In many plant species that remain leafless part of the year, CO2 fixation occurring in green stems represents an important carbon gain. Traditionally, a distinction has been made between stem photosynthesis and corticular photosynthesis. All stem photosynthesis is, sensu stricto, cortical, since it is carried out largely by the stem cortex. We proposed the following nomenclature: stem net photosynthesis (SNP), which includes net CO2 fixation by stems with stomata in the epidermis and net corticular CO2 fixation in suberized stems, and stem recycling photosynthesis (SRP), which defines CO2 ling in suberized stems. The proposed terms should reflect differences in anatomical and physiological traits. SNP takes place in the chlorenchyma below the epidermis with stomata, where the net CO2 uptake occurs, and it resembles leaf photosynthesis in many characteristics. SRP is found in species where the chlorenchyma is beneath a
well-developed stomata-free periderm and where reassimilation of internally respired CO2 occurs. SNP is common in plants from desert ecosystems, rates reaching up to 60% of the leaf photosynthetic rate. SRP has been demonstrated in trees from temperate forests and it offsets partially a carbon loss by respiration of stem nonphotosynthetic tissues. Reassimilation can vary between 7 and 123% of respired CO2, the latter figure implying net CO2 uptake from the atmosphere. Both types of stem photosynthesis contribute positively to the carbon economy of the species, in which they occur; they are advantageous to the plant because they allow the maintenance of physiological activity during stress, an increase of integrated water use efficiency, and they provide the carbon source used in the production of new organs., E. Ávila, A. Herrera, W. Tezara., and Obsahuje bibliografii
In the terrestrial bromeliad, Puya floccosa, a value of carbon isotopic composition (δ13C) of -22‰ has been previously reported, suggesting the operation of weak and/or intermediate (C3-CAM) crassulacean acid metabolism (CAM). In order to characterize the operation of CAM in P. floccosa and its possible induction by drought, plants were grown in Caracas and subjected to four independent drought cycles. Additionally, since plants of this species grow in Venezuela in a large range of elevations, leaf samples were collected at elevations ranging from 725 to 2,100 m a.s.l. in the Venezuelan Andes and the Coastal Range, in order to evaluate the effect of elevation on CAM performance. Even though nocturnal acid accumulation occurred in both watered and droughted plants, mean ΔH+ was higher in droughted than watered plants [ΔH+ = 60.17.5 and 22.9 ± 5.2 μmol g-1(FM), respectively]. The majority of plants from all the natural populations sampled had low values of δ13C not differing significantly from those of C3 plants collected as standards and δ13C did not change with elevation. We conclude that P. floccosa is capable of a weak CAM activity, with a large variability among populations and drought experiments probably due to local and temporal differences in microclimatic variables and drought stress; elevation bears no influence on values of δ13C in this species. and A. Herrera ... [et al.].
In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (PN) and stomatal conductance (gs) of attached leaves decreased as leaf water potential (Ψw) declined from -0.3 to -2.9 MPa. Although gs decreased over the whole range of Ψw, nearly constant values in the intercellular CO2 concentrations (Ci) were observed as Ψw decreased to -1.8 MPa, but Ci increased as Ψw decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (qNP) increased progressively. A highly significant negative relationship between qNP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 µmol m-2 s-1, the allocation of electrons from photosystem (PS) 2 to O2 reduction was increased by 51 %, while the allocation to CO2 assimilation was diminished by 32 %, as Ψw declined from -0.3 to -2.9 MPa. A significant linear relationship between mean PN and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas qP declined only at very severe stress and the excess photon energy was dissipated by increasing qNP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of qNP against photoinhibition in sunflower. and W. Tezara, S. Driscoll, D. W. Lawlor.
Seasonal changes in water relations, net photosynthetic rate (PN), and fluorescence of chlorophyll (Chl) a of two perennial C3 deciduous shrubs, Ipomoea carnea and Jatropha gossypifolia, growing in a thorn scrub in Venezuela were studied in order to establish the possible occurrence of photoinhibition during dry season and determine whether changes in photochemical activity of photosystem 2 (PS2) may explain variations of PN in these species. Leaf water potential (ψ) decreased from -0.2 to -2.1 MPa during drought in both species. The PN decreased with ψ in I. carnea and J. gossypifolia by 64 and 74 %, respectively. Carboxylation efficiency (CE) decreased by more than 50 and 70 % in I. carnea and J. gossypifolia, respectively. In I. carnea, relative stomatal limitation (Ls) increased by 17 % and mesophyll limitation (Lm) by 65 % during drought, while in J. gossypifolia Ls decreased by 27 % and Lm increased by 51 %. Drought caused a reduction in quantum yield of PS2 (ϕPS2) in both species. Drought affected the capacity of energy dissipation of leaves, judging from the changes in the photochemical (qP) and non-photochemical quenching (NPQ) coefficients. Photoinhibition during drought in I. carnea and J. gossypifolia was evidenced in the field by a drop in the maximum quantum yield of PS2 (Fv/Fm) below 0.8 and also by non-coordinated changes in ϕPS2 and quantum yield of non-photochemical excitation quenching (Yn). Total soluble protein content on an area basis increased with ψ but the ribulose-1,5-bisphosphate carboxylase/oxygenase content remained unchanged. A reduction of total Chl content with drought was observed. Hence in the species studied photoinhibition occurred, which imposed an important limitation on carbon assimilation during drought. and W. Tezara ... [et al.].
