Importancia ecofisiologica da reserva de xiloglucano e o controle de sua mobilização em cotiledones de Hymenaea courbaril L

AUTOR(ES)
DATA DE PUBLICAÇÃO

2002

RESUMO

Hymenaea courbarí/ is a climax leguminous tree species considered to be shade tolerant with wide neotropical geographic distribution. Seed cotyledons are rich in xyloglucan (XG), which is the main carbon reserve for the seedling growth. This polymer is a cell wall polysaccharide and its mobilisation mechanism, based on the co-ordinated action of four enzymes, has been shown to be rather complex. The objectives of this work were to characterise the ecophysiological importance of XG for the establishment of seedlings of Hymenaea courbarí/var. stí/bocarpa and to describe the control mechanism of the mobilisation process of this reserve. The results demonstrated that the importance of XG reserves range from seed imbibition up to the establishment of leaves that would be able to maintain autotrophic growth in understorey conditions. XG concentration presented an inverse relationship with the speed of seed imbibition. Considering the relatively large size of seeds of H. courbarí/ (ca. 5g), this control avoids premature initiation of germination during short periods of water availability and protect to dessecation after imbibition. As a source of carbon, XG is essential only after germination and emergence of the seedlings. (3050 days after the start of imbibition). During this period, the products of XG degradation are directed mainly to expanding eophylls and first metaphyll. During this same period, photosynthetic activity is established and was capable to fix carbon in low light such as 1% of full sun light (light compensation point = 12 Ilmol. m-2.s-1). The absence of this reserve by excision of cotyledons before xyloglucan mobilisation becomes a critica I point, mainly for seedlings growing in the forest, since in these conditions, a reduction of 77% of total leaf area was observed. This synchronism between XG degradation and leaf expansion was characterised through the analysis of the endogenous concentrations of 3-indol acetic acid (1M) in the cotyledons, which presented an exponential increase (ca. 8 fold) in step with the rise in XG hydrolase activities. We demonstrated that, during this period, 1M is transported mainly from the expanding eophylls to the cotyledons. This type of hormonal control is likely to be related with the synchronism observed among storage mobilisation, leaf expansion and light availability, therefore providing H. courbarí/with a powerful set of tools for establishment of its seedlings in the shaded understorey of the rain forest. These results also permit to speculate that, from the evolutionary pOint of view, storage XG degradation might have derived from the metabolism of primary cell walls

ASSUNTO(S)

ecofisiologia arvores - fisiologia parede celular vegetal auxina

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