Raimundo Santos Barros

Nitric oxide, stomatal closure, and abiotic stress

Various data indicate that nitric oxide (NO) is an endogenous signal in plants that mediates responses to several stimuli. Experimental evidence in support of such signalling roles for NO has been obtained via the application of NO, usually in the form of NO donors, via the measurement of endogenous NO, and through the manipulation of endogenous NO content by chemical and genetic means. Stomatal closure, initiated by abscisic acid (ABA), is effected through a complex symphony of intracellular signalling in which NO appears to be one component. Exogenous NO induces stomatal closure, ABA triggers NO generation, removal of NO by scavengers inhibits stomatal closure in response to ABA, and ABA-induced stomatal closure is reduced in mutants that are impaired in NO generation. The data indicate that ABA-induced guard cell NO generation requires both nitric oxide synthase-like activity and, in Arabidopsis, the NIA1 isoform of nitrate reductase (NR). NO stimulates mitogen-activated protein kinase (MAPK) activity and cGMP production. Both these NO-stimulated events are required for ABA-induced stomatal closure. ABA also stimulates the generation of H2O2 in guard cells, and pharmacological and genetic data demonstrate that NO accumulation in these cells is dependent on such production. Recent data have extended this model to maize mesophyll cells where the induction of antioxidant defences by water stress and ABA required the generation of H2O2 and NO and the activation of a MAPK. Published data suggest that drought and salinity induce NO generation which activates cellular processes that afford some protection against the oxidative stress associated with these conditions. Exogenous NO can also protect cells against oxidative stress. Thus, the data suggest an emerging model of stress responses in which ABA has several ameliorative functions. These include the rapid induction of stomatal closure to reduce transpirational water loss and the activation of antioxidant defences to combat oxidative stress. These are two processes that both involve NO as a key signalling intermediate.

Ecophysiology of coffee growth and production

, which together account for 99% of the world coffee bean production. Thisreview is organized into sections dealing with (i) climatic factors and environmental requirements, (ii) root and shootgrowth, (iii) blossoming synchronisation, fruiting and cup quality, (iv) competition between vegetative andreproductive growth and branch die-back, (v) photosynthesis and crop yield, (vi) physiological components of cropyield, (vii) shading and agroforestry systems, and (viii) high-density plantings.

DECLINE OF VEGETATIVE GROWTH IN COFFEA ARABICA L. IN RELATION TO LEAF TEMPERATURE, WATER POTENTIAL AND STOMATAL CONDUCTANCE

Abstract The decline of vegetative growth of Arabica coffee trees in Vicosa (20° 45′ S, 650 m altitude), Brazil, occurring from mid-March through late May, was observed in both irrigated and non-irrigated plants and did not show a direct relation with leaf water potential. Declining growth coincided with lowering temperatures, and particularly with increasing daily periods with low temperatures. Stomatal conductances in the morning (0800–0900 h) were relatively high until mid-March and decreased gradually in parallel with declining growth rates. During the quiescent growth phase in the cool season, starting from late May, stomata were closed for most of the diurnal period.

Photosynthesis in coffee (Coffea arabica and C. canephora) as affected by winter and summer conditions

Abstract Photosynthetic and biochemical responses of potted plants of Coffea arabica cv Red Catuai and C. canephora cv Kouillou were examined during winter and summer. There were large decreases in the photosynthetic rates of both cultivars from summer to winter, with more pronounced declines shown by Catuai. The photochemical efficiency of photosystem II, as evaluated by the ratio of variable to maximum chlorophyll fluorescence, decreased to a greater extent in Catuai than in Kouillou during winter. This probably was a secondary effect due to the loss of the photosynthetic capacity, which was mostly determined by non-stomatal factors. In response to winter conditions, Kouillou accumulated three times more proline and ascorbate with no changes in the content of malonedialdehyde (MDA). Catuai accumulated 160% more proline and 206% more MDA in winter in comparison to summer values, but there was no significant change in ascorbate levels. The content of chlorophylls and carotenoids did not change in response to natural seasonal acclimation. The leaf starch content in winter was higher by 53% in Catuai than in Kouillou. Photosynthetic rates, as measured by oxygen evolution, were well correlated with starch content in Catuai, but not in Kouillou. When winter-grown leaves were darkened for 96 h, their normal starch levels were halved; this was accompanied by the restoration of the photosynthetic rates to levels similar to those found in summer in both cultivars, with a concurrent resumption of the photosystem II efficiency.

Differential requirement for NO during ABA-induced stomatal closure in turgid and wilted leaves.

Abscisic acid (ABA)-induced stomatal closure is mediated by a complex, guard cell signalling network involving nitric oxide (NO) as a key intermediate. However, there is a lack of information concerning the role of NO in the ABA-enhanced stomatal closure seen in dehydrated plants. The data herein demonstrate that, while nitrate reductase (NR)1-mediated NO generation is required for the ABA-induced closure of stomata in turgid leaves, it is not required for ABA-enhanced stomatal closure under conditions leading to rapid dehydration. The results also show that NO signalling in the guard cells of turgid leaves requires the ABA-signalling pathway to be both capable of function and active. The alignment of this NO signalling with guard cell Ca(2+)-dependent/independent ABA signalling is discussed. The data also highlight a physiological role for NO signalling in turgid leaves and show that stomatal closure during the light-to-dark transition requires NR1-mediated NO generation and signalling.

