Introduction. Plant water stress
indicators have become valuable for moving towards deficit irrigation
strategies and saving water. In this case evapotranspiration (ET)
is below its maximum value for the crop and stage (ETc),
and a stress coefficient (Ks) is applied to obtain
actual ET (ETa). Predawn leaf water
potential (Yp) can be related to relative transpiration
(RT), the ratio between transpiration of a stressed
plot (T) and transpiration of a well-irrigated
plot (Tm). Estimating RT from
Yp allows calculating ETa for
determination of irrigation amounts, if deficit irrigation practices
are used, as RT corresponds approximately to Ks.
Materials and methods. RT and Yp were measured
with the aim of establishing a relationship to estimate RT under
moderate water stress for irrigation scheduling, in a peach orchard
in south Portugal. RT was calculated using sap flow
measurements (heat balance method) in two plots, one well-irrigated
(daily drip irrigation amounts calculated for Tm) and
another temporarily without irrigation. Results and discussion. A
high correlation was found between RT and Yp,
allowing the estimation of RT for the studied conditions.
Significant differences regarding the relationship RT-Y
p obtained for another peach orchard in the same
region and similar soil conditions were found for Yp in
the range between –0.11 and –0.45 MPa. The results suggest that the
differences resulted from the different irrigation systems: drip
and micro-sprinkling, as they determine different temporal and spatial
water distribution and therefore different geometry of root systems.
A formerly proposed equation to estimate RT from
Yp with a general form for different fruit tree
species was tested, proving to be adequate within some limits: when
RT is lowered to 0.7, the error was below 9%. The
threshold value RT = 0.7 was considered a minimum
as it was successfully tested in deficit irrigation practices for peach
orchards.