Leaf-level improvements in WUE

In the glasshouse experiment detailed in Chapter 2, an examination of the water transport system and stomatal regulation indicated that ryegrass has the capacity to transport more water than is required to maintain maximal A rates, and furthermore, water use could be reduced without permanent hydraulic damage. Accordingly, improved WUE was the result of three main factors: the non-linear relationship between A and gs; the capacity to respond to water inputs for much of the leaf functional water potential range; and maximal utilisation of hydraulic investment with stomata closing after 50 % loss of hydraulic conductivity. The apparent resilience of the grass plant with regards to diurnal exposure and recovery from hydraulic dysfunction was a unique finding as in most woody plants where the majority of hydraulic investigation has occurred, recovery of gas exchange and hydraulic function tends to be slow (Brodribb & Cochard 2009). Under soil drying the plant was still at risk of drought-induced dieback, which coincided with ~80 % loss of Kleaf and gs functionality at a

ΨPD of -1.5 MPa. However, dieback was avoided in the sustained drought treatment by

manipulating diurnal water availability such that water use was restricted during daylight hours, and high leaf water potential (leaf) at night ensured utilisation of carbon gain for

growth with minimal loss of water via transpiration.

The results as they apply to irrigation management suggest that:

1) soil water deficits causing dieback should be avoided as they reduce WUE and senescent herbage has a reduced nutritive value;

2) soil water deficits must persist to increase WUE through eliciting stomatal closure; but

3) in order to utilise carbon gain for growth, leaves must be well-hydrated.

Visual monitoring of pastures and calibration of irrigation trigger points with the onset of pasture senescence could be used to define the maximum soil water deficit at which WUE is maximised. However, manipulating diurnal soil water availability in the field to achieve both high leaf for growth overnight and reduced midday leaf to enhance WUE may be more

difficult to achieve on a diurnal basis.

Using soil moisture sensors to schedule irrigation events in the field, improvements to WUE were achieved over time by maintaining plants at the desired soil water deficit and regulating the amplitude in soil water availability experienced. This resulted overall in an increase in

148 irrigation efficiency of 0.24 t DM/ML for both well-watered and deficit irrigation practices, equating to a water saving of 20-33 % compared to where irrigation was scheduled according to a rainfall deficit, which uses estimated ET from meteorological data or an evaporative pan. However, in terms of instantaneous midday WUE, even well-watered plants were regulating at a relatively high WUE, compared to well-watered glasshouse plants in Chapter 2.

The disparity between well-watered glasshouse (Chapter 2) and field grown plants (Chapter 4) suggests that evaporative demand was greater in the field, and therefore despite soil water being readily available to maintain optimal leaf elongation rates, diurnal decline in Kleaf may

be occurring at much higher soil. Therefore increasing the soil water deficit further may

reduce the capacity to utilise carbon in growth by preventing plants from re-hydrating completely overnight and/or through increasing the period of time stomata are closed during the day, therefore limiting carbon assimilation. This hypothesis requires further testing, however greater sensitivity of Kleaf to declines under field conditions would support the

results reported in Chapter 4 of the linear reduction in DM yield with irrigation inputs and observations reported elsewhere (Merot et al. 2008; Smeal et al. 2005). Importantly, the saturating curve between yield and transpiration which is the basis for the use of deficit irrigation in cereal crops (Farre & Faci 2006; Kang et al. 2002), is not applicable to herbaceous vegetative crops. Furthermore, the curvilinear relationship between WUE and

leaf reported in Chapter 4 provides further evidence that water conservation will be largely

at the detriment of DM yield.

Drought resistance is commonly proposed as a method to improve WUE, however within the grass literature the definition of advantage in selecting for higher WUE has not always referred to improved DM yield. Furthermore, unlike in Mediterranean environments with persistent periods of dry weather, temperate environments are characterised by intermittent rainfall. Therefore drought resistance should also not be to the limitation of well-watered production potential. In Chapter 6, a further examination of the water transport system was instructive in defining dehydration tolerance and avoidance according to the water use traits

P50 and stomatal regulation respectively, and understanding the trade-offs in water use and DM yield according to maximum hydraulic conductivity and the hydraulic safety margin. This is the first methodological approach for forage grasses to examine the consequences of water shortage and production potential under well-watered conditions.

149 Variability between grass species pertained to dehydration avoidance characteristics, namely stomatal regulation rather than dehydration tolerance conferred by P50. Stomatal closure in

L. multiflorum occurred at less negative Ψleaf than F. arundinacea cultivars, resulting in

improved WUE under soil water deficit conditions compared to the well-watered situation. Dry matter returns were similar to F. arundinacea through higher maximal hydraulic conductivity and stomatal conductance, allowing for higher rates of carbon gain when leaf

was favourable. Whilst F. arundinacea consequently used more water than L. multiflorum

under the water deficit condition, the concomitant increase in carbon gain from closing stomata at a lower Ψleaf meant that overall WUE and DM yield were similar between the two

species. That is, the two hydraulic strategies were balanced under the sustained soil water- deficit conditions tested. However when plants were subject to soil drying to -200 KPa, the reduced hydraulic safety of F. arundinacea exposed leaves to increased proportions of leaf dieback.

The importance of these results relates to the specialisation of plant function for different spatial and temporal variation in soil moisture. Therefore for irrigation management, species variation provides the opportunity to matching the species grown to the environmental conditions and water availability scenario, so that DM yield is maximised and hence so too is irrigation efficiency. For example, given that F. arundinacea species tend to have larger root systems (Durand et al. 2007; Garwood & Sinclair 1979), prolonged stomatal opening of F. arundinacea may allow for continued utilisation of stored water deeper in the soil profile, and therefore larger infrequent irrigation scheduling strategies may maximise the plant‟s

characteristics. In comparison, where deeper water is not available, the conservative stomatal function of L. multiflorum may be more advantageous, and similarly respond better to short- interval, smaller irrigation applications.