The CO2 concentration in Earth's atmosphere may double during this century. Plant responses to such an increase depend strongly on their nitrogen status, but the reasons have been uncertain. Here, we assessed shoot nitrate assimilation into amino acids via the shift in shoot CO2 and O2 fluxes when plants received nitrate instead of ammonium as a nitrogen source (deltaAQ). Shoot nitrate assimilation became negligible with increasing CO2 in a taxonomically diverse group of eight C3 plant species, was relatively insensitive to CO2 in three C4 species, and showed an intermediate sensitivity in two C3-C4 intermediate species. We then examined the influence of CO2 level and ammonium vs. nitrate nutrition on growth, assessed in terms of changes in fresh mass, of several C3 species and a Crassulacean acid metabolism (CAM) species. Elevated CO2 (720 micromol CO2/mol of all gases present) stimulated growth or had no effect in the five C3 species tested when they received ammonium as a nitrogen source but inhibited growth or had no effect if they received nitrate. Under nitrate, two C3 species grew faster at sub-ambient (approximately 310 micromol/mol) than elevated CO2. A CAM species grew faster at ambient than elevated or sub-ambient CO2 under either ammonium or nitrate nutrition. This study establishes that CO2 enrichment inhibits shoot nitrate assimilation in a wide variety of C3 plants and that this phenomenon can have a profound effect on their growth. This indicates that shoot nitrate assimilation provides an important contribution to the nitrate assimilation of an entire C3 plant. Thus, rising CO2 and its effects on shoot nitrate assimilation may influence the distribution of C3 plant species.
The shoots of cultivated tomato ( Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling temperatures of around 5 ∞ ∞ ∞ ∞ C. Under the same treatment, a chillingtolerant congener ( Lycopersicon hirsutum LA 1778) maintains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure chambers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, however, in stomatal behaviour: in L. hirsutum , stomatal conductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 ∞ ∞ ∞ ∞ C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differential stomatal behaviour during root chilling has distinct shoot and root components.
Many plants of tropical or subtropical origin, such as tomato, suffer damage under chilling temperatures (under 10 degrees C but above 0 degrees C). An earlier study identified several quantitative trait loci (QTLs) for shoot turgor maintenance (stm) under root chilling in an interspecific backcross population derived from crossing chilling-susceptible cultivated tomato (Lycopersicon esculentum) and chilling-tolerant wild L. hirsutum. The QTL with the greatest phenotypic effect on stm was located in a 28 cM region on chromosome 9 (designated stm 9), and enhanced chilling-tolerance was conferred by the presence of the Lycopersicon hirsutum allele at this QTL. Here, near-isogenic lines (NILs) were used to verify the effect of stm 9, and recombinant sub-NILs were used to fine map its position. Replicated experiments were performed with NILs and sub-NILs in a refrigerated hydroponic tank in the greenhouse. Sub-NIL data was analyzed using least square means separations, marker-genotype mean t-tests, and composite interval mapping. A dominant QTL controlling shoot turgor maintenance under root chilling was confirmed on chromosome 9 using both NILs and sub-NILs. Furthermore, sub-NILs permitted localization of stm 9 to a 2.7 cM interval within the original 28 cM QTL region. If the presence of the L. hirsutum allele at stm 9 also confers chilling-tolerance in L. esculentum plants grown under field conditions, it has the potential to expand the geographic areas in which cultivated tomato can be grown for commercial production.
A chilling episode of a few hours damaged root ammonium absorption in a cultivated tomato (Lycopersicon esculentum cv. T-5), but not in a wild congener from high altitudes (Lycopersicon hirsutum LA1778). In the cultivar, ammonium influx was strongly temperature dependent and showed the residual effects of chilling, whereas ammonium efflux was nearly temperature invariant and showed no persistent effects. A 2 h exposure to 5°C significantly depressed subsequent ammonium absorption at 20°C, and about 12 h at 20°C was required for recovery. For both the cultivated and wild species, rerooted cuttings were slightly less sensitive to chilling than seedlings. The relative inhibition (mean ± SE) of ammonium absorption before and after chilling was 58·4 ± 2·5% for the cultivated species and 29·0 ± 9·1% for the wild species. The F 1 hybrid between the species showed a relative inhibition of 52·4 ± 3·6%, suggesting that chilling sensitivity may be dominant. In a backcross of the hybrid to L. esculentum, the phenotypic distribution of the relative inhibition of ammonium absorption indicated that this trait is segregating.Key-words: root ammonium uptake; tomato chilling tolerance. INTRODUCTIONTomato (Lycopersicon esculentum Mill.) is a classic example of a chilling-sensitive plant. One or two nights of temperatures below 10°C severely inhibit the growth and development of tomato at all life stages, and one or two nights of temperatures below 6°C inflict significant injury (Geisenberg & Stewart 1986). By contrast, Lycopersicon hirsutum, an interfertile wild species that inhabits the Peruvian Andes at altitudes up to 3300 m, may encounter temperatures near or below freezing every night during its growing season (Patterson, Paull & Smillie 1978). Comparisons between these species show that L. hirsutum accessions from high altitudes thrive at chilling temperatures that prove detrimental to L. esculentum (Patterson et al. 1978;Dalziel & Breidenbach 1982;Vallejos & Tanksley 1983;Wolf et al. 1986; Vallejos & Pearcy 1987). The physiological bases for this differential chilling sensitivity remain uncertain although several hypotheses have been put forward (Bowers 1994;Guy 1994;Li 1994;Kaye & Guy 1995;Nishida & Murata 1996).One hypothesis is that chilling causes transitions in membrane lipids from a fluid phase to a gel phase that impair membrane function (Lyons & Raison 1970). Not only has this hypothesis been subject to much debate (Martin 1986;Raison & Lyons 1986;Nishida & Murata 1996) but, in this specific case, phase transitions in leaf membrane lipids from chilling-sensitive and -tolerant genotypes of L. esculentum and L. hirsutum occur at similar temperatures (Dalziel & Breidenbach 1982;Low et al. 1984;Marangoni & Stanley 1989;Raison & Brown 1989). X-ray diffraction measurements on tomato fruits showed only trace amounts of gel phase lipid even after 20 d at 5°C (Sharom, Willemot & Thompson 1994). These results indicate that bulk lipid-phase transitions are not a major factor in the short-term chilling injury of tomato.Other po...
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