Eduardo Caruso Machado

Photosynthetic gas exchange, chlorophyll fluorescence and some associated metabolic changes in cowpea (Vigna unguiculata) during water stress and recovery

Abstract The responses of photosynthetic gas exchange and chlorophyll fluorescence along with changes in carbohydrate and proline levels were studied in cowpea ( Vigna unguiculata ) during water stress and recovery. Three experiments were conducted under greenhouse and laboratory conditions. Decreased CO 2 assimilation rates during water stress were largely dependent on stomatal closure, which reduced available internal CO 2 and restricted water loss through transpiration. During the initial phase of stress, photochemical activity was not affected, as revealed by lack of alterations in fluorescence parameters associated with photosystem II (PSII) activity. Development of non-radiative energy dissipation mechanisms was evidenced during stress by increases in non-photochemical quenching and decreases in efficiency of excitation capture by open centers. At an advanced phase of stress, a down-regulation of PSII activity was observed along with some impairment of photochemical activity, as revealed by decreases in the maximum quantum yield of PSII (Fv/Fm). However, this impairment did not limit the overall photosynthetic process, since assimilation rates recovered, upon rewatering, independent of the still present decreased Fv/Fm values. Complete recovery of all gas exchange and fluorescence parameters occurred 3 days after rewatering. However, on the first day after water stress relief, assimilation rates only partially recovered in spite of the availability of internal CO 2 , suggesting some non-stomatal limitation of photosynthesis. Accordingly, the downregulation of PSII activity observed during stress persisted at this time. Our results on carbohydrate metabolic changes revealed an accumulation of soluble sugars in water-stressed leaves, which also persisted for 1 day after rewatering. This finding suggest a transient end-product inhibition of photosynthesis, contributing to a minor non-stomatal limitation during stress and initial phase of recovery. Increases in proline level were small and their onset was delayed after stress imposition, so that it may rather be a consequence and not a stress-induced beneficial response.

Respostas biométricas e fisiológicas ao deficit hídrico em cana-de-açúcar em diferentes fases fenológicas

The aim of this work was to evaluate the biometric and physiological responses of sugarcane (Saccharum spp.) to water deficit (WD), during different phenological phases. Genotypes IACSP 94-2094 and IACSP 96-2042 were subjected to WD conditions during the initial, maximum and sucrose accumulation phases. The experiment was carried out in a completely randomized design. Susceptibility to WD was established by reduction in stalk dry matter and soluble solids. Water deficit reduced leaf gas exchange in all phenological phases of both genotypes. Lower plant height, less stalk dry matter and soluble solids, and reduction in number and length of internodes were only observed during the initial growth phase of the IACSP 96-2042 clone. In the initial growth phase, tolerance to WD was observed for IACSP 94-2094, with evidence of physiological acclimation, and for IACSP 96-2042 in reduction phytomass production and its soluble solid content, caused by lower stomatal conductance and lower apparent carboxylation efficiency which limit photosynthesis. Regardless of the phenological phase, genotype IACSP 94-2094 was tolerant to WD, since its phytomass production was maintained even with impairment of leaf gas exchange.

Photosynthesis and water relations of well-watered orange plants as affected by winter and summer conditions

The aim of this study was to evaluate how the summer and winter conditions affect the photosynthesis and water relations of well-watered orange trees, considering the diurnal changes in leaf gas exchange, chlorophyll (Chl) fluorescence, and leaf water potential (Ψ) of potted-plants growing in a subtropical climate. The diurnal pattern of photosynthesis in young citrus trees was not significantly affected by the environmental changes when compared the summer and winter seasons. However, citrus plants showed higher photosynthetic performance in summer, when plants fixed 2.9 times more CO2 during the diurnal period than in the winter season. Curiously, the winter conditions were more favorable to photosynthesis of citrus plants, when considering the air temperature (< 29 °C), leaf-to-air vapor pressure difference (< 2.4 kPa) and photon flux density (maximum values near light saturation) during the diurnal period. Therefore, low night temperature was the main environmental element changing the photosynthetic performance and water relations of well-watered plants during winter. Lower whole-plant hydraulic conductance, lower shoot hydration and lower stomatal conductance were noticed during winter when compared to the summer season. In winter, higher ratio between the apparent electron transport rate and leaf CO2 assimilation was verified in afternoon, indicating reduction in electron use efficiency by photosynthesis. The high radiation loading in the summer season did not impair the citrus photochemistry, being photoprotective mechanisms active. Such mechanisms were related to increases in the heat dissipation of excessive light energy at the PSII level and to other metabolic processes consuming electrons, which impede the citrus photoinhibition under high light conditions.

