Photosynthetic performance in jack bean [Canavalia ensiformis (L.) D.C.] under drought and after rehydration

Desempenho fotossintético de feijão-de-porco [Canavalia ensiformis (L.) D.C.] sob déficit hídrico e após reidratação

Abstracts

The effects of drought and rehydration on Canavalia ensiformis (L.) D.C. (jack bean) plants were evaluated using the following gas exchange parameters: net carbon assimilation rate (A), stomatal conductance (g s), Ci/Ca ratio and transpiration rate (E); chlorophyll a fluorescence: Fv/Fm and Fv/F0 ratio. The plants were cultivated under greenhouse conditions and after 30 days from the emergence, irrigation was suspended in the plants submitted to drought, to obtain the following predawn leaf water potential (psipd): -0.40 MPa (control), -1.00 MPa (moderate drought) and -2.30 Mpa (severe drought). Afterwards, the gas exchange and fluorescence analysis were initiated , and 24 h after rehydration the same analyses were repeated. The A, E, g s and Ci/Ca values decreased significantly under both drought treatments, without however changing the Fv/Fm and Fv/F0 values. The gas exchange parameters recovered after rehydration. It seems that drought affected photosynthesis by stomatal inhibition, as shown by the decreased gs and Ci/Ca values, besides the maintenance of PSII phtotochemical efficiency. The recovery of gas exchange after rehydration could be due to plant protection mechanisms.

fluorescence; moderate drought; photosynthesis; severe drought; stomatal conductance

Os efeitos do déficit hídrico e posterior reidratação foram avaliados em Canavalia ensiformis (L.) D.C., mediante parâmetros de trocas gasosas: fotossíntese (A), condutância estomática (g s), razão Ci/Ca e transpiração (E); fluorescência da clorofila a: razão Fv/Fm e Fv/F0. As plantas cresceram em casa de vegetação e após após 30 dias da emergência, suspendeu-se a irrigação naquelas submetidas ao déficit hídrico, obtendo-se os seguintes potenciais hídricos na antemanhã (psiam): -0,40 MPa (controle), -1,00 MPa (déficit hídrico moderado) e -2,30 MPa (déficit hídrico severo). Após, foram realizadas as análises de trocas gasosas e de fluorescência. As plantas foram reidradatas e, após um período de 24 h, as mesmas análises foram repetidas. Os regimes de déficit hídrico provocaram decréscimos significativos em A, E, g s e na razão Ci/Ca; contudo, não alteraram as razões Fv/Fm e Fv/F0. Após a reidratação houve o restabelecimento das trocas gasosas. Conclui-se que o déficit hídrico afetou negativamente a fotossíntese, mediante uma limitação estomática, o que se confirma pelos decréscimos em gs, na razão Ci/Ca e na manutenção da eficiência fotoquímica do FS II. Provavelmente, mecanismos de proteção tenham sido responsáveis pelo restabelecimento das trocas gasosas após a reidratação.

condutância estomática; déficit hídrico moderado; déficit hídrico severo; fluorescência; fotossíntese

SHORT COMMUNICATION

Photosynthetic performance in jack bean [Canavalia ensiformis (L.) D.C.] under drought and after rehydration

Desempenho fotossintético de feijão-de-porco [Canavalia ensiformis (L.) D.C.] sob déficit hídrico e após reidratação

Fábio ZanellaI, ^*; Tania Misae Watanabe^II; Ana Lúcia da Silva Lima^I; Marlene Aparecida Schiavinato^II

^ICentro Universitário Luterano de Ji-Paraná, Universidade Luterana do Brasil, CP 271, CEP 78961-970, Ji-Paraná-RO, Brasil

^IIDepartamento de Fisiologia Vegetal, Universidade Estadual de Campinas, CP 6109, CEP 13083-970, Campinas, SP, Brasil

ABSTRACT

The effects of drought and rehydration on Canavalia ensiformis (L.) D.C. (jack bean) plants were evaluated using the following gas exchange parameters: net carbon assimilation rate (A), stomatal conductance (g_s), C_i/C_a ratio and transpiration rate (E); chlorophyll a fluorescence: F_v/F_m and F_v/F₀ ratio. The plants were cultivated under greenhouse conditions and after 30 days from the emergence, irrigation was suspended in the plants submitted to drought, to obtain the following predawn leaf water potential (Y_pd): -0.40 MPa (control), -1.00 MPa (moderate drought) and -2.30 Mpa (severe drought). Afterwards, the gas exchange and fluorescence analysis were initiated , and 24 h after rehydration the same analyses were repeated. The A, E, g_s and C_i/C_a values decreased significantly under both drought treatments, without however changing the F_v/F_m and F_v/F₀ values. The gas exchange parameters recovered after rehydration. It seems that drought affected photosynthesis by stomatal inhibition, as shown by the decreased gs and Ci/Ca values, besides the maintenance of PSII phtotochemical efficiency. The recovery of gas exchange after rehydration could be due to plant protection mechanisms.

