Plants respond to drought stress with a series of complex mechanisms from genetic expression, biochemical metabolism, and physiological processes to plant morphological changes. There are several comprehensive reviews of plant responses to drought (Chaves et al., 2003; Farooq et al., 2009). In this research, we mainly investigated the morphological and physiological responses of perennial ryegrass to drought.
Yield reduction is the most dramatic and obvious response to drought. For forage grasses, the yield reduction is normally attributable to a lower rate of leaf expansion, leaf appearance, tiller appearance, and a greater rate of leaf senescence and tiller death (Barker & Caradus, 2001). In this research, it was observed that the tiller survival rate (visually scored) was significantly decreased under drought conditions compared to the irrigated plants in the rainout shelter experiment, while the leaf elongation rate was decreased and leaf senescence rate was increased in the glasshouse experiment. These morphological responses are not ideal from an agronomic point of view since this pattern of response necessarily involves yield loss. However, the reduced total leaf area, as a result of these responses, reduces the transpiration water loss and plant water demand, which favours the water status of
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the surviving tissues and enables those tissues to maintain their active metabolic activities. In the rainout shelter experiment, the major herbage yield reduction occurred in the second month after withholding irrigation while the surviving leaves were remaining well hydrated at this stage as indicated by the relative water content (RWC).
Soil water deficit is primarily perceived by roots. In this project, roots were able to maintain growth under water deficit, as indicated by similar new root formation and root organic matter between irrigated (I+) and non-irrigated (I−) plants in the rainout shelter experiment and glasshouse experiment, respectively. Root:shoot ratio (RSR) depends on the dynamics of shoot and root growth. In the glasshouse experiment, it was demonstrated that the RSR was increased under drought as a result of slightly increased root biomass compared to I+ plants. However, in another study, the increased RSR of drought-stressed plants was attributed to the larger decrease of shoot biomass than root biomass in response to drought (Huang et al., 1998b). These different results are probably due to different drought severities. No matter whether roots maintain or reduce growth under drought conditions, in both cases roots become a proportionately greater allocation priority under drought.
When roots sense drought, a chemical message, abscisic acid, is produced by roots and travels to shoots via the xylem and controls the stomatal aperture (Schroeder et al., 2001). In the glasshouse experiment, the stomatal conductance (Gs) was
decreased in response to drought, and as a consequence of stomatal closure, both transpiration and photosynthesis rate were reduced, and transpiration rate was inhibited more than photosynthesis rate, which would lead to increased water use efficiency (WUE), though lower productivity. It is known that Gs, WUE, and carbon
isotope discrimination (∆13C), crop yield are related to each other. For example, it has been found that: ∆13C is negatively correlated to WUE in many crop species such as wheat (Farquhar & Richards, 1984) and cool-season forage grasses (Johnson & Bassett, 1991); ∆13C is positively correlated to crop yield of bean (Zacharisen et al., 1999); and Gs is positively correlated to yield of cotton (Ulloa et al., 2000).
However, their relationships under different soil water conditions are not consistent (Jensen et al., 2002), and the reasons for this are not fully understood. To date, there is a lack of research investigating the relationships among these traits under different
145 water regimes in perennial ryegrass. In the rainout shelter experiment, only the ∆13C and herbage yield was measured, a positive correlation between ∆13C and herbage yield at both I+ and I− conditions was detected (Appendix 5).
The regrowth of perennial grass species after drought is very important for pasture re-establishment. In the rainout shelter experiment, compensatory growth following drought was observed, in the form of higher shoot DM in I− than I+ plants in May (Figure 4.2). As discussed in the Section 4.4, the compensatory growth was likely attributed to the remaining nutrients in the soil and/or OA. The total nitrogen capture was significantly decreased by water deficit (Figure 7.1). In the glasshouse experiment, it was shown that the degree of osmotic adjustment (OA) during drought was positively correlated with the plant regrowth during recovery from drought. It is important to recognise that OA is achieved by the accumulation of a multitude of solutes. Proline has been considered one of the osmolytes (Verbruggen & Hermans, 2008), however, not a major one. The role of proline in prevention of protein denaturation and cell membrane integrity may be more important than its role in OA (Hamilton & Heckathorn, 2001; Samuel et al., 2000). In the glasshouse experiment, proline accumulation contributed only 5% to the OA (calculated according to the Morse equation). In many plant species, water soluble carbohydrates are the major accumulated solutes contributing to the OA under drought (DaCosta & Huang, 2006; De Diego et al., 2013; Volaire & Lelievre, 1997). Fructans, synthesised and stored in the vacuole (Wagner et al., 1983), are the major carbohydrate storage compounds for forage grasses. It has been reported that fructan accumulation is increased when grasses are under drought conditions (Amiard et al., 2003; Thomas & James, 1999). It was planned to measure leaf carbohydrates in the glasshouse experiment but problems with equipment resulted in samples being lost. However, it was likely that the accumulated fructans act as an energy reserve for promoting plant regrowth during rehydration. It has been shown that total fructans decrease to a level similar to that of control plants during rehydration of cocksfoot (Dactylis glomerata) and perennial ryegrass (Amiard et al., 2003; Volaire & Lelievre, 1997). It has been reported that high OA breeding lines produced more seed/grain yield than low OA breeding lines under drought conditions in sunflower (Helianthus annuus L.) (Chimenti et al., 2002), castor (Ricinus communis L.) hybrids (Babita et al., 2010), sorghum (Sorghum bicolor L.) (Ludlow et al., 1990), barley (Hordeum vulgare L.)
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(Gonzalez et al., 2008) and wheat (Triticum aestivum L.) (Fischer et al., 2005). However, based on results from this project, it is deduced that selecting high OA in grass species that are harvested for herbage biomass might result in low herbage production during drought but improved herbage yield after drought, which is in agreement with statements of Munns (1988), who suggested that OA might be an adaptation for surviving stress rather than for growing during stress.
Malondialdehye (MDA) is one of the final products of peroxidation of unsaturated fatty acids in phospholipids (the major component of cell membranes), thus MDA accumulation and electrolyte leakage (EL) are commonly measured to assess the stress-induced injury of cell membrane (Agarie et al., 1995; Bajji et al., 2002; Bandurska & Jozwiak, 2010; Chai et al., 2010; Labudda, 2013). Both EL and MDA accumulation have been linked to the accumulation of reactive oxygen species (ROS) (Demidchik et al., 2014). In the glasshouse experiment, the cell membrane of I−
plants was injured at least to some extent as indicated by the significantly increased EL.