Caracterização fisiológica da resposta a adubação nitrogenada em duas cultivares de arroz

Abstract

An experiment was carried out to study the seasonal pattern of dry mass and nitrogen (N) distribution in the vegetative and reproductive parts of two rice cultivars of contrasting aerial architecture, as a function of the application of N in the maximum tillering stage. Substrate from the Ap horizon of a Red Yellow Podzolic soil was used as substrate. The cultivars Comum Branco (dryland) and IAC 4440 (irrigated) were combined with five application levels of N (as Ammonium Sulfate): 0, 2, 4, 6 and 8 g N m-2 in the maximum tillering, and grown in pots arranged in a completely randomized, greenhouse. During the plant cycle, seven successive collections were carried out in order to follow the phasic development as days after planting (DAP): beginning of tillering, maximum tillering, panicle differentiation, stem elongation, flowering, milky / pasty grain and physiological maturation of the grain, which corresponded to collections at 20, 40, 60, 80, 100, 115 and 135 DAP respectively. In each collection were determined the dry mass of: green and senescent leaves, stems and sheaths and roots. The green leaf area and root area were also determined. The N contents were determined in all these fractions by micro Kjeldhal technique. In the final collection the components of the production were evaluated: mass and number of panicle, viable and sterile grain, and their corresponding N contents. The experimental data were combined for the purpose of analysis of variance, in the form of a cultivar factorial x dose x DAP . Primary functions were adjusted from the data of dry mass and total N, foliar and root area, from which the growth and accumulation rates were derived, to obtain net assimilation rates and N influx rates. it was observed that during the period prior to flowering there were no differences in accumulation of total dry matter between the cultivars. There was a higher dry mass allocated in stalks and sheaths, and lower weight of senescent leaves mass in IAC 4440. The partition of dry mass in favor of roots was similar in both cultivars. The cultivar IAC 4440 established a potential productive superiority due to its greater number of tillers. As a consequence, it maintained a longer duration of photosynthetically active leaf area, resulting in a higher daily accumulation rate of dry matter and leaf area. Regarding the N accumulation patterns in the different biomass fractions, the same trend as that observed for dry mass accumulation was observed. However, the maximum rate of maximum accumulation of N occurred in both cultivars, rather than the maximum dry matter accumulation. Leaf N contents in both cultivars were similar until flowering. IAC 4440 had greater specific leaf area in the period, suggesting greater leaf N dilution. When compared to leaf area equality, the net assimilation rate was higher in IAC 4440, which indicates that the overproduction of area, affected the photosynthetic potential in this cultivar, to a greater degree than in Common White. In the post-flowering period, the dry matter partitioning pattern, which was rigidly fixed in pre-flowering, was manifested in a sustained remobilization of photoassimilates and N to the panicle in IAC 4440, evidenced by the reduction of dry mass and N contents of stalks and sheaths during the first 15 days post-flowering and additional contribution of N and C from marked leaf senescence in the maturation period. There was detention of the photo-assimilates and N remobilization of the panicle in Common White, after 15 days post-flowering, with consequent accumulation of dry matter and N in leaf stems and sheaths. The development of a secondary drainage originated in post-flowering neo-tillering may have subtracted photosynthates, otherwise able to be applied in the filling of the growing panicle. A higher grain weight per plant in IAC 4440 was explained by a higher number of panicles and a higher number of viable seeds, and lower crude protein levels, compared to White Common. Regarding the dry mass and nitrogen distribution patterns during the cycle of the cultivars, the effects resulting from the single application of N at the maximum tillering were diluted during the development of the plants, so that they had very little effect on the dynamics of retranslocation of C and N after flowering. This dilution effect was responsible for the non-significance of the interaction of practically none of the primary descriptive growth parameters of the cultivars. No effect of N on phasic development was also noted. The higher levels of N applied favored an increase in net assimilation due to an increase in leaf N content. At higher levels of N application, leaf mass increased without area promotion, which was evidenced by the increase in its specific weight. However, the N content did not significantly reduce leaf senescence; The application of N did not affect significantly the production, only affecting an increase in the percentage of sterility of the spikelets, which counterbalanced the gain of one panicle per plant. As a global result of the work, it was concluded that the patterns of dry matter production and C and N remobilization to the grain, are regulated genetically, showing cultivar specificity. Therefore, nitrogen fertilization is efficient only in order to ensure high rates of physiological activity of panicles and other organs related to grain filling, but not in changing the pattern itself.