Summary. Cashew
(Anacardium occidentale L.) is an emerging horticultural
crop in tropical northern Australia. Supplementary watering is required during
the dry season to achieve high yields but irrigation guidelines are not well
defined. As an introduction to large-scale field experiments which will aim to
define the irrigation requirements for cashew, this experiment was conducted
on small, container-grown cashew trees to examine their response to drying
soil and to evaluate a range of techniques for measuring tree water use and
photosynthesis with possible application in the proposed field experiments.
Measurements of sap flow, leaf chlorophyll fluorescence and leaf gas exchange
were made on all trees throughout the experimental period.
The water use of trees in drying soil was measured using Granier’s sap
flow system. Sap flux density (L/dm2 sapwood area .
h) of drying trees declined progressively over a 4-day period to a minimum
level that was only 10% of the sap flow in the well watered trees.
Measurements of leaf gas exchange showed similarly large reductions in
photosynthesis and transpiration which were associated with a low (0.05
mol/m2 . s) stomatal conductance in the drying
trees. After rewatering, sap flow and leaf gas exchange recovered to the high
levels of the well watered trees over 3–4 days. Similar behaviour was
observed during the second drying period. Measurements of the ratio of
variable to maximum fluorescence,
Fv :
Fm, an indicator of
photoinhibition, were made on dark-adapted leaves before dawn and during the
day. Fv :
Fm was in the range
0.65–0.80 with no large or sustained differences between drying and well
watered trees. When stomatal conductance and net photosynthetic rate
progressively declined during the period following irrigation, the quantum
yield of photochemical energy conversion in photosystem II,
ΦPSII, remained almost
constant. It is possible that by providing a pathway for electron flow as an
alternative to CO2 assimilation during this period,
photorespiration played an important role in avoiding photoinhibition.
Effect of prescribed burning on chlorophyll fluorescence and sap flow of Pinus laricio, a preliminary study
