Abstract
In the present work some dehydration-induced responses at whole plant and cellular levels were evaluated in three Vigna genotypes (V. glabrescens, Vg; V. unguiculata cv. EPACE-1; V. unguiculata cv. 1183) from different origins. Changes in leaf stomatal conductance (gs), membrane lipids composition and abscisic acid (ABA) content were determined under water stress. Membrane integrity was assessed through leaf discs dehydration with polyethylene glycol (PEG 8000, -1.3 MPa), and expressed as an injury index (I%).. In EPACE-1 and 1183, rapid stomatal closure at early stages of dehydration was related with the ability to accumulate endogenous ABA. On the contrary in Vg stomata closed at lower leaf relative water content as ABA increase was delayed. In 1183, high I% values occurred in PEG-dehydrated discs, and lipid degradation was observed even under moderate water stress (S1), becoming significant under more severe drought (S2). Also, along drought imposition this genotype presented a continuous leaf ABA increase that probably contributed to early visual tissue senescence. Vg and EPACE-1 presented lower I% values, probably denoting the ability to preserve membrane integrity. In EPACE-1 this was achieved through a quite stable lipid content, while in Vg new membrane lipids were synthesized. It is suggested that a higher protoplasmic tolerance in EPACE-1 and Vg might be related to stable and low endogenous ABA contents under increasing water deficit.
Highlights
Legumes are agronomically important crops mainly due to their nitrogen fixing capacity and to their high protein content
EPACE-1 and 1183 presented lower gs values than V. glabrescens (Vg). Such difference of behavior was maintained for relative water content (RWC) values around 80%, when gs remained high in Vg while EPACE-1 and 1183 presented already gs values below 100 mmol H2O m-2 s-1
In the second plants tolerate a lower water availability without strong gs reduction, as happened in V. glabrescens. Such stomatal closure at moderate dehydration (80% RWC), occurring in V. unguiculata cvs. 1183 and EPACE-1, was probably controlled by endogenous leaf abscisic acid (ABA) that increased in both genotypes
Summary
Legumes are agronomically important crops mainly due to their nitrogen fixing capacity and to their high protein content. Genetic variation in shoot sensitivity to ABA imported from the root may be important in mediating the root signal effect (Blum and Sinmena, 1995) This growth regulator is involved in triggering tolerance mechanisms at cellular level (Ingram and Bartels, 1996). ABA can directly affect membrane physical properties, like microviscosity (Shripathi et al, 1997), fluidity (Stillwell et al, 1988), and permeability to ions and solutes (Stillwell and Hester, 1984) Some of these modifications seem to be related to changes in lipid composition of cellular membranes (Shripathi et al, 1997; Campos and Pham Thi, 1997)
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