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

Effect of Preharvest Factors 15
between 7 and 8 in these compartments, the optimum for most enzyme reactions.
Potassium is required for enzyme activation and membrane transport processes.
There are mechanisms (pumps) at the plasma membrane and probably also at the
tonoplast for concentrating K + in the cytoplasm. Although several other univalent
cations can partially replace K + , they are toxic to intact cells at high concentra-
+ + tions (e.g. NH4 ) or are not abundant in nature (e.g. Rb ).
Potassium requirement for optimal plant growth is about 2–5% of the dry weight
of the vegetative parts, fleshy fruits and tubers. Potassium deficiency causes chlorosis
and necrosis in mature leaves and stems wilting and logging. The lower tolerance
of K + deficient plants to drought is related to a) the role of K + in stomatal regulation
which is a major mechanism controlling the water regime of higher plants
and b) the role of K + as the predominant osmotic solute in the vacuole, maintaining
a high tissue water level even under drought conditions. Plants receiving
inadequate K + are often more susceptible to frost damage, which at cellular level,
is related in some respect to water deficiency.
Frost damage is inversely related to the K + content of leaves (Marschner, 1989)
(Table 5). The changes in enzyme activity and organic compounds that take place
during K + deficiency are in part responsible for the higher susceptibility of plants
to fungal attack. They also affect the nutritional and technological (processing)
quality of harvested products. This is most obvious in fleshy fruits and tubers with
their high K + requirement for growth. In tomato fruits, the incidence of the ripening
disorder ‘greenback’ or ‘green shoulders’ increases with inadequate K + supply (Lune
and Goor, 1977), and in potato tubers a whole range of quality criteria are affected
by the K + level in the tuber tissue (Table 6). Too high K + supply affects plant
composition and interferes with the uptake and physiological availability of Mg2+ and Ca 2+ .
There are more than 50 enzymes which either depend on or are stimulated by
potassium (Suelter, 1970). Potassium and other univalent cations activate enzymes
by inducing conformational changes in enzyme protein. In general, potassium
induced conformational changes of enzymes increase the rate of catalytic reactions,
Vmax, and in some cases also the affinity for substrate, Km (Evans and Wildes,
1971). In K + deficient plants some gross chemical changes occur, including an
accumulation of soluble carbohydrates, a decrease in the levels of starch, and an
accumulation of soluble N compounds.
Potassium activates the following enzymes pyruvate kinase, phospofructokinase
Table 5. Relationship of potassium supply to potassium content in leaves, percentage of leaves damaged
by frost and tuber yield in potatoes.
Potassium supply Potassium content of Percentage foliage Tuber yield
(kg/ha) leaves (mg/g dry wt) damaged frost (tons/ha)
00 24.4 30 2.39
42 27.6 16 2.72
84 30 07 2.87
Based on Grewal and Singh (1980) and Marschner (1989).