Environmental stresses such as drought (Lauteri et al., 1997), salt stress (Bongi and Loreto, 1989), and leaf aging (Loreto et al., 1994) reduce conductance to CO 2 diffusion in the leaf mesophyll (mesophyll conductance). No information exists about possible increases of mesophyll conductance, such as when the stresses are alleviated. One obstacle to the investigation of this possibility is that mesophyll conductance reduction is frequently associated with the impairment of biochemical and photochemical characteristics of the leaf. The former is generally permanent, whereas the latter may recover slowly. However, it was recently shown that low salt accumulation (leaf Na concentration less than 15 mg g Ϫ1 ) primarily affects the conductance to CO 2 diffusion in spinach (Spinacia oleracea L.) leaves (Delfine et al., 1998). A coordinate reduction in stomatal and mesophyll conductance decreased the chloroplast CO 2 concentration of salt-stressed spinach. This, in turn, caused an inhibition of photosynthesis that was not associated with changes in biochemical or photochemical capacity when salt accumulation in the leaves was two to three times that of the controls.Mesophyll conductance reduction is also frequently associated with changes in leaf anatomy (Longstreth and Nobel, 1979;Bongi and Loreto, 1989;Evans et al., 1994;Syvertsen et al., 1995). This is likely to be a permanent effect, at least when leaf thickness is involved. However, low salt accumulation did not increase but slightly decreased the thickness of spinach leaves (Delfine et al., 1998). On the other hand, salt accumulation caused a 25% reduction of the intercellular spaces in the mesophyll of spinach leaves with respect to the controls. This could have caused a more tortuous path for CO 2 directed toward the chloroplast and was suggested to be responsible for the observed photosynthesis reduction associated with low mesophyll conductance in salt-stressed leaves (Delfine et al., 1998).The objectives of this work were to understand, under conditions that do not affect relevantly the biochemical and photochemical capacity of salt-stressed leaves, and are not able to change leaf anatomy significantly: (a) whether the reduction of mesophyll conductance can be reversed by alleviating the salt stress, and (b) how important changes in mesophyll conductance are in determining photosynthesis limitation. MATERIALS AND METHODS Plant Material and Experimental ConditionsFour groups of 30 spinach (Spinacia oleracea L. cv Matador) plants were grown in 3-dm 3 pots containing a mixture of soil, peat, and sand (1:1:1). When five to six leaves were fully expanded, the first group of plants (control) was grown under optimal water conditions by daily restoring the water lost through evapotranspiration. Evapotranspiration was estimated by weighing the pots daily. The second group of plants (salt stressed) was irrigated for 50 d with saline water (containing 1% [w/v] NaCl) when evapotranspiration was restored. The third group of plants (rewatered) was irrigated with saline w...
Spinach (Spinacia oleracea L.) leaves stressed by irrigation with water containing 1% (w/v) NaCl for 20 days had low conductance to CO2 diffusion both at the stomata and in the mesophyll. Mesophyll anatomy changed in salt-stressed leaves, which could have accounted for the decreased mesophyll conductance. Ribulose- 1,5-bisphosphate carboxylase/oxygenase in vitro activity and content were not affected by up to 20 days exposure to salinity but decreased when leaves were exposed to salt stress for longer than 20 days. Salt accumulation also caused a drop of Ca and Mg which might have decreased membrane stability and chlorophyll content, respectively. Measurements of chlorophyll fluorescence indicated that the 20-day-long salt stress did not directly affect photochemistry. We conclude that salinity reduces photosynthesis primarily by reducing the diffusion of CO2 to the chloroplast, both by stomatal closure and by changes in mesophyll structure which decreases the conductance to CO2 diffusion within the leaf. The capacity for carbon metabolism is eventually reduced but that occurs after substantial decreases in the conductance to CO2 diffusion.
