This research is part of an effort that the ICT (Institute of Tropical Cultivation) has been doing for several years tending to develop superior genotypes of cocoa (Theobroma cacao L.). That is why this study aims to find tolerant or moderately tolerant cocoa genotypes and accessions to water stress with resistance to pests and diseases and high production and industrial quality. Twenty genotypes of cocoa seedlings were investigated, during the period of 6 months, in a soil with sandy-loam texture under nursery conditions, of controlled irrigation. A split plot design was used, with 40 treatments and 3 repetitions. In addition, daily data of the micro climatic characteristics (T °, HR) were taken, in which different indicators of variable were evaluated such as the stomatal conductance (CE) that is greatly influenced by the T ° and HR. The results obtained indicate that the genotypes that showed moderate tolerance to water stress were UNG - 77, UNG - 53, ICT - 1281 and ICT - 1112; the non-tolerant ones were PAS - 93, CEPEC - 2002, ICT - 2142, ICT - 1092, CP - 2005 - C10, TSH - 1188, CCN - 51, IMC - 67, PH - 17, AYP - 15, ICS - 6, BN - 34, ICT - 1506, PAS - 91, PH - 990 and ICS - 1. Under optimal conditions, the most outstanding genotype was ICS-1, both in plant height, number of leaves, and stomatal conductance, this being proof that this genotype develops excellently and stands out if it has the right conditions and water availability.

Stomatal conductance and photosynthesis

Previous studies on the Physiology of stomata in higher plants suggest that stomata influence the rate of CO2 fixation in leaf mesophyll tissue. We believe that an equally important stomatal function has not been fully recognised; that stomatal aperture is determined by the capacity of the mesophyll tissue to fix carbon. We altered the capacity of leaves to fix carbon by various means, and found invariably that the diffusive conductance of the epidermis to CO2 transfer, g, (which mainly depends on the number and dimensions of the stomata) changes in nearly the same proportion as the rate of assimilation of CO2. Thus, the intercellular concentration of CO2 (ci), calculated as ci = ca–A/g (where ca is ambient concentration of CO2, A is assimilation rate of CO2), tends to remain constant providing ca is kept constant. We used routine techniques1 to measure A and estimate g in leaves placed singly in chambers. Conductance takes account of CO2 transfer through both stomata and leaf boundary layer, the conductance of the latter being 0.5 mol m−2 s−1.

Stomatal conductance correlates with photosynthetic capacity

Stomatal pores, each surrounded by a pair of guard cells, regulate CO2 uptake and water loss from leaves. Stomatal opening is driven by the accumulation of K+ salts and sugars in guard cells, which is mediated by electrogenic proton pumps in the plasma membrane and/or metabolic activity. Opening responses are achieved by coordination of light signaling, light-energy conversion, membrane ion transport, and metabolic activity in guard cells. In this review, we focus on recent progress in blue- and red-light-dependent stomatal opening. Because the blue-light response of stomata appears to be strongly affected by red light, we discuss underlying mechanisms in the interaction between blue-light signaling and guard cell chloroplasts.

Light Regulation of Stomatal Movement

INTRODUCTION 454 TECHNIQUES ....... 454 Quantitation of ABA ... 454 Synthesis of Tritiated ABA 456 BIOCHEMISTRY 456 ABA Biosynthesis .... 456 ABA Metabolism 457 Regulation 0/ Enzyme Synthesis 460 PHYSIOLOGY 462 Water Stress 462 The stimulus fOT Ttlising ABA levels ..... ........ ........ .... ..... ..•.. .... 462 Localization and regulation of ABA. synthesis •••••••••••••••••••. ......... .... .... ........ .... 463 Stomatal complexes and A.lJA. ...... ......... ....... .... ..... ........ .... 466 Mode of action 467 A.lJA. levels and stomatal aperture ........ ....... ....... ......... ..... 469 Drought mistance ..... 470 Root Geotropism 470 Seed Development and Germination 472 Seed development 472 Seed gennil/Qtion 473 Dormancy 474 Induction of dormancy 474 Maintenance of dormancy 475 Correlative Inhibition 0/ Buds 475 Ion Movement 477 ABA tran.spoTf 478 CONCLUDING REMARKS 479

Biochemistry and physiology of abscisic acid.

Plants both lose water and take in carbon dioxide through microscopic stomatal pores, each of which is regulated by a surrounding pair of guard cells. During drought, the plant hormone abscisic acid (ABA) inhibits stomatal opening and promotes stomatal closure, thereby promoting water conservation. Dozens of cellular components have been identified to function in ABA regulation of guard cell volume and thus of stomatal aperture, but a dynamic description is still not available for this complex process. Here we synthesize experimental results into a consistent guard cell signal transduction network for ABA-induced stomatal closure, and develop a dynamic model of this process. Our model captures the regulation of more than 40 identified network components, and accords well with previous experimental results at both the pathway and whole-cell physiological level. By simulating gene disruptions and pharmacological interventions we find that the network is robust against a significant fraction of possible perturbations. Our analysis reveals the novel predictions that the disruption of membrane depolarizability, anion efflux, actin cytoskeleton reorganization, cytosolic pH increase, the phosphatidic acid pathway, or K+ efflux through slowly activating K+ channels at the plasma membrane lead to the strongest reduction in ABA responsiveness. Initial experimental analysis assessing ABA-induced stomatal closure in the presence of cytosolic pH clamp imposed by the weak acid butyrate is consistent with model prediction. Simulations of stomatal response as derived from our model provide an efficient tool for the identification of candidate manipulations that have the best chance of conferring increased drought stress tolerance and for the prioritization of future wet bench analyses. Our method can be readily applied to other biological signaling networks to identify key regulatory components in systems where quantitative information is limited.

