A stimulation of an alternative photorespiratory CO₂ pathway by nitrogen in themeda triandra ‐ a possible explanation for its sensitivity to nitrogen

Abstract

Vacuum infiltration of inorganic nitrogen ions affected the photosynthetic rate and CO₂ compensation point of Themeda triandra and Zea mays in a similar way. However these two plants differed markedly in their photorespiratory mechanisms in response to inorganic nitrogen. Increased nitrogen levels in the in vitro reaction mixture stimulated T. triandra RuBP oxygenase activity (500%), but the similar enzyme from Z. mays showed no response to nitrogen. Using metabolic inhibitors it appears that glycolate and glyoxylate may serve as substrates for photorespiratory CO₂, producing formate. T. triandra leaf formate levels increased considerably with infiltration of increased inorganic nitrogen levels. This was not the case in Z. mays. In T triandra an alternative photorespiratory pathway appears to be operating under high nitrogen: this pathway would utilize inorganic nitrogen more efficiently with a greater loss of photorespiratory CO₂, resulting in decreased productivity. Z mays did not exhibit any of these responses to nitrogen, suggesting that the observed responses of T. triandra to nitrogen could in part account for its sensitivity to nitrogen inhibition of growth.

Uittreksel

Vakuum infiltrasie van anorganiese stikstof het die fotosintetiese aktiwiteit en CO₂ kompensasiepunt van Themeda triandra en Zea mays in die selfde manier verander. Maar die twee plante het verskillende reaksies tot anorganiese stikstof in hulle fotorespiratoriese meganismes getoon. RuBP oksigenase van ‘n T. triandra ekstrak is gestimuleer (500%) as stikstof by die in vitro buffer bygevoeg word. Met ‘n ekstrak van Z. mays was daar geen stimulering nie. Met die gebruik van metaboliese stremmers blyk dit dat glikolsuur en glioksaalsuur ook as fotorespiratoriese CO₂ substrate kan optree, met mieresuur as produk. Die mieresuur konsentrasie in T. triandra blare het gestyg met toenemende konsentrasies anorganiese stikstof infiltrasie. Dit was nie die geval in Z. mays nie. Met hoë konsentrasies stikstof lyk dit as of daar ‘n alternatiewe fotorespiratoriese pad in werking is: met dié pad word die anorganiese stikstof meer doeltreffend gebruik met ‘n groter verlies van fotorespiratoriese CO₂, wat ‘n vermindering in produktiwiteit sal veroorsaak. Z. mays het geen van bogenoemde reaksies met stikstof getoon nie, en dit lyk as of die reaksies van T. triandra met stikstof deels verantwoordelik kan wees vir sy sensitiwiteit tot stikstof groeibeperking.