Agriculture Reference
In-Depth Information
Photosynthesis, respiration,
and carbohydrate transport,
partitioning and storage
Introduction
The greater part of the dry weight of apple and pear trees is derived from
photosynthetically fixed carbon. Moreover, sugars comprise a critically im-
portant component of the economic product, the fruits, being a major factor
in controlling their taste.
Early in the season, around budburst, the carbohydrate needed to provide
energy for growth and to supply a basis for structural material comes from re-
serves stored in perennial tissues in the previous season or seasons. As the leaves
expand their photosynthesis supplies growing tissues and replenishes reserves.
One very important aspect of apple and pear carbohydrate metabolism
is that of the ripening and the post-harvest fruit. This is dealt with in later
chapters.
Photosynthesis
In photosynthesis energy from solar radiation is converted into chemical en-
ergy, which enables the reduction of carbon dioxide to produce carbohydrates,
according to equation (
.
).
light
−→
n
CO
+
n
H
O
chlorophyll
(CH
O)
n
+
n
O
+
n
H
O
(
.
)
This process involves both 'light' and 'dark' reactions in very close conjunc-
tion. In the 'light' reaction light energy is converted into chemical energy in the
form of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide
phosphate (NADPH). In the 'dark' reaction, which is closely coupled to the
'light' reaction, CO
is incorporated by carboxylation of ribulose phosphate by
ribulose
-bisphosphatecarboxylase-oxygenase(RuBPC/O).Thefirststable
products of this reaction are two molecules of glycerate
,
-phosphate (glycerate-
-P). Plants such as apple and pear in which the initial carboxylation results in
