Two weeks-old maize (Zea mays cv. XL-72.3) plants were exposed to Al concentrations 0 (Al0), 9 (Al9), 27 (Al27) or 81 (Al81) g m-3 for 20 d in a growth medium with low ionic strength. Thereafter, the Al concentration-dependent interactions on root nitrate uptake, and its subsequent reduction to ammonia in the leaves were investigated. Al concentrations in the roots sharply increased with increasing Al concentrations while root elongation correspondingly decreased. Root fresh and dry masses, acidification capacity, and nitrate and nitrogen contents decreased from Al27 onwards, whereas leaf nitrogen, nitrate, nitrite, and ammonia concentrations decreased starting with Al9. Electrolytic conductance increased by 60 % in root tissues from Al0 to Al81 but it did not increase significantly in the leaves. In Al9, Al27, and Al81 plants a decrease in shoot fresh and dry masses was observed. Al concentrations between 0 and 27 g m-3 increased net photosynthetic rate, stomatal conductance, and the quantum yield of photosynthetic electron transport, whereas the intercellular CO2 concentration was minimum in Al27 plants. In the leaves, nitrate reductase (E.C. 1.6.6.1) activity increased until Al27, and nitrite reductase (E.C. 1.6.6.4) activity until Al81. Hence there may be an Al mediated extracellular and intracellular regulation of root net nitrate uptake. Nitrate accumulation in the roots affects the translocation rates and, therefore, the nitrate concentration in the leaves. The in vivo reducing power generated by the photosynthetic electron flow does not limit nitrate to ammonia reduction, and the increase of maximum nitrate and nitrite reductase activities parallels the decreasing nitrate, nitrite, and ammonia concentrations. and F. C. Lidon, J. C. Ramalho, M. G. Barreiro.
In carob tree (Ceratonia siliqua) radiant energy saturated net photosynthetic rate (PN) during summer was about 10 % of the spring values. This was accompanied by a reduction in stomatal conductance (gs), which only partially explains the strong reduction in PN. Photosynthetic capacity (Pmax) and quantum yield (Φ), both measured under saturating CO2, had the maximum in spring (about 34 μmol m-2 s-1 and 0.08 mol mol-1, respectively) and both decreased in late summer to about 55 % of their spring values. Despite strong decreases in Φ, photoinhibition of photosystem 2 (PS2) was negligible or easily reversible in carob leaves subjected to summer drought, since Fv/Fm, measured in the morning, did not show appreciable changes. The recovery of affected parameters was very rapid after the first rains in late October. The chlorophyll (Chl) alb ratio in the end of the summer was 2.6, a value significantly lower than 3.6 obtained in the spring, suggesting that Chl a was preferentially reduced. and J. C. Ramalho, J. A. Lauriano, M. A. Nunes.
Drought was induced in chickpea (Cicer arietinum L.) genotypes (ChK 3226 and ILC 3279) differing in yield capacity. Water stress (S1, RWC around 55-50%; S2, RWC ≤ 40%) drastically reduced stomatal conductance (g s) and net photosynthetic rate (PN) in both genotypes. ILC 3279 showed greater photosynthetic capacity
(Amax) decreases. Maximum PSII photochemical efficiency (Fv/Fm), photochemical quenching (qP), total chlorophylls (Chls) and carotenoids (Cars) content showed stability in both genotypes under stress, but in S2 ILC 3279 presented an increase in basal fluorescence (F0) and a greater reduction in estimation of quantum yield of linear electron transport (Φe) than ChK 3226. Membrane damage evaluated by electrolyte leakage occurred earlier and was greater in ILC 3279. It also presented a decrease of total fatty acids (TFA) along drought, while in ChK 3226 greater amounts of TFA were observed in S1. In rehydration, PN of S1 plants completely recovered (ILC 3279) or remained slightly below control (ChK 3226). As regards S2 plants, ILC 3279 showed stronger PN and gs reductions than ChK 3226, despite both genotypes totally recovered Amax and chlorophyll (Chl) a fluorescence. ChK 3226 recovered more efficiently from membrane damage. Under control conditions, greater amounts of most of the studied soluble metabolites occurred in ChK 3226 plants. Malate and citrate decreased with water stress (S2) in both genotypes. Sucrose and pinitol (that had a higher concentration than sucrose in both genotypes) increased in ILC 3279 (S1 and S2), and decreased in ChK 3226 (S2). In ILC 3279 proline and asparagine followed similar patterns. Genotypes showed a similar shoot dry mass (DM) in control plants, but root DM was higher in ChK 3226. Drought reduced root and shoot DM in ChK 3226 already under S1, while in ILC 3279 root DM was unaffected by drought and shoot biomass decreased only in S2. Root/shoot ratio was always higher in ChK 3226 but tended to decrease under stress, while the opposite was observed in ILC 3279. No pods were obtained from control plants of both genotypes, or droughted ILC 3279 plants. ChK 3226 produced pods under S1 (higher yield) and S2. Under stress conditions, ChK 3226 was less affected in photosynthetic activity and membrane integrity, showing a better tolerance to drought. This agrees with the better yield of this genotype under water stress. Distinct strategies seem to underlie the different physiological responses of the two genotypes to water deficit. In spite of its significant solutes accumulation, ILC 3279 was more affected in photosynthetic activity and membrane integrity during water stress than ChK 3226, which showed better yield, under drought. A relation could not be established between solutes accumulation of ILC 3279 and yield., and M. C. Matos ... [et al.].
