Agriculture Reference
In-Depth Information
This may have been a consequence of the relative reduction in leaf area and
increase in roots as a result of water stress. It may also reflect a carried-forwards
effect on stomatal conductance.
RDI is clearly more relevant to arid than humid climates where control
of the period on deficit is difficult. The variability in responses suggests that
specific regimes may have to be devised for cultivars differing in the period
of flower bud differentiation and in the extent to which reduction in fruit size
reduces market value.
Partial rootzone drying
Another approach to minimizing water use with little or no adverse effect
on fruit growth and a useful checking of shoot growth is that of partial root
drying (PRD). This seeks to exploit the signals sent by roots in dry soil to
reduce leaf and shoot growth and induce partial stomatal closure (Davies
et al.
,
). ABA seems to be the major signalling chemical but is not necessarily
the only one. From work on other fruit crops it seems that PRD may exert a
particularly favourable differential effect on fruit and leaf growth where the
fruits are relatively isolated from the hydraulics of the rest of the plant, ABA in
the xylem sap being thought to be particularly effective. In apple, xylem flux
into the fruits declines through the season and its relative magnitude varies
with cultivar (Lang and Ryan,
) so some cultivars may respond better to
PRD than others.
PRD has been furthest developed in grapes, where it improves water use
efficiency (yield of crop per unit of water) by up to
% without significant
crop reduction (Stoll
et al.
,
). This is achieved by maintaining half of
the root system in a dry or drying state while irrigating the other half. The
wetted and dried sides of the root system are alternated on a
-day
cycle: this is easily done with trickle irrigation lines. PRD results in increased
xylem sap ABA and a reduction in zeatin and zeatin ribocide concentrations
in roots, shoot tips and buds. There is a nocturnal net fluxof water from
wetter roots to the roots in dry soil. The technique is being evaluated for
apples.
-
Over-tree irrigation for temperature and
humidity control
This has four main objectives: to reduce heat-stress in summer; to reduce
water-stress; to achieve evaporative cooling in winter or spring to prevent
'de-chilling' in warm winter areas and to delay blossoming in frost-prone
areas; and to give frost protection in winter by utilizing the latent heat of
fusion.
