Wilting contributes to genotypic differences in the response of transpiration rate to soil drying

VELÁZQUEZ, L; ALBERDI, I; AGUIRREZÁBAL, L.A.N; PEREYRA IRUJO, G.A

Mar del Plata

Conferencia; 18 th International Sunflower Confernece; 2012

ASAGIR - ISA

 Plants respond to soil drying with different mechanisms which ultimately lead to reduced transpiration rate. Numerous studies have shown the importance of stomatal control mechanisms in the regulation of this response. The effect of changes in the position of the leaves, however, have been less studied. In sunflower, the effect of leaf wilting on transpiration rate has not been quantified. The objective of this work was to quantify the contribution of the effect of wilting to genotypic differences in the response of transpiration rate to soil drying.  Two experiments were carried out: the first, for an evaluation of genotypic variability in wilting response, and the second, carried out in a growth chamber with the two most contrasting genotypes (HA89 and ND761), in which all the leaves of half the plants were kept in a horizontal position with a wire structure, while the other half of plants were untreated (control treatment). Plants were grown in a greenhouse in 2.7L pots filled with soil. For measurements, pots were irrigated to different soil water contents, covered with polyethylene to prevent water loss by evaporation, and left to dry down to a water potential of about -2 MPa. In both experiments, whole-plant transpiration rate, stomatal conductance and angle of the youngest fully-expanded leaf were measured. In experiment 2, quantum yield of photosystem II and leaf temperature were also measured.  In both experiments, genotype HA89 showed a more linear response of transpiration rate to soil water deficit than ND761, along with a more marked wilting response. Keeping leaves in a horizontal position -thus eliminating the effects of wilting- increased transpiration rates at low soil water contents. The magnitude of the effect of wilting on transpiration rate was higher for HA89 than for ND761 and was closely related to the cosine of measured leaf angle. Leaf angle and temperature together explained 79% of the variability in the quantum yield of photosystem II, which decreased with increased temperature in horizontal leaves, and increased with decreasing leaf angle in control leaves. Keeping the leaves in a horizontal position increased the curvature of the fitted function of transpiration rate vs. soil water content for HA89, making it more similar to that of ND761; the effect of wilting increased this genotypic difference by 50%.  Wilting contributes, in a genotype-dependent manner, to the response of whole-plant transpiration rate to soil drying by changing the shape of the response function. About one-third of the genotypic difference in the curvature of the response was explained by this mechanism. Our results confirm that this mechanism allows sunflower plants to conserve more soil water, and protects them from thermal stress and high radiation loads. This study was carried out under controlled conditions; under field conditions, the effect of wilting on transpiration would be expected to be increased by higher incident radiation but decreased by wind.  These results contribute to understanding the response of transpiration rate to soil drying in sunflower, and point out a mechanism, in addition to stomatal control, regulating this response. Such adaptive traits have the potential of increasing performance under stress without yield penalties under favourable conditions. Characterization of this trait, together with the availability of public inbred lines with contrasting response, could help breeders in obtaining new sunflower varieties with increased adaptation to water deficit scenarios.