THE EFFECT OF TRANSITION DROUGHT PERIOD O N TOTAL AND PROTEIN NITROGEN AND NITRATE CONTENTS IN POTATO-TUBERS

In three-years' greenhouse trial (1990 to 1992) with four varieties (R esy, Karin, Desiree, Kamyk) we ''posed potato crop, in individual deve lopmental stages in nine treatments, to various water deficit (Tab. I) . Details about trial are given in earlier work (Zrust, Hold, 1994). A fter physiological crop maturity, tubers were analyzed for nitrate con tent, total and protein nitrogen content. On average of varieties and years, irrigated control had the highest values of nitrate content dur ing the whole vegetation on 75 % relative humidity. Drought was effect ive in any period of vegetation decreased their content in 1992 and on average of all three years (Fig. 1). On contrary, the lowest values o n average of three years, had treatments, in which the drought was eff ective in the season from planting to crop emergence and at lower stre ss in other period from emergence to bud formation visible by eyes. Th e tubers had higher values in treatments, where the drought was effect ive in the season from bud formation to the end of vegetation. Treatme nt sequence according to nitrate content size changed in individual ye ars. Differences among final treatments were from 20.5 mg.kg-1 in 1991 to 100.1 mg.kg-1 in 1992, on average of three years there was differe nce between them 30.9 mg.kg-1. In 1992 among treatments and varieties were highly significant differences (Tab. II). In previous years, the differences among treatments and varieties were lower and not always s ignificant. The weather effect on nitrogen dynamics in soil and in pla nt is known from many works. The question consists in precipitation ma inly. In literature, we used to meet mainly global assessment of water need for amount of some organic compound containing nitrogen. Our res ults, concerning the effect of terminal drought period on nitrate cont ent in potato tuber, correspond to the results as published by Neubaue r et al. (1991) and Augustin et al. (1977). Cold weather, small freque ncy of sunny days and abundant precipitation decrease in photosyntheti c activity and they lead in their consequence to nitrate accumulation exceeding the admissible maximum level. Nitrate accumulation in plant is the result of disproportion between nitrogen uptake and its utiliza tion in plant metabolism. In the trial we confirmed relation of variet y vegetation period length to find nitrate content in tubers. Very ear ly variety Resy had the highest NaNO3 values, late variety Kamyk had t he lowest values. Total nitrogen content was increased by drought, if it was effective from the full flowering to the end of vegetation. The lowest values on average for both years had the first treatment - con trol, even when it did not place separately as the last by its values in each year (Fig. 2). The range of significant values, recorded in 19 92, is given in Tab. III. Similar significant values were found out ev en in previous years. Varieties, without respect to values of individu al treatments, did not keep the sequence according to above-measured d ata in both years. The lowest values in total nitrogen content at opti mum irrigation of control obtained by us agree with more authors, whic h also observed decreasing of nitrogen content, mostly significant in all varieties classified in the trial (Necas, 1962; Necas, Zrust,1964; Zanker et al.,1975; Ekeberg,1986; Vetter et al.,1991), under increase d soil humidity in individual periods of onthogenesis. Treatment seque nce according to the above reached values of protein nitrogen content was in average of both years similar to total nitrogen content (Fig. 2 ). Also tubers from treatment 9 had significantly higher values than a ll other treatments. Significantly higher values compared with the res t ones had even three treatments, in which the drought was effective f rom bud formation to the end of vegetation. The control treatment had the lowest value, as in the previous qualitative index Even all these data were mutually highly significant (Tab. III). In individual years, variant sequence according to protein nitrogen content changed and wa s different from treatment sequence in total nitrogen content, especia lly in 1991. Increasing protein nitrogen content in potato tubers unde r drought effect was observed also by Davies et al. (1989).