Agriculture Reference
In-Depth Information
The axillary roots will absorb both water and nutrients, facilitating the
utilization of foliarly applied nutrients. The main absorptive area of the root is
in the unlignifi ed white tissue of the root tip. Roots without white tissues at the
root tips are not actively growing, are inei cient in absorbing water and may
have been lost because of root rot and nematode feeding.
Leaves
Leaves are produced in a spiral around the stem in a tight rosette (Fig. 12.1). A
dormant axillary bud is found at the midpoint of each leaf on the main stem.
By counting from a leaf or bud at a selected node on the stem upwards in a
spiral, the 13th leaf or bud will be directly above the initial leaf or bud and fi ve
turns around the stem will have been made, resulting in a 5/13 phyllotaxy
(Bartholomew and Kadzimin, 1977).
The number of leaves increases regularly, with an average of fi ve or
six leaves per month. Old leaves do not abscise, and a mature plant may
have 70-80 active leaves. The most frequently mentioned developmental
index for carbohydrate and nutrient status is the whorl of leaves designated
and described as the D-leaves by Sideris and Krauss (1936) and fi rst used by
Nightingale (1942). Usually, the D-leaves are in the fourth whorl from the
base of the plant, and not more than three leaves would qualify as D-leaves
at any one time. These are the longest (80-100cm) and the youngest nearly
physiologically mature set of leaves. In non-fruiting plants, the leaf base of
these leaves is only slightly broader than the green blade. In 'Smooth Cayenne'
a small group of marginal spines is found near the leaf base and near the
tip. Also, a short area of spines is formed on new growth of the leaves after
temporary cessation of leaf growth due to adverse conditions. There is always
a slight constriction at the point where leaf expansion was interrupted.
There are a number of structures in pineapple plants and other
bromeliads that contribute to their strong resistance to moisture stress.
The position and trough shape of the leaves, the presence of trichomes, and
stomata located in furrows beneath trichomes on the underside of the leaf
are features that enhance drought resistance. Long, multicellular trichomes
are found in abundance on the lower side (abaxial) of the leaf and less on the
upper (adaxial) side. It is generally believed that the trichomes absorb moisture
and nutrient solution and reduce water loss through the stomata by forming a
dense covering over the stomata.
A unique internal feature of bromeliads, including the pineapple leaf,
is a water-storage tissue that is colourless and translucent. This tissue can
be identifi ed with the naked eye in the adaxial part of transverse sections of
the leaf and contrasts with the chlorophyllous mesophyll tissue beneath. The
width of both tissues in leaf cross section varies with age and environmental
factors. In the pineapple-growing district of Wahiawa, Hawaii, the water-
