Resumo -O objetivo deste trabalho foi avaliar as respostas biométricas e fi siológicas de cana-de-açúcar (Saccharum spp.) ao defi cit hídrico (DH), em diferentes fases fenológicas. Os genótipos IACSP 94-2094 e IACSP 96-2042 foram submetidos a DH nas fases de crescimento inicial, crescimento máximo e de acúmulo de sacarose no colmo. O delineamento experimental foi inteiramente casualizado. A suscetibilidade ao DH foi determinada pela redução de matéria seca do colmo e do conteúdo de sólidos solúveis no caldo. O defi cit hídrico causou redução nas trocas gasosas, nas três fases fenológicas, em ambos os genótipos. Foi observada menor altura das plantas, menor acúmulo de matéria seca do colmo e de sólidos solúveis, e redução no número e comprimento de entrenós, apenas na fase de crescimento inicial, no clone IACSP 96-2042. Na fase de crescimento inicial, observou-se tolerância ao DH no genótipo IACSP 94-2094, com evidências de aclimatação fi siológica, e redução na produção de fi tomassa e de sólidos solúveis no genótipo IACSP 96-2042, como resposta à menor condutância estomática e à menor efi ciência aparente de carboxilação da fotossíntese. Independentemente da fase fenológica, o genótipo IACSP 94-2094 foi tolerante ao defi cit hídrico, pois manteve a produção de fi tomassa mesmo com redução das trocas gasosas.Termos para indexação: Saccharum, crescimento, fotossíntese, seca. Biometric and physiological responses to water defi cit in sugarcane at different phenological stagesAbstract -The aim of this work was to evaluate the biometric and physiological responses of sugarcane (Saccharum spp.) to water defi cit (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 defi cit 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 effi ciency 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.
Resumo -Foram avaliadas as respostas das trocas gasosas à irradiância, à temperatura, ao déficit de pressão de vapor e à concentração interna de CO 2 em plantas jovens de laranjeira 'Valência', tangor 'Murcote' e lima ácida 'Tahiti', sob condições controladas. As taxas máximas de assimilação de CO 2 foram de 9,8, 12,8 Photosynthetic responses of three citrus species to environmental factorsAbstract -Gas exchange responses to irradiance, temperature, air vapor pressure deficit and intercellular CO 2 concentration were evaluated in young plants of sweet orange 'Valência', tangor 'Murcote' and acid lime 'Tahiti' plants, under controlled conditions. Maximum rates of CO 2 assimilation were around 9.8, 12.8 and 13.0 µmol m -2 s -1 , respectively, for 'Valência', 'Murcote' and 'Tahiti', and these differences were related to stomatal conductance and instantaneous carboxylation efficiency. Light saturation of photosynthesis was around 750 µmol m -2 s -1 for 'Valência', whereas 'Murcote' and 'Tahiti' did not show evident light saturation, exhibiting small increases of CO 2 assimilation above 1,000 µmol m -2 s -1 . The CO 2 compensation point was 4.8, 5.8 and 5.4 Pa for 'Valência', 'Murcote' and 'Tahiti', respectively, indicating differences in photorespiration of these citrus species. Leaf temperatures between 25 o C and 30 o C were optimum for photosynthesis of 'Valência', whereas this optimum was around 30 o C for 'Murcote' and 'Tahiti'. At temperatures above or below the optimum range, CO 2 assimilation was reduced by partial decrease of stomatal conductance and instantaneous carboxylation efficiency. Reduced CO 2 assimilation rate was also caused by increasing vapor pressure deficit from 1.5 to 3.5 kPa, and this effect enhanced when temperature increased from 28 o C to 35 o C.
Xylem vulnerability to embolism represents an important trait to determine species distribution patterns and drought resistance. However, estimating embolism resistance frequently requires time‐consuming and ambiguous hydraulic lab measurements. Based on a recently developed pneumatic method, we present and test the “Pneumatron”, a device that generates high time‐resolution and fully automated vulnerability curves. Embolism resistance is estimated by applying a partial vacuum to extract air from an excised xylem sample, while monitoring the pressure change over time. Although the amount of gas extracted is strongly correlated with the percentage loss of xylem conductivity, validation of the Pneumatron was performed by comparison with the optical method for Eucalyptus camaldulensis leaves. The Pneumatron improved the precision of the pneumatic method considerably, facilitating the detection of small differences in the (percentage of air discharged [PAD] < 0.47%). Hence, the Pneumatron can directly measure the 50% PAD without any fitting of vulnerability curves. PAD and embolism frequency based on the optical method were strongly correlated (r2 = 0.93) for E. camaldulensis. By providing an open source platform, the Pneumatron represents an easy, low‐cost, and powerful tool for field measurements, which can significantly improve our understanding of plant–water relations and the mechanisms behind embolism.
