Agriculture Reference
In-Depth Information
Apple fruits have chloroplasts in the green tissue of the hypodermal and
inner perivascular tissues (Blanke and Lenz,
). Hypodermal chloroplasts
in the five to sixlayers below the epidermis are smaller than in the inner tissues,
resemble those found in leaves and exhibit grana throughout fruit develop-
ment (Phan,
). Inner central tissues in young fruits are more photosyn-
thetically active than in mature fruits but still less active than the hypodermis.
The fruit surface has stomata but these are only about
% as frequent as
those in leaves. The fruits also, however, possess a system which re-fixes CO
from the mitochondrial respiration of predominantly imported carbon. This
pathway produces malate by the action of phosphoenolpyruvate carboxylase
(PEPC). Fruit photosynthesis appears to be intermediate in status between
C non-autotropic and C /CAM photosynthesis. Fruit photosynthesis often
compensates for respiratory CO loss in the light but, due to respiration in
the dark, there is a continual net loss of CO from fruits throughout their
development. Nevertheless, given the very high proportion of the tree mass
which is made up of fruits, their ability to compensate for respiratory losses in
the light is very important.
-
Respiration
Respiration is the process in which carbon substrates are utilized in the pro-
duction of energy.
C H O +
O −→
CO +
H O
+
energy
Low-energy bonds in the carbon substrates are converted to high-energy
bonds in reduced nucleotides (NADH, NADPH and FADH ) and ATP. The
oxidation of one glucose molecule has the potential to yield
ATP molecules.
Respiration is divided into that associated with the biosynthesis of new plant
tissue (growth respiration) and that needed for organ maintenance (main-
tenance respiration). These definitions have to be used with care and are,
to some extent, oversimplifications. Not all growth has the same energy re-
quirement: in particular, the cell division phase may involve much more en-
ergy use than that of cell expansion. Furthermore, processes such as those
involved in senescence, and translocation and storage of metabolites which
cannot be construed as growth, are energy-intensive. Measurements of respi-
ration are of particular value as an indexof when energy-requiring processes
are taking place or being inhibited. They are also used to help construct
carbon-balance sheets and help in understanding the effects of environmen-
tal factors on these, and, consequently, on potential growth and cropping.
Attention has been concentrated on effects of temperature and on seasonal
Search WWH ::




Custom Search