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.].
The seasonal changes of photosynthesis of cones of Japanese larch (Larix kaempferi Carr.) trees showed that gross photosynthetic rate of young cones (G) was 2-3 µmol m-2 s-1 at surface area unit and PG / RD (dark respiration of cones) peaked about 0.7 in the same period, indicating that 70 % of respiratory CO2 was re-fixed. With maturation, PG and PG / RD sharply decreased. Chlorophyll content in cones was 3-20 % of that in leaves, which made it a limiting factor for photosynthesis and its content was closely correlated with photosynthetic capacity. Although sunken and linearly arranged stomatal organs were found on the scale of young cones, differently from the significant regulation of leaf photosynthesis, these stomata tended to be non-functional since CO2 is not limiting factor for cone photosynthesis. Thus photosynthesis of larch cones is an additional contribution to their development. and W.-J. Wang ... [et al.].
Twelve-year-old Norway spruce (Picea abies [L.] Karst.) trees were exposed to ambient (AC) or elevated (EC) [ambient + 350 µmol(CO2) mol-1] CO2 concentrations in open-top-chamber (OTC) experiment under the field conditions of a mountain stand. Short-term (4 weeks, beginning of the vegetation season) and long-term (4 growing seasons, end of the vegetation season) effects of this treatment on biochemical parameters of CO2 assimilation were evaluated. A combination of gas exchange, fluorescence of chlorophyll a, and application of a mathematical model of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity was used. The analysis showed that the depression of photosynthetic activity by long-term impact of elevated CO2 was mainly caused by decreased RuBPCO carboxylation rate. The electron transport rate as well as the rate of ribulose-1,5-bisphosphate (RuBP) formation were also modified. These modifications to photosynthetic assimilation depended on time during the growing season. Changes in the spring were caused mainly by local deficiency of nitrogen in the assimilating tissue. However, the strong depression of assimilation observed in the autumn months was the result of insufficient carbon sink capacity. and O. Urban, M. V. Marek.
CO2 exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000-2001 using a closed IRGA system. Stand CO2 uptake and release, soil respiration rate (Rs), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO2 uptake and release were -3.240 and 1.903 μmol m-2 s-1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum Rs were 1.470 and 0.226 μmol(CO2) m-2 s-1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than Rs, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type. and J. Balogh ... [et al.].
The effects of summer and winter stress on the chlorophyll and carotenoid contents and photosystem 2 efficiency were examined in six Mediterranean scrub species. These six species belong to two different plant functional types: drought semi-deciduous (Halimium halimifolium L., Rosmarinus officinalis L., Erica scoparia L.) and evergreen sclerophylls (Juniperus phoenicea L., Pistacia lentiscus L., Myrtus communis L.). Two sites with different water availability were chosen. In the xerophytic site, despite they belong to two different functional types, R. officinalis and J. phoenicea showed a similar response. These were the most affected species in summer. H. halimifolium showed optimal values of Fv/Fm and non-significant seasonal changes in xanthophyll content. In the mesic site, E. scoparia and M. communis were apparently the most affected species by winter climatic conditions. P. lentiscus presented a pattern similar to H. halimifolium, except for elevated F0 values. In all the studied species, lutein plus zeaxanthin content was negatively correlated with Fv/Fm in summer and with leaf water potential, thus indicating that the thermal dissipation of energy was a general pattern for all species. Under stress, plant response is more species-specific than dependent on its functional type. and F. Ain-Lhout ... [et al.].
