Plants with citrus variegated chlorosis (CVC), a disease caused by the xylem-limited bacteria Xylella fastidiosa, have leaves with water deficiency symptoms and are associated with decreases on the net photosynthesis and transpiration rates. Using healthy and CVC-affected 'Pêra' sweet orange plants on 'Rangpur' lime rootstock, the leaf gas exchange variables were measured with an open-gas portable photosynthesis system. All plants were watered and the leaf water potential (Ψ w ) was measured by isopiestic thermocouple psychrometric technique. The net photosynthesis (A) vs. internal leaf CO 2 concentration (A/Ci curves) was analyzed. The relative effect of stomatal resistance on photosynthesis (S%) and the estimation of carboxylation efficiency were calculated. The rates of photosynthesis and transpiration, stomatal conductance, and internal leaf CO 2 concentration (Ci) were also measured while varying the photosynthetic photon flux density (PPFD). The S% values were approximately 30 % greater in infected plants when compared to healthy ones. The light compensation point for diseased plants was higher than in the healthy ones, and the saturation light point in plants with CVC was twofold lower. The lower Ψ w in diseased plants favours the hypothesis of xylem occlusion, which probably caused a lower water supply to the mesophyll, thus decreasing the photosynthesis and transpiration rates. Nevertheless, there was also a reduction in the photosynthetic metabolic activities, represented by lower carboxylation efficiency and photochemical disturbances that were detected in diseased plants.Key words: A/Ci curves, citrus variegated chlorosis, Citrus sinensis L. Osbeck, photosynthetic photon flux density, photosynthetic metabolism, stomata.Assimilação do CO 2 , curvas de resposta fotossintética à luz e relações hídricas de plantas da laranja doce 'Pêra' infectadas com Xylella fastidiosa: Plantas com clorose variegada dos citros (CVC), uma doença causada pela bactéria Xylella fastidiosa, restrita ao xilema, apresentam folhas com sintomas de deficiência hídrica, associados com diminuição na fotossíntese líquida e na taxa de transpiração. Utilizando-se plantas sadias e com CVC de laranjeira 'Pêra' sobre limoeiro 'Cravo', as variáveis de trocas gasosas foram medidas com sistema portátil aberto de fotossíntese. Todas as plantas foram irrigadas e o potencial da água na folha (Ψ w ) medido pela técnica psicrométrica de termopar isopiéstico. Fotossíntese líquida (A) vs. concentração interna de CO 2 na folha (curvas A/Ci) foram analisadas. O efeito relativo da resistência estomática sobre a fotossíntese (S%) e a estimativa da eficiência de carboxilação foram calculados. Taxas de fotossíntese e transpiração, condutância estomática e concentração interna de CO 2 na folha (Ci) foram também medidas quando se variou a densidade de fluxo de fótons fotossinteticamente ativos (DFFFA). Os valores de S% foram 30 % maiores nas plantas doentes, comparados às sadias, e seu ponto de compensação de luz também foi maior e o ponto de saturação de...

*Autor correspondente RESUMO: Em espécies perenes podem ocorrer variações nas taxas de trocas gasosas e nas relações hídricas em função da variação das condições ambientais, durante os diferentes meses do ano. Avaliaram-se, em laranjeira ´Valência´ enxertada sobre quatro espécies de porta-enxerto, mantida sem deficiência hídrica, as taxas de fotossíntese (A) e de transpiração (E), a condutância estomática (g) e o potencial da água na folha (Ψ Ψ Ψ Ψ Ψ f ), medidos nos períodos da manhã (9h00 às 11h00) e da tarde (13h00 às 15h00) nos meses de janeiro, março e julho em Campinas -SP. As espécies de porta-enxertos não tiveram efeitos sobre as variáveis medidas. Independente do porta-enxerto A, g e Ψ Ψ Ψ Ψ Ψ f foram menores no período da tarde. A queda de A deve estar relacionada com a queda de g que diminuiu em resposta ao aumento do déficit de pressão de vapor entre o ar e a folha (DPV ar-folha ) nos horários mais quentes do dia. Apesar de ocorrer fechamento parcial dos estômatos no período da tarde E foi similar ao período da manhã, devido ao aumento do DPV ar-folha . Também observou-se queda em A e em g no sentido de janeiro para julho. Sugere-se que a queda em A e em g ocorrida em março em comparação a janeiro esteja relacionada à queda da atividade de crescimento da planta, afetando as relações fonte-dreno, visto que as condições ambientais nestes dois meses foram semelhantes. As quedas de A e de g observadas em julho, em relação à janeiro e março, parecem estar relacionadas tanto à queda na temperatura noturna quanto à queda na atividade de crescimento. Palavras-chave: Citrus sinensis, trocas gasosas, transpiração, crescimento SEASONAL VARIATION OF PHOTOSYNTHETIC RATES, STOMATAL CONDUCTANCE AND LEAF WATER POTENTIAL IN 'VALENCIA' ORANGE TREESABSTRACT: Seasonal variation in environmental conditions may influence gas exchange rates as well as water relations in perennial species. This work was carried out to evaluate photosynthetic rates (A), transpiration (E), stomatal conductance (g) and leaf water potential (Ψ Ψ Ψ Ψ Ψ f ) in 'Valencia' orange trees grafted on four different rootstocks. Measurements were made twice a day: from 9h00 to 11h00 a.m. and from 1h00 to 3h00 p.m., during January, March and July. A and g were significantly lower and Ψ Ψ Ψ Ψ Ψ f was significantly more negative, in the afternoon. The decrease in A may be related to the reduction in g, due to the increase in the vapor pressure deficit between the air and the leaf (VPD air-leaf ) in the afternoon, when temperatures are higher. In spite of the partial stomatal closure in the afternoon, the values for E were approximately the same as those measured in the morning, due to the increase in the VPD air-leaf . A decrease in A and g could also be noted from January to July, that is, from the hot and humid summer months, to the colder and drier winter ones. It was suggested that the decrease in A and g observed from January through March, may be related to the decrease in plant growth rates, which could have influenced the sourc...