In this review we re-visit and discuss the current knowledge on ecophysiology of citrus trees, addressing the influence of environmental conditions on citrus photosynthesis. Knowledge of physiological responses of citrus trees to their surrounding environment is essential in order to improve crop production and plant development, both being consequences of appropriate horticultural management in citrus orchards. In this context, citrus photosynthesis is addressed as the primary source of carbon and energy for plant growth and development. The photosynthetic activity on both a daily and a seasonal scale is reviewed, taking into consideration the physiological aspects related to seasonal variation of photochemical and biochemical activities, stomatal conductance and leaf water potential. These aspects are treated for citrus plants growing in subtropical climates with varying environmental conditions, such as moderate to severe drought during the winter season. In addition, the possible inhibitory/stimulatory effects of carbohydrate metabolism on citrus photosynthesis are discussed with regard to the source-sink relationship. Field experimentation that enhances knowledge concerning citrus ecophysiology in subtropical climates is highlighted. Among interesting subjects to be unraveled by future research, we may point out the effects of low temperatures on citrus photosynthesis and water relations, the nature of the relationship between leaf carbohydrate content and photosynthesis, and the significance of photosynthesis in different canopy layers and positions in relation to the total carbon gain in mature citrus trees.
RESUMOEm experimento desenvolvido em casa de vegetação e em câmara de crescimento avaliou-se o efeito do alumínio (Al) na fotossíntese de seis cultivares de cafeeiro. As plantas foram cultivadas em solução nutritiva aerada continuamente, contendo duas concentrações de Al, 0 e 0,148 mmol L -1 , fornecidas como Al 2 (SO 4 ) 3 . Após 97 dias mediram-se as taxas de assimilação de CO 2 (A) e transpiração (E), a condutância estomática (gs), a concentração interna de CO 2 (Ci), eficiência instantânea de carboxilação (Φc) e variáveis de fluorescência da clorofila. Em todas as cultivares, a presença de Al causou quedas significantes em A, gs, Φc, ocorrendo aumento em Ci. Também se observou aumento significativo na fluorescência basal (Fo) e queda na eficiência quântica máxima do fotossistema II (Fv/Fm), sugerindo injúrias na estrutura dos tilacóides causadas pelo Al. Na curva de indução de fotossíntese, observou-se que o Al causou queda no coeficiente de extinção fotoquímica da fluorescência e aumento no coeficiente de extinção não fotoquímico. Os resultados desse estudo indicaram que a queda de A foi devida à queda da condutância estomática, nas atividades bioquímicas e fotoquímicas. Palavras-chaves:Coffea arabica, toxidez de alumínio, fotossíntese, condutância estomática, indução da fotossíntese. ABSTRACT GAS EXCHANGE AND CHLOROPHYLL FLUORESCENCE IN SIX COFFEE CULTIVARS UNDER ALUMINUM STRESSExperiments were carried out under greenhouse and growth chamber to evaluate the effects of aluminum (Al) on several photosynthetic characteristics in six coffee cultivars. Plants were grown in nutrient solution aerated continuously, containing two Al concentration, 0 and 0.148 mmol L -1 , supplied as Al 2 (SO 4 ) 3 . After 97 days of treatament, measurements of CO 2 assimilation rate (A), transpiration rate (E), stomatal conductance (gs), internal CO 2 concentration (Ci), instantaneous carboxylation efficiency (Φc) and chlorophyll fluorescence related characteristics were performed. All six cultivars showed decrease in A, gs and Φc and increase in Ci. The basal chlorophyll fluorescence yield (Fo) increased and the maximum quantum efficiency (Fv/Fm) decreased suggesting structural damage in thylakoid. The photosynthetic induction curve revealed that Al caused decreases in photochemical quenching of fluorescence as well as increases of non-photochemical quenching. Our results indicated that A reduction was related to decreases in both stomatal conductance, and biochemical and photochemical damages. Key-words:Coffea arabica, aluminum toxicity, photosynthesis, stomatal conductance, photosynthesis induction.( 1 ) Recebido para publicação em 5
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 CO 2 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 ºC), 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 CO 2 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.
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