Photosynthetic response of citrus grown under reflective aluminized polypropylene shading nets

Abstract High temperatures and high atmospheric vapor pressure deficits (VPDs) are usually encountered in greenhouses in hot climates. For citrus, these environmental conditions can lead to decreases in photosynthetic activity with detrimental effects on plant growth. The aim of this study was to evaluate the use of reflective aluminized polypropylene shading nets on photosynthetic performance of citrus plants, by measuring CO2 assimilation, transpiration rate, stomatal conductance and chlorophyll a fluorescence. Incident photosynthetically active radiation (PAR) levels and leaf temperatures were reduced when the reflective nets were used. Higher stomatal conductance and higher CO2 assimilation rates were observed in shaded plants, so that integrated daily net CO2 uptake was approximately 20% higher than in exposed plants. The better performance of shaded plants, however, was observed only during the middle of the day, being PAR-limited in early morning and late afternoon. The reflective net was also effective in preventing photoinhibition of photosynthesis in shaded plants, which sustained higher maximum (Fv/Fm) and effective (ΔF/Fm′) quantum yield with higher apparent electron transport rates (ETRs) than exposed plants. Observed photoinhibition in exposed plants was transient, probably reflecting photosynthetic regulatory responses to excess absorbed light energy. Therefore, the results clearly showed that photosynthetic performance of citrus cultivated in greenhouses can be improved by the use of reflective nets. Favorable effects comprised not only the maintenance of proper stomatal aperture for gas exchange but also a better functioning of the photosystem II (PSII) under non-photoinhibitory conditions.

Early photosynthetic responses of sweet orange plants infected with Xylella fastidiosa

Abstract Photosynthetic processes of sweet orange plants [Citrus sinensis (L.) Osbeck cv. Pera] infected with Xylella fastidiosa were investigated. Simultaneous measurements of leaf gas exchanges and chlorophyll a fluorescence were taken in healthy and infected leaves before development of symptoms. The photosynthetic oxygen evolution response to different light intensities and curves of photosynthesis induction were carried out in leaf discs extracted from the same leaves used for gas exchange measurements. Infected plants showed decreased CO2 assimilation, transpiration and stomatal conductance. No differences between healthy and infected intact leaves regarding chlorophyll a fluorescence parameters were observed, except for the potential quantum yield of photosystem II which was higher in infected plants. Values of photosynthetic oxygen evolution, effective quantum yield of photosystem II, apparent electron transport rate, and photochemical fluorescence quenching were higher in healthy leaf discs than in infected ones. Our results suggest that low photosynthetic rates of non-symptomatically sweet orange leaves infected with X. fastidiosa were caused by low stomatal conductance, biochemical alterations of photosynthetic machinery and increase in alternative electron sinks.

Superoxide dismutase and ascorbate peroxidase improve the recovery of photosynthesis in sugarcane plants subjected to water deficit and low substrate temperature.

The physiological responses of C4 species to simultaneous water deficit and low substrate temperature are poorly understood, as well as the recovery capacity. This study investigated whether the effect of these abiotic stressors is cultivar-dependent. The differential responses of drought-resistant (IACSP94-2094) and drought-sensitive (IACSP97-7065) sugarcane cultivars were characterized to assess the relationship between photosynthesis and antioxidant protection by APX and SOD isoforms under stress conditions. Our results show that drought alone or combined with low root temperature led to excessive energetic pressure at the PSII level. Heat dissipation was increased in both genotypes, but the high antioxidant capacity due to higher SOD and APX activities was genotype-dependent and it operated better in the drought-resistant genotype. High SOD and APX activities were associated with a rapid recovery of photosynthesis in IACSP94-2094 plants after drought and low substrate temperature alone or simultaneously.

Some aspects of citrus ecophysiology in subtropical climates: re-visiting photosynthesis under natural conditions

Received: 14 November 2007; Accepted: 12 December 2007In this review we re-visit and discuss the current knowledge on ecophysiology of citrus trees, addressing the influenceof environmental conditions on citrus photosynthesis. Knowledge of physiological responses of citrus trees to theirsurrounding environment is essential in order to improve crop production and plant development, both beingconsequences of appropriate horticultural management in citrus orchards. In this context, citrus photosynthesis isaddressed as the primary source of carbon and energy for plant growth and development. The photosynthetic activityon both a daily and a seasonal scale is reviewed, taking into consideration the physiological aspects related to seasonalvariation of photochemical and biochemical activities, stomatal conductance and leaf water potential. These aspects aretreated for citrus plants growing in subtropical climates with varying environmental conditions, such as moderate tosevere drought during the winter season. In addition, the possible inhibitory/stimulatory effects of carbohydratemetabolism on citrus photosynthesis are discussed with regard to the source-sink relationship. Field experimentationthat enhances knowledge concerning citrus ecophysiology in subtropical climates is highlighted. Among interestingsubjects to be unraveled by future research, we may point out the effects of low temperatures on citrus photosynthesisand water relations, the nature of the relationship between leaf carbohydrate content and photosynthesis, and thesignificance of photosynthesis in different canopy layers and positions in relation to the total carbon gain in maturecitrus trees.