Key words: fluorescence, moderate drought, photosynthesis, severe drought, stomatal conductance.

RESUMO

Os efeitos do déficit hídrico e posterior reidratação foram avaliados em Canavalia ensiformis (L.) D.C., mediante parâmetros de trocas gasosas: fotossíntese (A), condutância estomática (g_s), razão C_i/C_a e transpiração (E); fluorescência da clorofila a: razão F_v/F_m e F_v/F₀. As plantas cresceram em casa de vegetação e após após 30 dias da emergência, suspendeu-se a irrigação naquelas submetidas ao déficit hídrico, obtendo-se os seguintes potenciais hídricos na antemanhã (Y_am): -0,40 MPa (controle), -1,00 MPa (déficit hídrico moderado) e -2,30 MPa (déficit hídrico severo). Após, foram realizadas as análises de trocas gasosas e de fluorescência. As plantas foram reidradatas e, após um período de 24 h, as mesmas análises foram repetidas. Os regimes de déficit hídrico provocaram decréscimos significativos em A, E, g_s e na razão C_i/C_a; contudo, não alteraram as razões F_v/F_m e F_v/F₀. Após a reidratação houve o restabelecimento das trocas gasosas. Conclui-se que o déficit hídrico afetou negativamente a fotossíntese, mediante uma limitação estomática, o que se confirma pelos decréscimos em gs, na razão Ci/Ca e na manutenção da eficiência fotoquímica do FS II. Provavelmente, mecanismos de proteção tenham sido responsáveis pelo restabelecimento das trocas gasosas após a reidratação.

Palavras-chave: condutância estomática, déficit hídrico moderado, déficit hídrico severo, fluorescência, fotossíntese.

During centuries, the jack bean [Canavalia ensiformis (L.) D.C.] legume has been used by local inhabitants of the southwest United States, Central America, Mexico, Brazil, Peru, Equator and of the west of India. The great adaptability of C. ensiformis to adverse conditions, mainly soil related, has been of great relevance for the high protein production in regions inept for agriculture. Besides the grains being a good source of protein and its use as livestock feed, this plant is also used in soil recovery in several countries (Lynd and Ansman, 1989).

For most plants, drought is one of the factors that most limits photosynthesis (García-Plazaola and Becerril, 2000). The intensity of water deficit is commonly evaluated by the leaf water potential (Y_w). Values of -0.9; -1.5 and -1.3 MPa represented moderate drought for bean (Cornic et al., 1992), coffee (Da Matta et al., 1997) and tomato (Haupt-Herting et al., 2001), respectively. The values of -2.7 MPa for coffee (Da Matta et al., 1997) and -1.8 MPa for tomato (Haupt-Herting et al., 2001), were assumed to be severe drought.

The stomatal closure is among the first responses to the water stress, and is assumed to be the main cause of impaired photosynthesis induced by drought, since the stomatal closure limits CO₂ availability to the mesophyll (Chaves, 1991). In view of this, a decrease in net photosynthesis under drought depends more on the availability of CO₂ in the chloroplast than of leaf water potential (Sharkey, 1990). This fact can be interpreted as a direct adjustment of photosynthesis to CO₂ availability, which acts by regulating the activity of Rubisco (Perchorowicz and Jensen, 1983).

Under natural conditions drought usually occurs in association with high temperatures and high irradiance (Pereira and Chaves, 1993), resulting in photoinhibition of photosynthesis, characterized by a decrease in the PSII photochemistry efficiency. Certainly, PSII is the main target of photoinhibitory damage (Barber and Anderson, 1992), although Genty et al. (1987) and Havaux (1992) have inferred that drought has little effect on PSII functioning. Though an evaluation of photosynthetic performance, the aim of the present work was to investigate the effect of drought and rehydration in jack bean plants.

The experiment was carried out at the Plant Physiology Department of the State University of Campinas (UNICAMP), SP, Brazil (22º54´S and 47º05´W), from September to October 2002, in the greenhouse under natural light and temperature conditions. Seeds of jack bean [Canavalia ensiformis (L.) D.C.] were sown in trays filled with vermiculite. After emergence the seedlings were inoculated by immersing roots in a suspension of previously selected Rizobium, and then transferred to 5 L polyethylene pots with vermiculite as substrate. The plants were supplied with Hoagland and Arnon (1950) N-deficient nutrient solution twice weekly, and water as required. Thirty days after emergence, watering was suspended for the first lot of 15 plants, in order to induce severe drought (SD). Seven days later, the same procedure was carried out for another lot of 15 plants, to induce moderate drought (MD). By this means, 26 and 19 days after suspending water, the following predawn leaf water potentials (Y_pd) were obtained: -1.0 MPa (MD) and -2.30 MPa (SD). In the control plants (C) which remained watered, the value of Y_pd was -0.4 MPa.

