gross photosynthesis,measured by the ¹⁴C assimilated during 40 s, was greater thannet photosynthesis by some 10 per cent at 13 ?C and 17 per centat 28 ?C. These data indicate that photorespiration was relativelygreater at higher temperatures.     

Ribulose-1,5-Diphosphate Carboxylase Protein during Flag Leaf Senescence

Ribulose-l,5-diphosphate (RuDP) carboxylase protein and activitywere determined in relation to net photosynthetic rate duringthe senescence of intact flag leaves of wheat on the plant.Initially the decrease in RuDP carboxylase activity was greaterthan the decline in net photosynthesis. The major decrease inRuDP carboxylase activity over this period resulted from a decreasein enzyme specific activity from 11 to 2 µmol CO₂ fixedh^–1 mg^–1 protein. Loss of RuDP carboxylase proteindid not occur until late in senescence by which time chlorophyllconcentration had decreased by more than 50%. Treatment of flagleaves at weekly intervals with either 1000 parts 10^–62-chloro-ethyltrimethylammonium chloride or 100 parts 10^–6gibberellic acid with 1 part 10^–6 kinetin did not significantlyaffect net photosynthetic rate, RuDP carboxylase protein oractivity during senescence.     

The Mechanism of Photosynthesis in the Tea Plant (Camellia sinensis L.)

Leaves of the tea plant photosynthesizing in ¹⁴CO₂ incorporatedmuch radioactivity into intermediates of the glycolate pathwayand little into C₄ acids. Increased O₂ in the atmosphere decreasedphotosynthesis, stimulated photorespiration, and increased theCO₂ compensation point. In air the rate of photorespirationwas 19% of net photosynthesis. These observations indicate aC₃ rather than a C₄ mechanism of photosynthesis.     

Chimaeras of the Rubisco Large Subunit from Wheat and Anacystis do not Assemble into Active Enzyme in E. coli

A new restriction site was engineered in the cloned gene codingfor the large subunit polypeptide of ribulose 1, 5-bisphosphatecarboxylase (Rubisco) of the cyanobacterium Anacystis nidulans.This change resulted in the mutation of a phenylalanine residueto an isoleucine residue in the encoded polypeptide but hadno effect on the assembly or biochemical properties of Rubiscocontaining the polypeptide. The mutation was in a loop regionlinking highly structured domains at the N and C termini ofthe complete large subunit. Using the new restriction site, and a corresponding EcoRl restrictionsite in the cloned gene for the native large subunit polypeptideof wheat Rubisco, chimaeric genes were made encoding the polypeptidewith either the 140 residues of the N-terminal part of the wheatlarge subunit fused to the 336 residues forming the C-terminalregion of the A. nidulans large subunit, or the alternativeof 136 residues comprising of the N-terminal chains of A. nidulanssubunit and the 338 residue chain at the C-terminus of the wheatlarge subunit polypeptide. The chimaeric proteins expressedin E. coli, together with the small subunit of the A. nidulansRubisco, formed an insoluble inactive aggregate mainly in inclusionbodies. The possible reasons for the failure to obtain activeholoenzyme are discussed. Key words: Rubisco, protein engineering, site-directed mutagenesis     

The Soluble Nucleotide Content of Germinating Wheat Embryos

Estimates are given of the amounts of adenine, uracil, guanine,and cytosine present in the soluble nucleotides of wheat embryosduring germination in the dark. After 48 hours at 25° Cthe nucleotide content per gramme of dry tissue reaches a maximumlevel when the content of bases is approximately 3.5 µmolesof uracil, 2.8 µmoles of adenine, 0.6 µmoles ofguanine, and 0.5 µmoles of cytosine. The soluble nucleotidecontent of embryos growing at 0 to 5° C is lower than thatof embryos of the same dry weight grown at 25° C. Initiallythe amount of adenine present is greater than the amount ofuracil but after 20 hours of germination at 25° C uracilbecomes the predominant base in the soluble nucleotide fraction.Ion-exchange resin chromatography was used to separate the chiefsoluble nucleotides present in the extract of embryos grownfor four days at 25° C. Uridine diphosphate glucose accountsfor the major part of the uracil and adenosine monophosphatefor most of the adenine. Nicotinamide-adenine dinucleotide wasdetected and identified and also a little uridine 5'-mono-phosphate.The possibility is discussed that most of the adenosine monophosphateis produced by degradation of reduced nicotinamide-adenine dinucleotidephosphate.