Seasonal changes in leaf gas exchange, assimilation response to light and leaf area were monitored in bearing and nonbearing pistachio shoots. Shoot bearing status did not directly affect leaf photosynthetic rate. However, photosynthetic light-response curves strongly varied during the season demonstrating the dominant effect of the tree’s seasonal phenology on assimilation. Early in the season low photosynthetic rates were associated with high rates of dark respiration indicating limited photosynthesis in the young leaves. As leaves matured, dark respiration decreased and assimilation reached maximum values. Photosynthetic efficiency was strongly reduced late in the season due to leaf age and senescence. Fruit load precipitated an early leaf senescence and drop that resulted in a 53% decrease in leaf area in bearing vs. nonbearing shoots, strongly decreasing the seasonal photosynthetic performance of bearing shoots. Bearing shoots produced a 26% lower seasonal carbon gain compared to nonbearing shoots., G. Marino, M. La Mantia, T. Caruso, F. P. Marra., and Obsahuje bibliografii
Photochemical efficiency of photosystem 2 (PS2), assessed from in situ chlorophyll (Chl) fluorescence measurements, was seasonally monitored in five evergreen sclerophyll and five malacophyllous drought semi-deciduous species, co-occurring in the same Mediterranean field site. In evergreen sclerophylls, a considerable drop in the variable (Fv) to maximum (Fm) Chl fluorescence ratio coincided with the lowest winter temperatures, indicating low PS2 efficiency during this period. Summer drought caused a comparatively slight decrease in Fv/Fm and only in three of the five evergreen sclerophyll species tested. In drought semi-deciduous shrubs, the winter drop in Fv/Fm was much less conspicuous. During the summer, and in spite of the severe and prolonged desiccation of their malacophyllous leaves, Fv/Fm was maintained high and only in one species the PS2 efficiency was transiently suppressed, when the leaf relative water content became lower than 30 %. Thus evergreen sclerophylls are more prone to photoinhibition by low winter temperatures, while the sensitivity of drought semi-deciduals depends on the extent and duration of summer drought. and S. Karavatas, Y. Manetas.
Some photosynthetic attributes of leaves and stems were seasonally followed in the small-leaved, summer-deciduous, green-stemmed Mediterranean shrub Calicotome villosa. Both leaves and stems displayed similar photon energy-saturated photosystem 2 (PS2) efficiencies with a minimum during winter. A second minimum in stems during the leafless summer period could be ascribed to sustained photoinhibition. Yet, stems were slightly inferior in photon capture, resulting partly from lower chlorophyll (Chl) contents and partly from higher reflectance due to pubescence. As a result, photon energy-saturated linear electron transport rates were slightly higher in leaves. However, when the total leaf and stem areas were taken into account, this superiority was abolished during autumn and winter and more than overturned during spring. Given that during summer the stems were the only photosynthetic organs, the yearly photosynthetic contribution of stems was much higher. Chl contents in stems displayed a transient and considerable summer drop, accompanied by an increase in the carotenoid to Chl ratio, indicating a photo-protective adaptation to summer drought through a decrease of photo-selective capacity, typical for leaves of many Mediterranean plants. and C. Yiotis, G. K. Psaras, Y. Manetas.
Leaf chlorophyll (Chl) content, net photosynthetic rate (P^) and biomass production were estimated at monthly intervals in Chukrasia tabularis, Dolichandrone atrovirem, Eugenia Jambolana, Gmelina arborea, Lannea coromandelica, Terminalia arjuna and Terminalia bellerica from September 1990 to August 1991. The leaves of all the seven tree species showed significantly higher Chl content during summer than in winter, when it was expressed on area and/or fresh mass basis. Chl concentration showed marked variations from month to month and these differed from one plant species to the other. Clear positive correlations were found between Chl content and and/or biomass production in all the seven deciduous forest tree species studied. The scope for screening and selecting the right type of tree species prospective for energy plantations can be based on a simple technique such as estimation of leaf Chl content and PN.
Seasonal variations in photosynthesis of cassava cv. Rayong 9 (RY9) under irrigated and rain-fed conditions were evaluated at the age of three and six months after planting (MAP). Photosynthetic light-response (PN/I) curves revealed that cassava leaves attained the highest maximum net photosynthetic rates (PNmax) in the rainy season, followed by the hot one, while the lowest PNmax was found in the cool season. Photosynthetic potential of the 3-month-old plants was mostly higher than that of the 6-month-old plants, and the seasonal variation in photosynthetic capacity was also more apparent in the younger plants. PN/I curves were used to predict daily net photosynthetic rate (PN) for each season based on daily average solar radiation data. The predicted PN were considerably lower than the PNmax values. This indicated that solar radiation is a limiting factor for photosynthesis, particularly in the rainy season. The data provided basic information for breeding cassava genotypes with enhanced photosynthesis during the period of unfavorable environment. Furthermore, the data are potentially useful in modeling photosynthesis and crop growth as affected by environmental factors., K. Vongcharoen, S. Santanoo, P. Banterng, S. Jogloy, N. Vorasoot, P. Theerakulpisut., and Obsahuje bibliografii