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Aluminum (Al) is toxic to most plants. Although inhibition of root elongation can occur even under 10 M Al, above 1000 M damage to photochemical performances has been reported, reducing the CO 2 assimilation rate (A). However, Al is retained in the roots of most plants, including Citrus limonia ('Rangpur' lime) with no direct explanation for the low A. In this study, 'Rangpur' lime plants were cultivated hydroponically in a nutrient solution with 1480 M Al, and we expected to determine the time range within which Al-induced decrease in A starts. To our surprise, low stomatal conductance (gs) rather than low photochemical performances was evident 45 days after exposing the roots to Al. Aluminum was mostly retained in the roots and histochemically observed in the stele, suggesting reduction in water uptake/transport in the xylem that became fibrous. We also found a 35% reduction in the midday leaf water potential in plants exposed to Al, indicating association between the fibrous xylem vessels and low gs, which could explain the Al-induced decrease in A in 'Rangpur' lime plants.

Stomatal aperture generally increases in response to low vapor pressure deficit (VPD) and decreases at high VPD. Aluminum (Al) inhibits root growth, indirectly exposing the roots to low water availability, which may decrease leaf hydration and, consequently, the stomatal conductance (gs). In this study, Citrus limonia ('Rangpur' lime) was grown in nutrient solution with 1480 μM Al for 90 days, and we expected that the presence of Al could prevent gs from responding to VPD. As expected, gs did not respond to the increase in VPD in plants exposed to Al. Aluminum also reduced the relative water content and midday leaf water potential (Ψmd) after 60 and 90 days. The CO 2 assimilation rate (A) followed the same response pattern exhibited by gs, the estimation of the carboxylation efficiency was not reduced in plants exposed to Al and measured under drier air, while photochemical responses were slightly reduced in plants exposed to Al, indicating that the Al-induced decrease in A was dependent on gs and less ascribed to low photochemical performance. Like in drought conditions, the longterm exposure to Al reduces leaf hydration and compromises gs responses to the atmosphere, eventually impairing A in 'Rangpur' lime plants.

RESUMO: Resultados de pesquisa envolvendo aspectos fisiológicos da pupunheira (Bactris gasipaes Kunth), fruteira nativa da América Tropical, são escassos. Procurando completar essa lacuna, um experimento sob deficiência hídrica foi conduzido em casa de vegetação, durante um período de 13 dias, utilizando plantas de 12 meses de idade. O objetivo principal foi avaliar as respostas da pupunheira à deficiência hídrica. As variáveis observadas foram: taxa de assimilação de CO 2 , transpiração, condutância estomática e potencial de água das folhas. As coletas dos dados foram realizadas diariamente em laboratório e sob fluxo de 1200 µm -2 s -1 . Os resultados foram submetidos à análise de variância e de regressão. Verificou-se decréscimo no potencial de água da folha e nas trocas gasosas quando a irrigação foi interrompida por mais de seis dias. Valores mínimos foram obtidos no décimo dia, com redução de 92% da fotossíntese líquida, 87% da condutância estomática e 70% da transpiração. O menor potencial de água nas folhas (-1,9 MPa) foi também observado nesse período. Houve recuperação total de todas as variáveis dois dias após reirrigação, com exceção da condutância estomática. A diminuição da condutância estomática e a queda mais rápida da taxa de transpiração que a queda na fotossíntese, indicam a existência de mecanismos de aclimatação em pupunheira, no sentido de diminuir as perdas de água, quando sob condição de estresse hídrico moderado. Palavras-chave: Bactris gasipaes, deficiência hídrica, fotossíntese PHOTOSYNTHESIS, STOMATAL CONDUCTANCE AND TRANSPIRATION IN PEACH PALM UNDER WATER STRESSABSTRACT: 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: CO 2 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 µm -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 -1.9 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.

To understand the effect of summer and winter on the relationships between leaf carbohydrate and photosynthesis in citrus trees growing in subtropical conditions, ‘Valencia’ orange trees were subjected to external manipulation of their carbohydrate concentration by exposing them to darkness and evaluating the maximal photosynthetic capacity. In addition, the relationships between carbohydrate and photosynthesis in the citrus leaves were studied under natural conditions. Exposing the leaves to dark conditions decreased the carbohydrate concentration and increased photosynthesis in both seasons, which is in accordance with the current model of carbohydrate regulation. Significant negative correlations were found between total non-structural carbohydrates and photosynthesis in both seasons. However, non-reducing sugars were the most important carbohydrate that apparently regulated photosynthesis on a typical summer day, whereas starch was important on a typical winter day. As a novelty, photosynthesis stimulation by carbohydrate consumption was approximately three times higher during the summer, i.e. the growing season. Under subtropical conditions, citrus leaves exhibited relatively high photosynthesis and high carbohydrate levels on the summer day, as well as a high nocturnal consumption of starch and soluble sugars. A positive association was determined between photosynthesis and photoassimilate consumption/exportation, even in leaves showing a high carbohydrate concentration. This paper provides evidence that photosynthesis in citrus leaves is regulated by an increase in sink demand rather than by the absolute carbohydrate concentration in leaves.

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