The predawn leaf water potential was evaluated by means of pressure chamber (PMS). The gas exchange parameters, net carbon assimilation rate (A), stomatal conductance (g_s) and transpiration rate (E), were measured under natural light at about 8:30 am (temperature: 29ºC; PAR: 801 µmol.m^-2.s^-1), using a portable open-system infrared gas analyzer (LCA4, Analytical Development Company). The internal CO₂ concentration (C_i) and the ambient CO₂ concentration (C_a) values were used in order to calculate the C_i/C_a ratio. The chlorophyll a fluorescence, in the form of F_v/F_m and F_v/F₀ ratios, was measured at room temperature using a portable mini-pulse amplitude modulation fluorometer (MiniPAM, Walz), in dark-adapted leaves for 30 min. All the analyses were carried using the middle foliole of the third expanded trifoliate leaf from the apex. The plants were distributed in a completely randomized layout, with five replicates. Each experimental plot was composed of one plant per tray. Statistical significance of mean differences were analyzed by Duncan's test, at P < 0.05.

All the gas exchange parameters decreased under both drought treatments, but they recovered within 24 h after rehydration (figure 1). The observed decline in g_s and E (figure 1), indicates that drought caused stomatal inhibition with negative reflexes on the photosynthetic CO₂ uptake and transpiration rate. The decrease in the C_i/C_a ratio in the present of drought is consistent with this interpretation (figure 1), since increases in this ratio demonstrate decreased uptake of CO₂ due to non-stomatal inhibition of photosynthesis. According to Baker (1993), there is a direct relationship between the reduction of intercellular CO₂ concentration, due to stomatal closure, and decreases in CO₂ assimilation.

Although drought impaired photosynthesis it did not damage the photosynthetic apparatus, as demonstrated by the F_v/F_m and F_v/F₀ ratios (table 1). F_v/F_m values were 0.78, 0.81 and 0.80 in control, MD and SD treatments, respectively, while the F_v/F₀ ratio ranged from 3.8 to 4.3. According to Flexas et al. (2002) the F_v/F_m ratio in plants of grapevine under drought remained around 0.8, as in the present work in spite of significant decreases in the stomatal conductance.

The reduction in photosynthesis in plants of Myracrodruon urundeuva under drought occurred mainly because of stomatal closure rather than damage to PSII (Queiroz et al., 2002). In coffee plants under drought conditions partial maintenance of the quantum yield of PSII was observed in spite of photosynthesis suppression. In this case, some processes could contribute to the maintenance of electron flow, such as the Mehler reaction and the photorespiratory process (Lima et al., 2002). In this context, the fall of net carbon assimilation rate caused by drought was not accompanied by decreases in F_v/F_m, showing that PSII is resistant to drought, as has been shown for the cotton plant (Genty et al., 1987), coffee tree (Da Matta et al., 1997) and Casuarina equisetifolia (Sanchez-Rodriguez, 1997). However, photoinhibitory damages to PSII were found by He et al. (1995), who verified degradation of the D1 and D2 proteins (mainly D2) in plants under drought. In olive trees, the photosynthetic apparatus was resistant to both weak and moderate drought, stomatal closure being the main factor limiting photosynthesis. However, under severe drought, decreases in the photosynthesis were attributed to photoinhibitory phenomena associated with an over-excitation of PSII (Angelopoulos et al., 1996).

In view of this decline in photosynthesis, less photochemical energy would be spent on CO₂ assimilation. Consequently, the photochemical energy would need to be consumed by alternative pathways. One possibility is the loss of energy by heat (Krause and Weis, 1991). Data on non-photochemical quenching (NPQ) gives information on the fraction of luminous energy lost as heat (Lima et al., 2002). In this regard, Casper et al. (1993) suggest that under drought, protection mechanisms such as the zeaxanthine cycle could be active and thereby prevent damage to the photosynthetic apparatus.

Considering the results of this study, we conclude that the two drought treatments negatively affected photosynthesis and all gas exchange parameters, and these could be re-established within 24 h after rehydration. The inhibitory effect of drought on photosynthesis could be attributed to some stomatal limitation. Neither of the two drought-imposed treatments affected the maintenance of PSII photochemistry efficiency, which could be attributed to protection mechanisms.

Acknowledgments: Fábio Zanella and Ana Lúcia da Silva Lima held doctoral fellowships granted by CNPq and CAPES, respectively.

Received: 21/06/2004, Accepted: 17/11/2004

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Publication Dates

• Publication in this collection
  14 Feb 2005
• Date of issue
  Dec 2004

History

• Accepted
  17 Nov 2004
• Received
  21 June 2004