Production Practices and Quality Assessment of Food Crops. Vol. 1

16 R. M. Madakadze and J. Kwaramba
Table 6. Effect of increasing potassium concentrations in potato tubers, on the composition and
quality of the tubers.
Type of change Effect of increasing Responsible mechanism
Potassium
Water content Increase Osmoregulation
Reducing sugars Decrease Osmoregulation
Citric acid Increase Cation-anion balance
Starch Decrease
Black spot disorder Decrease Lower polyphenol oxidase activity
Darkening of press sap Decrease High citric acid/low polyphenol oxidase
Discoloration after cooking Decrease High citric acid/low chlorogenic acid
Storage loss Decrease Lower respiration and fungal diseases
Adapted from Marschner (1989).
(Lauchli and Pfluger, 1978), starch synthase (Nitsos and Evans, 1969), and
membrane bound ATPases (which requires Mg 2+ but are further stimulated by K + ).
Potassium is required for protein synthesis in higher plants. The role of K + in protein
synthesis is reflected in the accumulation of soluble nitrogen compounds, (e.g. amino
acids, amides and nitrate) in K-deficient plants (Mengel and Halal, 1968).
In higher plants K + affects photosynthesis at various levels. K + is the dominant
counter ion to light induced H + flux across the thylakoid membranes and the
establishment of the transmembrane pH gradient necessary for the synthesis of
ATP (photophosphorylation) (Lauchli and Pfluger, 1978). An increase in the leaf
potassium content is accompanied by increased rates of photosynthesis, photorespiration
and RuBP carboxylase activity, but a decrease in dark respiration. In most
plant species K + also has the major responsibility for turgor changes in the guard
cells during stomatal movement. An increase in the K + concentration in the guard
cells results in the uptake of water from the adjacent cells and thus stomatal opening
(Humble and Raschke, 1971).
3.3.3. Boron
Boron occurs mainly as boric acid, H 3BO 3 a very weak acid in aqueous solution.
At physiological pH (< 8) mainly the undissociated boric acid is present in the
soil or nutrient solution. This is also the preferred form taken up by the roots.
Boron is strongly complexed to cell wall constituents even in the roots (Thelier et
al., 1979). Long distance transport of boron from the roots to the shoot is confined
to the xylem, and uptake and translocation are closely related not only to the mass
flow of water to the root surface but also to the xylem water flow. There is no
indication that boron is an enzyme component and there is only little evidence
(Birnbaum et al., 1977) that the activity of any enzyme is enhanced or inhibited
by boron. Interactions between boron and other mineral nutrients during uptake
and in the plant itself seem to be of minor importance (Marschner, 1989).
Bo deficiency is a widespread nutritional disorder in vegetables. Boron deficiency
usually occurs under: