In the practice of deficit irrigation, less water is applied than is needed to meet full losses from ET, thus creating a soil water deficit and exposing plants to mild water stress. Deficit irrigation has the potential to improve irrigation efficiency through either increasing rainfall utilisation and/or through reducing stomatal conductance, and the capacity to achieve this without negative impact on DM yield, depends on rainfall amount and distribution pattern (White 2007), and non-linearity in the relationship between assimilation and stomatal conductance (Fereres & Soriano 2007).
Under glasshouse conditions, a watering regime that restricted daily water use indicated that it was possible to augment WUEl at the leaf level without significantly reducing DM yield,
due to well-watered plants using water in excess of that required to maximise DM yield (Chapter 2). However one of the criticisms in translating pot-derived results to the field is that the rooting zone of potted plants is constricted, and consequently the soil drying rate tends to be more rapid than in the field and roots are unable to explore the soil profile for deeper water (Begg & Turner 1976; Jones et al. 1980). As a result, potted plants do not always have the same opportunity to acclimate to changing soil moisture conditions.
However, close alignment of WUEl in relation to midday leaf was found between acclimated
potted plants and field tested plants (Fig. 4.4), suggesting that well-managed pot experiments may be an acceptable means of testing the effects of moisture stress. Importantly, a
curvilinear relationship between leaf and WUEl (Fig. 4.4) was observed in the field in further
support of the understanding that water-deficit increases WUE at the leaf level up to a point beyond which further stress results in complete stomatal closure (Comstock 2002; Dingkuhn
87 In Chapter 2 it was shown that only a small watering event under non-transpiring conditions was required to fully-hydrate leaves, despite the majority of the soil being substantially drier. By restricting the nightly watering amount, the time spent in the hydrated state was reduced, and thus also the unbeneficial use of water during daylight hours. A similar explanation may also be provided to the augmentation in the IWUI when irrigation was scheduled according to soil sensors (Table 4.2). In the field context, the use of soil sensors helped both maintain plants at the desired soil water deficit, particularly evident between Sens2 and Evap2, and regulate the amplitude in soil water availability experienced, as evidenced by Sens1
compared with Evap1 (Fig. 4.1b and 4.1a). It is by these mechanisms that the soil sensors are suggested to have helped regulate leaf level WUE. When the IWUI was analysed separately for each zone, the effect of the scheduling method was less obvious, which may suggest that the variation between reps was too large to differentiate treatment effects. However in biological terms the difference between the average IWUI between scheduling methods was still 0.2 t DM/ML of irrigation applied in zone A and 0.26 t DM/ML in zone B (data not shown).
The potential to augment leaf-level WUE in the field however may not be as significant as indicated in Chapter 2, as even under the well-watered irrigation strategy (Sens1 and Evap1), plants in the field were maintained within a soil range of around -20 to -40 KPa in zone A
(Fig. 4.1a), compared with the most hydrated treatment in the glasshouse trial which maintained the soil at field capacity (-10 KPa). As a result, leaf was much higher in the
glasshouse (-0.5 MPa) to that estimated from the linear regression of soil and leaf in the
field (-1.1 MPa) (Fig. 4.2), and consequently field plants were on average operating at a higher WUEl than well-watered glasshouse plants (Fig. 4.4).
This may explain why there was a general linear decline in DM consumed with irrigation inputs in the field (Fig. 4.5) and not the glasshouse (Chapter 2), which is consistent with the notion that the relationship between biomass production and water use is conservative (Steduto et al. 2007), as has been demonstrated in other pasture studies (Merot et al. 2008; Smeal et al. 2005). Thus limitations to improving WUEl through irrigation management may
be the constraint to DM yield. Therefore at the paddock scale, reducing water losses via runoff and deep drainage may be able to achieve greater water savings without the subsequent negative effects to yield (Hsiao et al. 2007).
88 Deep drainage was not explicitly measured in this study, and therefore rainfall utilisation cannot easily be quantified. However there were few occasions where sensors registered values above -10 KPa indicating saturation of the root-zone. This was particularly true of sensors in zone B (Fig 4.1a and 4.1b), although there was no significant zone effect on the IWUI to suggest an advantage to DM yield in the case that rainfall utilisation had increased in zone B (Table 4.2). Consequently, due to the reduction in irrigation rate from zone A to zone B, a DM yield decline was observed (Table 4.2).
Despite the fact that the minimum soil observed in zone B ranged from -35 KPa in treatment
Evap1 to as dry as -112 KPa in treatment Sens2 (data not shown), there was no significant difference in the DM yield reduction between zones A to zone B across treatments (Table 4.2). One of the risks associated with practising deficit irrigation is in order to achieve higher WUE plants tend to be regulated closer to the threshold where water deficits cause significant declines in production. Therefore under situations of high spatial variability or poor
irrigation uniformity, deficit irrigated crops are more prone to yield variation in response to similar water inputs, i.e. variable WUE (Grove & Oosthuizen 2010). However for grasses, deviation from the linear response of DM yield to water appears to only occur when severe water stress causes leaf senescence. This was identified in Chapter 2 as coinciding with a midday leaf of around -2 MPa. Thus the consistency in the DM yield reduction in zone B
across treatments suggests the plant water stress level was not severe enough to cause dieback and therefore the reduction in DM remained in proportion to the reduction in water applied between zones (Table 4.2). In terms of absolute DM yield however, a soil water threshold of -75 KPa resulted in a significant reduction in yield compared to Evap1 and Sens1. Therefore for ensuring maximum yields, results from the current study recommend a -30 KPa threshold as measured at the base of the root-zone.