We studied the responses of leaf gas exchange and growth to an increase in atmospheric CO2 concentration in four tropical deciduous species differing in carbon fixation metabolism: Alternanthera crucis, C3-C4; Ipomoea carnea, C3; Jatropha gossypifolia, C3; and Talinum triangulare, inducible-CAM. In the first stage, plants were grown in one open-top chamber at a CO2 concentration of 560±40 μmol mol-1 (EC), one ambient CO2 concentration chamber (AC), and one unenclosed plot (U). In the second stage, plants were grown in five EC chambers (CO2 concentration = 680±30 μmol mol-1), five AC chambers, and five unenclosed plots. During the first weeks under EC in the first stage, plants of all the species had a very marked increase in their maximal net photosynthetic rates (Pmax) of 3.5 times on average; this stimulatory effect was maintained for 11-15 weeks, rates dampening afterward to values still higher than controls for 37 weeks. After a suspension of CO2 enrichment for 6 weeks, an increase in Pmax of EC plants over the controls was found in plants of all the species until week 82 of the experiment. Stomatal conductance (g) showed no response to EC. Carboxylation efficiency decreased in all the species under EC and this was correlated with a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) content in all the species except for T. triangulare. During drought Pmax was higher in all species except for 7 triangulare, grown under EC relative to controls.
Ecosystem photosynthetic rates at EC were higher than in the controls during the second stage under irrigation as well as after 30 d of drought. and M. D. Fernández ... [et al.].
In order to determine whether stomatal closure alone regulates photosynthesis during drought under natural conditions, seasonal changes in leaf gas exchange were studied in plants of five species differing in life form and carbon fixation pathway growing in a thorn scrub in Venezuela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 deciduous shrubs), Alternanthera crucis (C4 deciduous herb), and Prosopis juliflora and Capparis odoratissima (evergreen phreatophytic trees). Xylem water potential (Ψ) of all species followed very roughly the precipitation pattern, being more closely governed by soil water content in I. carnea and A. crucis. Maximum rate of photosynthesis, Pmax, decreased with Ψ in I. carnea, J. gossypifolia, and A. crucis. In I. carnea and J. gossypifolia stomatal closure was responsible for a 90 % decline in net photosynthetic rate (PN) as Ψ decreased from -0.3 to -2.0 MPa, since stomatal conductance (gs) was sensitive to water stress, and stomatal limitation on PN increased with drought. In A. crucis, PN decreased by 90 % at a much lower Ψ (-9.3 MPa), and gs was relatively less sensitive to Ψ. In P. juliflora and C. odoratissima, Pmax, gs, and intercellular CO2 concentration (Ci) were independent of soil water content. In the C3 shrubs stomatal closure was apparently the main constraint on photosynthesis during drought, Ci declining with Ψ in I. carnea. In the C4 herb, Ci was constant along the range of Ψ values, which suggested a coordinated decrease in both gs and mesophyll capacity. In P. juliflora Ci showed a slow decrease with Ψ which may have been due to seasonal leaf developmental changes, rather than to soil water availability. and W. Tezara ... [et al.].
We studied the seasonal changes in water relations, chlorophyll a fluorescence, and leaf saccharide contents of the tropical flood-tolerant trees Acosmium nitens, Campsiandra laurifolia, Eschweilera tenuifolia, Symmeria paniculata, and Psidium ovatifolium. Xylem water potential increased with flooding to a larger extent than leaf sap osmotic potential in all the species, and soluble sugars contributed up to 66 % of osmotic potential at maximum flooding. Starch was accumulated in leaves. Maximum quantum yield of photosystem 2 decreased in emerged leaves, values being always higher than 0.76. Daily maximum net photosynthetic rate and leaf conductance decreased in all the species. This reduction was associated in all the species but S. paniculata with the absence of a compensatory increase in non-photochemical quenching. and E. Rengifo, W. Tezara, A. Herrera.