Cytokinin Activity in Stolons and Tubers of Solanum tuberosum during the Period of Tuberization

In water-culture experiments with potato plants (Solanum tuberosum L. cv. Ostara), changes in cytokinin activity in the stolon tips and newly formed tubers during tuberization were studied. Tuberization was induced by withdrawing nitrogen from the nutrient solution. — The cytokinin activity was low in the stolon tips prior to tuberization, but increased considerably in both stolon tips and young tubers during tuberization. At the same time qualitative changes in the cytokinin spectrum occurred. These qualitative changes are reversible if ‘regrowth’ of young tubers is brought about by a sudden high supply of nitrogen. — Despite the close correlation between tuberization and cytokinin activity, it is assumed that cytokinins are not directly responsible for the onset of tuberization, although they play an important role in tuber growth.     

Relation between Nitrogen Nutrition, Cytokinin Activity and Tuberization in Solanum tuberosum

In water-culture experiments with potato plants (Solanum tuberosum L. cv. Ostara), the influence of tuberization initiated by a 7-day period of nitrogen withdrawal (discont. N) on the cytokinin activity in shoots, roots and exudate was studied. Plants with a constant supply of nitrogen (cont. N) were used as control. — Whereas no tuberization could be observed with cont. N, discont. N led to tuberization already 2 days after nitrogen withdrawal and all plants had been induced after another 4 days. In the roots of plants with discont. N, there was a temporary increase in cytokinin activity, whereas the activity decreased steadily with cont. N. In the exudate, cytokinin activity was greatly reduced during nitrogen withdrawal, whereas this activity in the exudate increased steadily with cont. N. — In the shoot with cont. N cytokinin activity decreased steadily, but with discont. N, after an initial decrease, the activity increased steeply. This increase is mainly or exclusively caused by a shift between the water-soluble and butanol-soluble fractions of the cytokinins in favour of the latter. The shift in cytokinin activity of the shoot is assumed to be in causal connection with an increased photosynthetic activity after the onset of tuber growth as ‘sink’ for assimilates.     

Influence of Ni Supply on Growth and Nitrogen Metabolism ofBrassica napusL. Grown with NH4NO3or Urea as N Source

The significance of nickel (Ni), which is essential for ureaseactivity, for growth and nitrogen (N) metabolism ofBrassicanapusgrown in nutrient solution with either NH₄NO₃or urea assole N source was investigated. Although Ni contents were below25 µg kg^-1d. wt, growth of plants relying on NH₄NO₃wasnot affected by the Ni status. However, supplementing the growthmedium with 0.04 µMNi enhanced dry matter production ofurea-grown plants significantly. Urease activity was significantlyreduced in leaves and roots of plants grown without supplementaryNi irrespective of N source. Plants grown with urea withoutadditional Ni accumulated large amounts of urea and had loweramino acid contents indicating impaired usage of the N supplied,while those grown with NH₄NO₃under Ni-deprived conditions accumulatedendogenous urea in their older leaves. It is suggested thatNi may not be strictly essential for plants receiving mineralN, or that the critical level is well below 25 µg kg^-1d.wt. These results confirm that Ni is required for urease activityand thus for growth of plants on urea-based media, as well asfor recycling endogenous urea.Copyright 1999 Annals of BotanyCompany. Brassica napusvar.annua, amino acids, N nutrition, nickel, spring rape, urea, urease activity.     

Effects of the Temperature of the Rooting Zone on the Growth and Development of Roots of Potato (Solanum tuberosum)

In solution culture the effect of optimal (20 °C) and supra-optimal(30 °C) root-zone temperatures on root growth and root morphologyof six potato clones (Solanum tuberosum L spp) was studied Growthwas compared with sweet potato (Ipomoea batatas L) and cassava(Manihot esculenta Crantz) Significant genotypical differencesin the responses of potato roots to supra-optimal temperatureswere observed, indicating the potential for selecting heat tolerantpotato clones In both heat tolerant and heat sensitive clones,the size of the root system was reduced by supra-optimal root-zonetemperature This was principally a result of decreased numberand length of lateral roots The first symptom of heat damagewas a reduction in the rate of cell division, followed by cessationof root elongation Bending of the apical root-zone togetherwith the formation of root hairs on the inner (concave) andlateral roots on the outer (convex) side were other symptoms,these coincided with the loss of starch granules in the root-cap Potato, genotypical differences, root morphology, root growth, temperature, Solanum tuberosum L     

Nitrogen Nutrition and Cytokinin Activity in Solanum tuberosum

In water culture experiments with potato plants (Solanum tuberosum L. cv. Ostara), the influence of continuous nitrogen nutrition (constant supply of NO₃^?) and discontinuous nitrogen nutrition (interruption of NO₃^? supply, i. e., nitrogen withdrawal for 6 days) on the endogeneous cytokinin level in the roots, shoots and exudate of decapitated plants was studied. Harvests took place at intervals of 3 days. The chlorophyll formation test (cucumber cotyledons) and soya callus test were used to determine the cytokinin activity. With continuous nitrogen, the cytokinin activity decreased slightly with time in both roots and shoots but rose in the exudate. With discontinuous nitrogen, the nitrogen withdrawal led to a temporary, pronounced increase in cytokinin activity in the roots; at the same time, the cytokinin activity in the exudate decreased sharply. It is assumed that this temporary increase in cytokinin activity in the roots is a reflection of increased meristem activity in the roots. In the shoots, the cytokinin activity decreased during the nitrogen withdrawal period. These nitrogen-induced changes in cytokinin activity in the roots and shoots of potato should presumably have an important influence on the physiological age of the shoot, with all its consequences in the further development of the plant. Zeatin riboside was likely the main cytokinin component involved.     

Interrelation Between Growth Rate of Individual Potato (Solanum tuberosum L.) Tubers and Their Cell Number

In potato plants fast and slow growing tubers develop on thesame plant. A hypothetical causality between tuber growth rateand tuber cell number was investigated by determining the tubercell number with the aid of an automatic counting procedure.Our data show a close correlation between tuber size and cellnumber over the whole range of tuber volumes considered (3–28cm³). If the influence of tuber size on cell number is eliminatedby means of a partial correlation analysis, the cell numberof the entire tuber is not significantly correlated with itsgrowth rate. An exclusive consideration of the smaller cells(10–30 µm) in the apical tuber region, where thecell division rate in potato tubers is highest, reveals a loosebut significant partial correlation to tuber growth rate (r= 0.383, P < 0.05). The growth rate of the slow growing tubers of any potato plantmay be enhanced by removing the fast growing tubers. In thefirst few days this enhanced growth rate is not due to a stimulationof cell division rate, but rather due to cell expansion. Potato, Solanum tuberosum L., tuber growth rate, tuber cell number