MICRO-IRRIGATION SYSTEMS FOR POT-IN-POT ORNAMENTAL NURSERY PRODUCTION - Management, Performance, and Applications of Micro Irrigation Systems - page 304

21.5 SUBSTRATE MOISTURE BASED IRRIGATION STRATEGIES FOR PIP
21.5.1 DAILY WATER USE REPLACEMENT (DWU)
The DWU irrigation system was developed to replace water used during the previ-
ous day's use for container production [51]. DWU is the combined loss of water due
to plant transpiration and evaporative water loss from the substrate [47]. Irrigation
based on DWU has been conducted on nursery crops using daily pre and postirrigation
substrate moisture probe or gravimetric measurements to determine DWU [51, 56].
Warsaw et al. [51] used a SMS to determine daily water use requirements in container
production for 24 taxa of commonly used woody shrubs. Additionally, irrigating based
on 100% DWU or alternating with up to 2 days at 75% DWU followed by 100%
DWU did not reduce plant growth of woody ornamental shrubs in 10-L containers and
possibly increased nutrition of plants compared to a well-watered control [51]. Using
DWU to schedule irrigation reduced the amount of irrigation applied by as much as
75%, runoff volume by up to 79%, nitrate quantity by up to 59% and phosphate quan-
tity moving in runoff by as much as by 74% compared to standard nursery irrigation
practices (19 mm per application) [51].
21.5.2 SET POINT ON-DEMAND IRRIGATION
A plant demand-based irrigation system models plant response to environmental
changes to predict amount and timing of irrigation. A simple on-demand irrigation
scheduling system has been developed to calculate an irrigation set point adjusted to
the substrate water content at which plant photosynthetic rate began to be reduced by
water deficit [12]. It is assumed that growth is not be compromised when an irriga-
tion set point was used based on the substrate water content where photosynthesis
begins to decline due to water stress. Because photosynthesis is closely linked with
stomatal conductance, and stomatal conductance is controlled by both root to shoot
signaling and the environment, photosynthesis is a sensitive indicator of water status.
An irrigation model was developed using Hibiscus rosa-sinensis [13]. An irrigation
set point was established that reflected the substrate water content at which photosyn-
thesis began to drop (photosynthetic rate was reduced to 98% of maximum), which
corresponded with a reduction in stomatal conductance. By maintaining the substrate
moisture content just above this set point, a crop could be produced using 27% less
water than the control, and without adversely impacting quality, or production time
[14]. Development of an irrigation system based on photosynthetic rates would require
a minimum of data collection for model development and could easily be modified for
use with other species. The plant on-demand irrigation scheduling system has since
been employed successfully on a number of crops including dogwood ( Cornus sp. ),
oakleaf hydrangea ( Hydrangea quercifolia ), and boxwood ( Buxus sp.) for example,
in Boxwood (Buxus microphylla 'Green Ice'), overall water use as well as water use
efficiency (WUE) were improved using either the DWU or demand-based scheduling
system without reducing plant biomass and plant quality under outdoor nursery condi-
tions. An irrigation system predicated on maximizing photosynthesis has also been
developed for container-grown apple ( Malus x domestica Borkh.) trees. This system
uses sap flow and stem diameter variations to predict the amount of water required