Based on the SWC of I− pots and the amount of water that can be held in the pot (see details in Section 8.3.2), it was estimated that plants in I− pots consumed 768 ml water in average from each pot during the drought period (soil evaporation from the pot is negligible as pots were fully covered by plants, Figure 8.1). Since plant size was large when drought treatment was initiated, the daily water consumption was 200–250 ml (estimated during the experiment in a sunny day), thus plants in the I−
pots likely consumed a majority of the soil available water in the first three days after withholding irrigation.
Plant growth was restricted by water deficit as indicated by decreased LER and increased LSR (Table 8.1 and Figure 8.3). The LSR was even higher than the LER of
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I− plants (Figure 8.3), which would eventually result in herbage yield reduction. However, since the drought period was relatively short (only two weeks), the herbage accumulation of I+ plants and I− plants was not significantly different (Table 8.1 and Figure 8.3).
Stomatal closure is usually the earliest plant response to water deficit (Schroeder et al., 2001), which enables plants to reduce the transpiration water loss, but with a concomitant reduction of photosynthesis due to restricted CO2 uptake, as indicated by decreased Gs, Pn and Tr of I− plants compared to I+ plants (Table 8.3 and Figure 8.6). When water supply cannot meet the water demand of plants, plants will get dehydrated. Here, the RWC of I− plants decreased from 90% of I+ plants to 70% (Figure 8.5). The more severe plant dehydration, the more the LER was limited and the greater LSR, as indicated by negative correlation between LER and RWC and positive correlation between LSR and RWC (Figure 8.10a and b). Reactive oxygen species are usually over produced and cause oxidative damage of organic molecules when plants are under drought stress, resulting in damage to cell membranes, as indicated by increased EL this experiment (Table 8.4 and Figure 8.7). The non- significant increase of MDA in I– plants compared to I+ plants suggests that lipid oxidation is less sensitive to drought stress. It may be that lipid oxidation occurs only under more severe or more prolonged drought stress.
Among plant responses, OA development is generally considered to be one of the plant adaptation traits to drought stress (Blum, 1996; Turner, 1986; Zlatev & Lidon, 2012), as OA has the effect to maintain cell turgor and at the same time, certain types of accumulated solutes help to protect cellular proteins, enzymes, and cellular membrane (Chaves et al., 2003; Farooq et al., 2009). Here, the TP was maintained even though plants were dehydrated (Table 8.2 and Figure 8.5), which could be attributed to the function of OA. However, if LWP and OP were measured in the midday, the TP between I+ and I− plants would probably be different, as the water deficit induced TP decrease is more pronounced in the midday than in the early
139 morning (Jones et al., 1980a). Diurnal measurements of LWP and OP for I+ and I−
perennial ryegrass field swards can be found in Figure 2.3.The positive correlation between OA and RLDM (Figure 8.10d) suggests that the accumulated solutes benefited plant regrowth during recovery from drought. The RLDM was also negatively correlated with LER during drought (Figure 8.10c), which indicates that the more plant growth was restricted during drought, the more solutes accumulated in plant cells. It has been claimed that the accumulation of solutes during water deficit is because the cell expansion and elongation rates fall more rapidly than the photosynthesis rate, thus resulting in the supply of photosynthate exceeding its utilisation (Munns & Weir, 1981; Van Volkenburgh & Boyer, 1985; Wardlaw, 1969). The relationship between OA and crop yield was reviewed by Serraj and Sinclair (2002), who pointed out that most published papers indicated no effect or a negative influence of OA on crop yield during drought stress. In the current experiment, it was seen that the higher OA level benefited plant growth in rehydration but with a sacrifice of yield during the drought stress.
Plant species that inhabit dry sites generally have higher RSR than those of wet habitat, and the RSR is usually increased under drought stress as reviewed by Wu and Cosgrove (2000). Blum (2005) claimed that the increased RSR mainly arises from reduced shoot growth rather than increased root dry matter; therefore, the RSR should not be considered a good indicator of drought tolerance nor a selection criteria for drought tolerant plants. However, in this experiment, it was shown that the increased RSR was mainly due to increased root OM rather than decreased shoot DM, as indicated by significant correlation between RSR and root OM and non- significant correlation between RSR and shoot DM under drought conditions (Table 8.6). This experiment also demonstrated that the increased root OM and RSR under drought had no effect on delaying dehydration, as indicated by the non-significant correlations between RSR or root OM and RWC (Table 8.6), which agrees with Blum’s statement that root biomass and RSR should not be considered as selection
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criteria for drought tolerance. However, it should be noted that drought in this experiment was only of two weeks duration. Sinclair and Muchow (2001) assessed traits for crop yield under drought conditions and found that an increase in rooting depth consistently increased crop yield. Therefore, instead of root total biomass and RSR, rooting depth could be one of the selection criteria for enhancing drought tolerance of perennial ryegrass.