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

18 R. M. Madakadze and J. Kwaramba
areas where irrigation water contains high levels of boron. The critical boron
concentration in irrigation water varies between 1 and 10 mg/L for sensitive and
tolerant crops respectively. It is between 2 and 3 mg/L for wheat and 4 and 6
mg/L for peas (Chauhan and Powar, 1978). The critical toxicity levels expressed
as mg boron per kg dry weight of leaves is 400 for cucumber and 1000 for squash
(El-Sheik et al., 1971). Typical boron toxicity symptoms on mature leaves are
marginal or tip chlorosis or both. They reflect the distribution of boron in shoots,
following the transpiration stream.
3.3.4. Phosphorus
The phosphorus requirement for optimal growth of vegetables is in the range of
0.3 to 0.5% of the plant dry weight during the vegetative stages of growth. Plants
suffering from P deficiency exhibit retarded growth, and often a reddish colouration
occurs because of enhanced anthocyanin formation. P-deficient plants also often
have a darker green colour than do normal plants. Deficiency leads to a general
reduction of most metabolic processes including cell division and expansion,
respiration and photosynthesis because of the functions of P in the growth and
metabolism of plants (Terry and Ulrich, 1973). The regulatory function of
inorganic phosphate P i in photosynthesis and carbohydrate metabolism of leaves can
be considered to be one of the major factors limiting growth particularly during
the reproductive stage.
After uptake at physiological phosphate either remains as inorganic phosphate
(P i) or is esterified to a simple phosphate esteri or attached to another phosphate
by the energy – rich pryophosphate bond e.g. in ATP. Phosphate forms a bridging
group connecting units to more complex or macromolecular structures using another
type of phosphate bond (C-P-C). In both RNA and DNA, phosphate forms a bridge
between ribonuclesiode units to form macromolecules. Phosphorus is responsible
for the strongly acidic nature of nucleic acids and thus for exceptionally high
cation concentration in DNA and RNA structures. The bridging form of P diester
is also abundant in the phospholipids of biomembranes where it forms a bridge
between a diglyceride and another molecule (amino acid, amine or alcohol). In
biomembranes the amine choline is often the dominant partner, forming phosphatidyl
choline (lecithin). Most phosphate esters are intermediates in metabolic pathways
of biosynthesis and degradation. Their function and formation is directly related
to the energy metabolism of the cells and to energy rich phosphates. The energy
required, for example, for biosynthesis or for ion uptake is supplied by an energyrich
intermediate or coenzyme, principally ATP. Energy liberated during glycolysis,
respiration, or photosynthesis is utilized for the synthesis of the energy-rich
pyrophosphate bond. ATP is the principal energy-rich phosphate required for starch
synthesis.
Inorganic phosphate is also either a substrate or an end-product (e.g. ATP –
ADP + P i). Inorganic phosphate controls some key enzyme reactions. In fruit tissue
of tomato, P i released from the vacuoles into the cytoplasm can stimulate phosphofructokinase
activity (Woodrow and Rowan, 1979). Phosphofructokinase is the
key enzyme in the regulation of substrate flux into the glycolytic pathway. An