/pdf/predicting-essential-components-of-signal-transduction-114qg54czg.pdf

Predicting Essential Components of Signal Transduction Networks: A Dynamic Model of Guard Cell Abscisic Acid Signaling

Epidermal strips with closed stomata were exposed to malic acid labelled with (14)C either uniformly or in 4-C only. During incubation with [U-(14)C]malate, radioactivity appeared in products of the tricarboxylic-acid cycle and in transamination products within 10 min, in sugars after 2 h. Hardly any radioactivity was found in sugars if [4-(14)C]malate had been offered. This difference in the degree of labelling of sugars indicates that gluconeogenesis can occur in epidermal tissue, involving the decarboxylation of malate. Epidermis incubated with labelled malate was hydrolyzed after extraction with aqueous ethanol. The hydrolysate contained glucose as the only radioactive product, indicating that starch had been formed from malate. Microautoradiograms were black above stomatal complexes, showing that the latter were sites of starch formation. In order to follow the fate of malate during stomatal closure, malate was labelled in guard cells by exposing epidermes with open stomata to (14)CO2 and then initiating stomatal closure. Of the radioactive fixation products of CO2 only malate was released into the water on which the epidermal samples floated; the epidermal strips retained some of the malate and all of its metabolites. In the case of rapid stomatal closure initiated by abscisic acid and completed within 5 min, 63% of the radioactivity was in the malate released, 22% in the malate retained, the remainder in aspartate, glutamate, and citrate. We conclude that during stomatal closing guard cells can dispose of malate by release, gluconeogenesis, and consumption in the tricarboxylic-acid cycle.

Malate metabolism in isolated epidermis of Commelina communis L. in relation to stomatal functioning.

Isolated epidermes of Tulipa gesneriana L and Commelian communis L were exposed to 14CO2 in the light and in darkness, when stomata were either closed or open The labelling patterns did not differ: the main products of CO2 fixation were malate and aspartate Small amounts of radioactivity appeared also in acids of the tricarboxylic-acid cycle and their transamination products Since the capacity of epidermis to assimilate CO2 is known to reside in the guard cells, we can state that guard cells continuously take up CO2 if present, and are thus able to recognize the presence of CO2 in their environment at all times Epidermal samples exposed to 14CO2 in the light contained only small amounts of radioactive 3-phosphoglyceric acid (3-PGA) and sugar phosphates, or none at all Epidermal samples from Commelina communis did not contain labelled 3-PGA if all adhering mesophyll cells had been removed before exposure to 14CO2 Homogenates of clean epidermal strips of Commelina communis were able to convert exogenous ribulose diphosphate to 3-PGA at a low rate, but could not catalyze the conversion of exogenous ribulose-5-phosphate to ribulose diphosphate Guard cells of Commelina communis, and probably also those of Tulipa gesneriana, appear not to possess the reductive pentosephosphate pathway, despite the presence of chloroplasts In such species, the guard cells will have to rely on import in order to maintain their carbon balance Earlier findings of photosynthetic reduction of CO2 by epidermal tissues were probably obtained with samples that were contaminated with mesophyll cells

[(14)C]Carbon-dioxide fixation by isolated leaf epidermes with stomata closed or open.

Isolated epidermis of Commelina communis L. and Tulipa gesneriana L. assimilated 14CO2 into malic acid and its metabolites but not into sugars or their phosphates; epidermis could not reduce CO2 by photosynthesis and therefore must be heterotrophic (Raschke and Dittrich, 1977). If, however, isolated epidermis of Commelina communis was placed on prelabelled mesophyll (obtained by an exposure to 14CO2 for 10 min), radioactive sugars appeared in the epidermis, most likely by transfer from the mesophyll. Of the radioactivity in the epidermis, 60% was in sucrose, glucose, fructose, 3-phosphoglyceric acid and sugar phosphates. During a 10-min exposure to 14CO2, epidermis in situ incorporated 16 times more radioactivity than isolated epidermal strips. Isolated epidermis of Commelina communis and Tulipa gesneriana took up 14C-labelled glucose-1-phosphate (without dephosphorylation), glucose, sucrose and maltose. These substances were transformed into other sugars and, simultaneously, into malic acid. Carbons-1 through-3 of malic acid in guard cells can thus be derived from sugars. Radioactivity appeared also in the hydrolysate of the ethanol-insoluble residue and in compounds of the tricarboxylic-acid cycle, including their transamination products. The hydrolysate contained glucose as the only radioactive compound. Radioactivity in the hydrolysate was therefore considered an indication of starch. Starch formation in the epidermis began within 5 min of exposure to glucose-1-phosphate. Autoradiograms of epidermal sections were blackened above the guard cells. Formation of starch from radioactive sugars therefore occurred predominantly in these cells. Epidermis of tulip consistently incorporated more 14C into malic and aspartic acids than that of Commelina communis (e.g. after a 4-h exposure to [14C]glucose in the dark, epidermis, with open stomata, of tulip contained 31% of its radioactivity in malate and aspartate, that of Commelina communis only 2%). The results of our experiments allow a merger of the old observations on the involvement of starch metabolism in stomatal movement with the more recent recognition of ion transfer and acid metabolism as causes of stomatal opening and closing.

K. Raschke

Papers

Malate metabolism in isolated epidermis of Commelina communis L. in relation to stomatal functioning.

[(14)C]Carbon-dioxide fixation by isolated leaf epidermes with stomata closed or open.

Uptake and metabolism of carbohydrates by epidermal tissue.