Photosynthetic responses of tropical tree species from different successional groups under contrasting irradiance conditions

This study evaluated the photosynthetic responses of seven tropical trees of different successional groups under contrasting irradiance conditions, taking into account changes in gas exchange and chlorophyll a fluorescence. Although early successional species have shown higher values of CO2 assimilation (A) and transpiration (E), there was not a defined pattern of the daily gas exchange responses to high irradiance (FSL) among evaluated species. Cariniana legalis (Mart.) Kuntze (late secondary) and Astronium graveolens Jacq. (early secondary) exhibited larger reductions in daily-integrated CO2 assimilation (DIA) when transferred from medium light (ML) to FSL. On the other hand, the pioneer species Guazuma ulmifolia Lam. had significant DIA increase when exposed to FSL. The pioneers Croton spp. trended to show a DIA decrease around 19%, while Cytharexyllum myrianthum Cham. (pioneer) and Rhamnidium elaeocarpum Reiss. (early secondary) trended to increase DIA when transferred to FSL. Under this condition, all species showed dynamic photoinhibition, except for C. legalis that presented chronic photoinhibition of photosynthesis. Considering daily photosynthetic processes, our results supported the hypothesis of more flexible responses of early successional species (pioneer and early secondary species). The principal component analysis indicated that the photochemical parameters effective quantum efficiency of photosystem II and apparent electron transport rate were more suitable to separate the successional groups under ML condition, whereas A and E play a major role to this task under FSL condition.

REVEALING DROUGHT-RESISTANCE AND PRODUCTIVE PATTERNS IN SUGARCANE GENOTYPES BY EVALUATING BOTH PHYSIOLOGICAL RESPONSES AND STALK YIELD

This study was conducted to investigate the physiological response of sugarcane genotypes to drought and its consequence for stalk yield. Sugarcane genotypes IACSP94-2094, IACSP96-2042 and SP87-365 were subjected to water deficit during the initial growth phase by withholding water. Resistance and sensitivity patterns were defined by the impact of drought on the stalk yield and content of soluble solids in the stalk juice. IACSP94-2094 and SP87-365 were considered drought-resistant genotypes, as the stalk dry matter production and yield of soluble solids were not reduced by the water deficit. Although drought caused reductions in leaf gas exchange in all the genotypes, IACSP96-2042 was most affected when considering the cumulative reduction in photosynthesis throughout the experimental period. This photosynthetic impairment of IACSP96-2042 was related to both non-stomatal and stomatal limitations, whereas photosynthesis in SP87-365 and IACSP94-2094 were only stomatally limited under drought. In general, a reduced photosynthetic sensitivity to water deficit was an important physiological trait for dry matter production in sugarcane plants, and the concentrations of soluble carbohydrates, sucrose, starch and proline in the leaves did not reveal consistent differences between the patterns of resistance and sensitivity. Even though IACSP96-2042 was severely affected by water shortage, this genotype presented a similar stalk yield under drought and the highest stalk yield under well-watered conditions when compared to the other genotypes. This response to variable water conditions is interesting for regions with seasonal drought, whereas the pattern of drought resistance is more appropriate for regions in which drought occurs for long periods during the crop season. Our findings are also discussed from the point of view that increases in sugarcane yield and sustainable agriculture may be reached by choosing the best genotype for each specific environmental condition.

Fotossíntese, condutância estomática e transpiração em pupunheira sob deficiência hídrica

Research results on physiological aspects of peach palm (Bactris gasipaes Kunth), a native fruit tree from tropical America, are scarce. Trying to fill this gap, a water deficit experiment was performed under nursery conditions during 13 days, utilizing 12 months old plants. The main objective was to evaluate peach palm responses to water deficit. The measured variables were: CO2 assimilation rate, transpiration rate, stomatal conductance and leaf water potential. Data were collected daily in a laboratory, under a photosynthetic photon flux (PPF) of 1200 mm-2 s-1, and studied by variance and regression analysis. Significant decreases of leaf water potential values and gas exchange rates were verified when water was withhold for more than six days. The smallest values were found at the tenth day without water replacement, with a reduction of 92% of the net photosynthetic rate, 87% of the stomatal conductance and 70% of the transpiration. By that time, the smallest measured leaf water potential was ¾1.9 MPa. Recovering from water stress was accomplished two days after rewatering, except for stomatal conductance. The partial closing of the stomata (decrease in stomatal conductance) and the reduction of photosynthesis, suggest the existence of an acclimation mechanism of the peach palm, diminishing water loss under moderate stress.