RATES OF PHOTOSYNTHESIS AND DRY-MATTER AC CUMULATION OF VERY EARLY POTATO VARIETIES

The field trials were established in two regions with different weathe r conditions (Zabcice u Brna - 184 m of elevation, Valecov u Havlickov a Brodu - 460 m of elevation). Four potato varieties were selected for the observations (Tab. I). The spacing chosen for the tuber planting was identical, 750 x 250 mm, with the same stand density of 53 300 pla nts.ha(-1) The trials were evaluated by the method of the vertical div ided parts with four replications in two variants with fertilizer rate s 60 and 120 kg N.ha(-1). Net photosynthetic rate P-N was measured in the period of visible beginning of bud formation when P-N Of potatoes was on the top and a week later. The variety Ukama exceeds the rest of varieties with two exceptions in experimental years 1994 and 1995. Th e variety Krystala showed the lowest P-N (Fig. 1). The level of nitrog en did not appear prominently in varieties especially during dry condi tions of 1994. The difference between varieties decreased with the hig her level of nitrogen approximately about a half in 1995. The differen ce among varieties with the lowest and the highest values of P-N Was a t 60 kg N.ha(-1) over 10 mg (d. m.).dm(-2).h(-1) which represented 0.2 867 mg when converted into m(2).s(-1). Significant differences of crop growth rate CGR were found out at the higher nitrogen level (Tab. II) . The week sequence of full blossom stand is shown in Tab. II. From al l the varieties observed on both locations, the highest value was foun d in the variety Impala at 120 kg N.ha(-1). The highest values CGR wer e found on the location Zabcice. The variety Impala achieved the highe st value of the LAI, especially with nitrogen level 120 kg.ha(-1) and the variety Koruna had the lowest value with both levels of nitrogen ( Fig. 2). On an average the highest values of LAI were found in mutual comparison of both nitrogen levels, locations and years with higher le vel of nitrogen on the location Zabcice in 1995. These differences wer e caused by higher rate of nitrogen which positively affected the leng th of closed assimilation apparatus, in production activity formed mor e favourable soil and climatic conditions of location placed in the ma ize - production region and higher total amount of rainfall during the growing season in 1995. Factors mentioned above influenced favourably the values of leaf area duration LAD (Tab. III). The variety Impala a chieved largely the highest value on both locations in both years. On the contrary, the variety Koruna attained largely the lowest values of LAD. The varieties Krystala and Ukama did not show an essential diffe rence on the location Zabcice while the higher values of variety Ukama were reached in potato production region conditions. The total yield of tubers was affected by values of the above-mentioned parameters (Ta b. IV). The variety Impala showed the highest yield and the most favou rable values of growth analytical characteristics on both locations du ring both years. The effect of higher rates of nitrogen applied positi vely affected all the varieties observed, on both locations and both y ears (Fig. 3). The differences (Tab. V) in tuber yield of some varieti es in the period from last sampling to harvest were caused by the diff erent late blight infection level on potato plant leaves.