The effect of drought on plant water relations and photosynthesis of Vigna glabrescens (Vg) and Vigna unguiculata (cvs. 1183, EPACE-1 and Lagoa), which differ in their drought resistance, was compared. With the increase of drought severity, Vg showed a more gradual stomatal closure and maintained significantly higher levels of stomatal conductance (gs) and photosynthetic activity (PN) than the other genotypes even when minimum relative water content (RWC) values were observed. Furthermore, Vg was the only genotype able to accumulate significant amounts of proline already under moderate water deficit, what could explain the lower osmotic potential (ψs) values observed in these plants. The three V. unguiculata cultivars presented a similar stomatal control under increasing water deficit. A mesophyllic impairment of photosynthetic capacity (Pmax) was detected for cv. 1183 from the beginning of drought onset (85-75 % RWC) while in the Vg plants the values remained unaffected along the whole drought period, indicating that PN decrease observed in this genotype is mainly a consequence of stomatal closure. Such Pmax maintenance suggests the existence of a high mesophyllic ability to cope with increasing tissue dehydration in Vg. and P. Scotti Campos ... [et al.].
Effect of drought on the mechanisms of energy dissipation was evaluated in two-month-old Arachis hypogaea cvs. 57-422, 73-30, and GC 8-35. Plants were submitted to three treatments: control (C), mild water stress (S1), and severe water stress (S2). Photosynthetic performance was evaluated as the Hill and Mehler reactions. These activities were correlated with the contents of the low and high potential forms of cytochrome (cyt) b 559, plastoquinone, cyt b 563, and cyt f. Additionally, the patterns of carotenoids and chlorophylls (Chls), as well as the alterations of Chl a fluorescence parameters were studied. Under mild water stress the regulatory mechanism at the antennae level was effective for 57-422 and GC 8-35, while in the cv. 73-30 an overcharge of photosynthetic apparatus occurred. Relative to this cv., under S1 the stability of carotene and the dissipative cycle around photosystem (PS) 2 became an important factor for the effective protection of the PS2 reaction centres. The cyclic electron flow around PS1 was important for energy dissipation under S1 only for the cvs. 57-422 and 73-30. and J. A. Lauriano ... [et al.].
The photosynthetic response of three Arachis hypogaea L. cultivars (57-422, 73-30, and GC 8-35) grown for two months was measured under water available conditions, severe water stress, and 24, 72, and 93 h following re-watering. At the end of the drying cycle, all the cultivars reached dehydration, relative water content (RWC) ranging between 40 and 50 %. During dehydration, leaf stomatal conductance (gs), transpiration rate (E), and net photosynthetic rate (PN) decreased more in cvs. 57-422 and GC 8-35 than in 73-30. Instantaneous water use efficiency (WUEi) and photosynthetic capacity (Pmax) decreased mostly in cv. GC 8-35. Except in cv. GC 8-35, the activity of photosystem 1 (PS1) was only slightly affected. PS2 and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were the main targets of water stress. After re-watering, cvs. 73-30 and GC 8-35 rapidly regained gs, E, and PN activities. Twenty-four hours after re-watering, the electron transport rates and RuBPCO activity strongly increased. PN and Pmax fully recovered later. Considering the different photosynthetic responses of the studied genotype, a general characterisation of the interaction between water stress and this metabolism is presented. and J. A. Lauriano ... [et al.].
Pachyrhizus ahipa (Wedd.) Parodi, originally from Latin America, is an agronomy interesting legume crop due to high seed protein content and saccharides-rich tuber root. Its capacity of adaptation to Mediterranean climate, where heat and water stress are frequently associated, is being tested. Two accessions of P. ahipa (AC 102 and AC 524) differing in field production were compared as concerns the effects of water stress and high temperature on photosynthetic performance. Membrane integrity was also evaluated through electrolyte leakage (injury index, I%), lipid composition, and ultrastructure observations. Short-term heat stress (40 °C) did not affect net photosynthetic rate (PN), stomatal conductance (gs), and most of fluorescence parameters in both accessions, what was consistent with low electrolyte leakage. However, photosynthetic capacity (Pmax) showed a significant reduction, AC 524 being more affected than AC 102. Relative water content (RWC) below 70 % caused a drastic decrease in PN and gs. Fluorescence parameters, Pmax, and I% were affected in the two accessions, which also presented a strong reduction (42 %) in total fatty acids (TFA). Contents of galactolipids were drastically reduced, and changes in their saturation also occurred, namely a decrease in linolenic acid (C18:3) percentage of monogalactosyl-diacylglycerol (MGDG) in both accessions. Thylakoid ultrastructure in AC 524 submitted to drought showed disorganisation of grana stacking. Mitochondria presented signs of injured cristae. When water-stressed plants were subjected to high temperature, photosynthesis and fluorescence parameters did not show significant additional changes in both accessions. The exposure of drought stressed plants to 40 °C further increased electrolyte leakage in AC 524, but not in AC 102. Chloroplasts, mitochondria, and plasmalemma showed an increased disorganisation. Vesicles appeared in the cytoplasm, which became electron-transparent, reflecting a strong reduction in the number of ribosomes. Hence AC 102 was less affected than AC 524 as regards some components of photosynthetic process, namely Pmax and membrane integrity. This could account for its better yield production previously observed in field grown plants. and M. C. Matos ... [et al.].