The significance of photorespiration in drought-stressed plants was studied by withholding water from wild-type barley (Hordeum vulgare L.) and from heterozygous mutants with reduced activities of chloroplastic glutamine synthetase (GS-2), glycine decarboxylase (GDC) or serine : glyoxylate aminotransferase (SGAT). Wellwatered plants of all four genotypes had identical rates of photosynthesis. Under moderate drought stress (leaf water potentials between -1 and -2 MPa), photosynthesis was lower in the mutants than in the wild type, indicating that photorespiration was increased under these conditions. Analysis of chlorophyll a fluorescence revealed that, in the GDC and SGAT mutants, the lower rates of photosynthesis coincided with a decreased quantum efficiency of photosystem II and increased non-photochemical dissipation of excitation energy. Correspondingly, the de-epoxidation state of xanthophyll-cycle carotenoids was increased several-fold in the drought-stressed GDC and SGAT mutants compared with the wild type. Accumulation of glycine in the GDC mutant was further evidence for increased photorespiration in droughtstressed barley. The effect of drought on the photorespiratory enzymes was determined by immunological detection of protein abundance. While the contents of GS-2 and Pand H-protein of the GDC complex remained unchanged as drought stress developed, the content of NADH-dependent hydroxypyruvate reductase increased. Enzymes of the Benson-Calvin cycle, on the other hand, were either not affected (ribulose-1,5-bisphosphate carboxylase-oxygenase and plastidic fructose-1,6-bisphosphatase) or declined (sedoheptulose-1,7-bisphosphatase and NADPdependent glyceraldehyde-3-phosphate dehydrogenase). These data demonstrate that photorespiration was enhanced during drought stress in barley and that the control exerted by photorespiratory enzymes on the rate of photosynthetic electron transport and CO 2 fixation was increased.Key-words: drought stress; glutamine synthetase; glycine decarboxylase; hydroxypyruvate reductase; mutants; photorespiration; photosynthesis; serine : glyoxylate aminotransferase; xanthophyll cycle.Abbreviations: C i , intercellular CO 2 concentration; F v /F m , quantum efficiency of excitation capture by open photosystem II centres; FBPase, fructose-1,6-bisphosphatase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GDC, glycine decarboxylase; GS-2, chloroplastic glutamine synthetase; HPR, hydroxypyruvate reductase; PFD, photon flux density; ΦCO 2 , quantum efficiency of CO 2 assimilation; ΦPSII, quantum efficiency of photosystem II electron transport; ψ, water potential; q N , non-photochemical chlorophyll a fluorescence quenching; q P , photochemical chlorophyll a fluorescence quenching; RuBP, ribulose-1,5-bisphosphate; Rubisco, ribulose-1,5-bisphosphate carboxylase-oxygenase; SBPase, sedoheptulose-1,7-bisphosphatase; SGAT, serine : glyoxylate aminotransferase. INTRODUCTIONDrought stress leads to a substantial reduction in the rate of photosynthetic CO 2 assimilation. Under mild to...

The capture of photons by the photosynthetic apparatus is the first step in photosynthesis in all autotrophic higher plants. This light capture is dominated by pigmentcontaining proteins known as light-harvesting complexes (LHCs). The xanthophyll-carotenoid complement of these LHCs (neoxanthin, violaxanthin, and lutein) is highly conserved, with no deletions and few, uncommon additions. We report that neoxanthin, considered an integral component of LHCs, is stoichiometrically replaced by lutein-5,6-epoxide in the parasitic angiosperm Cuscuta reflexa, without compromising the structural integrity of the LHCs. Lutein-5,6-epoxide differs from neoxanthin in that it is involved in a light-driven deepoxidation cycle similar to the deepoxidation of violaxanthin in the xanthophyll cycle, which is implicated in protection against photodamage. The absence of neoxanthin and its replacement by lutein-5,6-epoxide changes our understanding of the structure-function relationship in LHCs, has implications for biosynthetic pathways involving neoxanthin (such as the plant hormone abscisic acid), and identifies one of the early steps associated with the evolution of heterotrophy from autotrophy in plants.

Cells capable of photosynthesis in the parasitic angiosperm Cuscuta re¯exa Roxb. (dodder) are highly localized. Immunolocalization of ribulose-1,5 bisphosphate carboxylase-oxygenase (Rubisco) and au-to¯uorescence of chlorophyll in transverse sections of stems showed that they were largely restricted to a band of cells adjacent to the vascular bundles, consequently, the concentrations of Rubisco and chlorophyll were low per unit area or fresh weight. When 14 CO 2 was supplied to stem segments of C. re¯exa it preferentially accumulated in these cells adjacent to the vasculature. Although the conductance for CO 2 movement to the cells containing chlorophyll and Rubisco was very low, both the light reactions and dark reactions of photosynthesis appeared to be functional. De-epoxidation of the xanthophyll-cycle pigments after exposure to high light, and the chlorophyll¯uorescence parameters, photochemical quenching (qP), non-photochemical quenching (NPQ) and the quantum eciency of photosystem II (/PSII) responded normally to changes in photon¯ux density, indicating functional light-driven electron transport. The response of CO 2 exchange to photon¯ux density followed a typical hyperbolic curve, and positive rates of CO 2 ®xation occurred when external CO 2 was increased to 5%. We propose that CO 2 for carbon assimilation is derived from internally respired CO 2 and that this layer of photosynthetic cells makes a positive contribution to the carbon budget of C. re¯exa. Abbreviations and symbol: NPQ non-photochemical quenching; qP photochemical quenching; /PSII quantum eciency of photosystem II Planta (1998) 205: 506±513

Despite the enormous diversity in plant form, structure and growth environment across the seed-bearing plants (angiosperms and gymnosperms), the chloroplast genome has, with few exceptions, remained remarkably conserved. This conservation suggests the existence of universal evolutionary selection pressures associated with photosynthesis-the primary function of chloroplasts. The stark exceptions to this conservation occur in parasitic angiosperms, which have escaped the dominant model by evolving the capacity to obtain some or all of their carbon (and nutrients) from their plant hosts. The consequence of this evolution to parasitism is a relaxation of the evolutionary constraints associated with the need to maintain photosynthetic function, the very function that drove early stages of the ancient symbiotic relationship that produced the contemporary chloroplast. Extreme examples of reductionism among parasitic angiosperms reveals major alterations in chloroplast function with the loss of photosynthetic capacity and, with that, massive alterations in chloroplast genome content. This review highlights emerging patterns in reported gene loss and gene retention in the chloroplast genomes of parasitic plants. Some gene losses appear to occur in the early stages of parasitic evolution, even before the loss of photosynthetic capacity, like the chlororespiratory (ndh) genes. This contrasts with unexpected gene retentions, like that of the rbcL gene responsible for photosynthetic carbon dioxide fixation, and belies current understanding of gene function. The review relates gene retention to current knowledge of protein function and gene processing that has implications to broader aspects of genome conservation in organelles.

Nitrogen assimilation was studied in the deciduous+ is assimilated in the absence of any NO 3 -assimilation. These data show that the potential for nitrate assimilation in C. vitalba is induced by a nitrogenous compound in the absence of its substrate and suggest that NO 3 -assimilation in C. vitalba may have a significant role beyond the supply of reduced N for growth.