Photosynthetic performance of two coffee species under drought

Coffea arabica cv. Red Catuaí and C. canephora cv. Kouillou were grown in pots beneath a plastic shelter. When they were 14 months old, irrigation was withheld until the leaf pre-dawn water potential was about -1.5 and -2.7 MPa (designated mild and severe water stress, respectively). Under mild stress, net photosynthetic rate (PN) decreased mainly as a consequence of stomatal limitations in Kouillou, whereas such decreases were dominated by non-stomatal limitations in Catuaí. Under severe drought, further decreases in PN and apparent quantum yield were not associated to any changes in stomatal conductance in either cultivar. Decreases were much more pronounced in Catuaí than in Kouillou, the latter maintained carbon gain at the expense of water conservation. In both cultivars the initial chlorophyll (Chl) fluorescence slightly increased with no changes in the quantum efficiency of photosystem 2. In response to rapidly imposed drought, the Chl content did not change while saccharide content increased and starch content decreased. Photoinhibition and recovery of photosynthesis, as evaluated by the ratio of variable to maximum fluorescence and by the photosynthetic O2 evolution, were unaffected by mild drought stress. Photoinhibition was enhanced under severe water deficit, especially in Catuaí. In this cultivar the O2 evolution did not resume upon reversal from photoinhibition, in contrast to the complete recovery in Kouillou.

Limitations to photosynthesis in coffee leaves from different canopy positions

Limitations to photosynthesis were explored in leaves from four canopy positions of field-grown, unshaded coffee (Coffea arabica L.), a tropical tree species classified as shade-obligatory. Overall, compared to shade (lower) leaves, sun (upper) leaves had higher net carbon assimilation rate (A) (4.5 against 2.0 micromol m(-2)s(-1) at most) associated with higher electron transport rate (due to a greater irradiance availability) but unrelated to stomatal and mesophyll conductances, which were similar regardless of leaf position. Neither physiological variable directly involved with photosynthetic carbon gain nor those involved with light capture were able to adjust themselves to match the capacity of the photosynthetic machinery to the light supply. We concluded that: (i) there was no major difference in photosynthetic capacity between sun and shade leaves; (ii) the intrinsic low A in coffee was greatly associated with remarkable low diffusive limitations rather than with biochemical or photochemical constraints; and (iii) morphological (e.g., variations in specific leaf area and leaf inclination) or anatomical plasticity should be of greater acclimative value than physiological plasticity as a mean of coffee leaves to respond to changing irradiance.

Ethylene and polyamine production patterns during in vitro shoot organogenesis of two passion fruit species as affected by polyamines and their inhibitor

Levels of ethylene and polyamines (PAs) were measured during organogenesis of hypocotyl explants of two species of passion fruit (Passiflora cincinnata Masters and Passiflora edulis Sims f. flavicarpa Degener ‘FB-100’) to better understand the relationships of these regulators and their influence on cell differentiation and morphogenesis. Moreover, histological investigation of shoot ontogenesis was conducted to characterize the different events involved in cell redifferentiation and regulation of PA and ethylene levels. A delay was observed in morphogenic responses of P. edulis f. flavicarpa as compared to P. cincinnata, and these changes coincided with production of elevated levels of polyamine and ethylene levels. During differentiation, cells showed high rates of expansion and elongation, and high ethylene levels were associated with high PA levels, suggesting that the two biosynthesis pathways were highly regulated. Moreover, their interaction might be an important factor for determining cell differentiation. The addition of PAs to the culture medium did not promote organogenesis; however, the incorporation of the PA inhibitor methylglyoxal bisguanylhydrazone in the culture medium reduced shoot bud differentiation, suggesting the need to maintaining a minimum level of PAs for morphogenic events to take place.

Role of nitric oxide in regulating stomatal apertures

During stomatal closure, nitric oxide (NO) operates as one of the key intermediates in the complex, abscisic acid (ABA)-mediated, guard cell signaling network that regulates this process. However, data concerning the role of NO in stomatal closure that occurs in turgid vs. dehydrated plants is limited. The data presented demonstrate that, while there is a requirement for NO during the ABA-induced stomatal closure of turgid leaves, such a requirement does not exist for ABA-enhanced stomatal closure observed to occur during conditions of rapid dehydration. The data also indicate that the ABA signaling pathway must be both functional and to some degree activated for guard cell NO signaling to occur. These observations are in line with the idea that the effects of NO in guard cells are mediated via a Ca2+-dependent rather than a Ca2+-independent ABA signaling pathway. It appears that there is a role for NO in the fine tuning of the stomatal apertures of turgid leaves that occurs in response to fluctuations in the prevailing environment.

Accumulation of proline and quaternary ammonium compounds in mature leaves of water stressed coffee plants (Coffea arabica and C. canephora)

SummaryWater stressed coffee plants of five genotypes of C. arabica and one of C. canephora were compared with watered controls for their accumulation of proline and quaternary ammonium compounds. These solutes, mainly proline, accumulated in mature leaves under water stress, their concentration being related to the osmotic potential at zero turgor and to the osmotic adjustment either in well watered or droughted plants. Proline accumulated to a greater extent than quaternary ammonium compounds under water stress.