Among European Neottieae, Limodorum abortivum is a common Mediterranean orchid. It forms small populations with a patchy distribution in woodlands, and is characterized by much reduced leaves, suggesting a partial mycoheterotrophy. We have investigated both the photosynthetic abilities of L. abortivum adult plants and the diversity of mycorrhizal fungi in Limodorum plants growing in different environments and plant communities (coniferous and broadleaf forests) over a wide geographical and altitudinal range. Despite the presence of photosynthetic pigments, CO2 fixation was found to be insufficient to compensate for respiration in adult plants. Fungal diversity was assessed by morphological and molecular methods in L. abortivum as well as in the related rare species Limodorum trabutianum and Limodorum brulloi. Phylogenetic analyses of the fungal internal transcribed spacer (ITS) sequences, obtained from root samples of about 80 plants, revealed a tendency to associate predominantly with fungal symbionts of the genus Russula. Based on sequence similarities with known species, most root endophytes could be ascribed to the species complex encompassing Russula delica, Russula chloroides, and Russula brevipes. Few sequences clustered in separate groups nested within Russula, a genus of ectomycorrhizal fungi. The morphotypes of ectomycorrhizal root tips of surrounding trees yielded sequences similar or identical to those obtained from L. abortivum. These results demonstrate that Limodorum species with inefficient photosynthesis specifically associate with ectomycorrhizal fungi, and appear to have adopted a nutrition strategy similar to that known from achlorophyllous orchids.
Summary• The aim of the paper was to determine nitrogen compounds contributing to leaf cell osmoregulation of spinach ( Spinacia oleracea ) submitted to increasing salt stress.• Sodium, free amino acids and glycine betaine contents were determined in the last fully expanded leaf of plants stressed by daily irrigation with saline water (0.17 M NaCl).• After 20 d of treatment, when Na + content was c . 55 umol g − 1 f. wt above the control, and the reduction of stomatal conductance lowered photosynthesis to c . 60% of the control, the free amino acids of the leaves, especially glycine and serine, strongly increased. Proline and glycine betaine also increased significantly. After 27 d of treatment, when the Na + content was c . 100 umol g − 1 f. wt above the control and photosynthesis was 33% of the control, the free amino acid content, especially glycine and serine, declined. Gycine betaine, but not proline, increased further.• Glycine betaine comprised c . 15% of the overall nitrogen osmolytes at mild saltstress, but represented 55% of the total, when the stress became more severe. The increase of glycine betaine balanced the decline in free amino acids, mainly replacing glycine and serine (the precursors of glycine betaine) in the osmotic adjustment of the cells. Photorespiration, which increased during salt stress, was also suggested to have a role in supplying metabolites to produce compatible osmolytes.
-The aim of this study was to monitor the effect of foliar application of humic acid on plant growth, photosynthetic metabolism and grain quality of durum wheat grown in a Mediterranean-type climate. Four fertilization treatments were applied: a non-fertilized control, a crop fertilized with foliar application of humic acid, a crop fertilized with mineral N on soil at sowing, tillering and stem elongation, and a crop fertilized with foliar application of N (ammonium-nitrate solution). Aboveground plant mass accumulation was measured throughout two growing seasons, and grain quality parameters were tested at harvest time. Gas exchange, leaf protein content and Rubisco activity were monitored at different stages of plant development. Differences between years were often relevant due to weather conditions. The foliar application of humic acid caused a transitional production of plant dry mass with respect to unfertilized control and split soil N application. This effect was also evident for grain yield, spike fertility and grain protein content during the two years of the study. Humic acid never affected photosynthesis or stomatal conductance, while Rubisco activity and leaf protein content showed intermediate responses between unfertilized control and split soil N application. We conclude that humic acid had limited promoting effects on plant growth, grain yield and quality, and photosynthetic metabolism of durum wheat crops grown in a typical Mediterranean-type agro-ecosystem of southern Italy, with respect to split soil N application.humic acid / durum wheat / gas exchange / nitrogen / protein / Rubisco / grain quality
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