CHAPTER 1: LITERATURE REVIEW
1.9 The Drought Problem
An agricultural drought is defined as lack of adequate soil moisture for a given crop to grow and thrive during a particular time. Apart from reducing agricultural productivity leading to food security problems, drought has some ripple effects on the agriculture- dependent sub-Saharan economies (Richardson, 2003). Banziger and Diallo (2002) reported that 93% of maize production was on dry land. Thus, there is very limited use of irrigation in the region where drought is rampant. In Southern Africa, the most devastating drought was recorded in 1991/1992 season and reduced grain production about 60% (Rosen and Scott, 1992).
1.9.1 Managing Drought
On the farm level, sustainable strategy for mitigating yield losses due to drought should be based on use of tolerant cultivars. According to Boyer (1992), breeding for high water use efficiency improves economic yield. Improvements for drought stress tolerance results in cultivars with better yield and growth under drought condtions.
Boyer (1992) classified mechanisms of drought tolerance in cultivars as dehydration avoidance and dehydration tolerance under drought conditions. Structural mechanisms such as improved rooting depth and increased cuticle thickness delay dehydration, but grain yield is reduced due to increased partitioning of dry mass towards production of structures. According to Pingali and Pandey (2001), farmers in drought prone areas can by plant early such that their cultivars would flower during high moisture conditions and thus escape the drought. Use of conventional tillage increases water infiltration into the soil, encourages development of deep roots.
Breeding for early maturing cultivars would be feasible the flowering traits such as anthesis to silking interval and days to flowering are highly heritable, even under drought stress conditions (Pingali and Pandey, 2001). The problem reported by Pingali and Pandey (2001) that these early maturing cultivars incurred a yield penalty when grown under favourable rainfall conditions. Thus, it is important to study physiological mechanisms that condition cultivar resistance to low moisture stress.
1.9.2 Physiological Basis of Yield Reduction
Scientists are in agreement that early reproductive development is most vulnerable to
coincides with flowering (Boyer, 1992; Bolanos and Edmeades, 1993a). Thus highest yield reduction occurs at flowering stage because of abnormal floral, ear and kernel development (Westgate and Boyer, 1986; Lafitte and Edmeades, 1995; Edmeades et al., 1999; Zinselmeier et al., 2002). Low moisture stress at flowering reduces the rate of photosynthesis to almost zero under mild and severe stress (Schussler and Westgate, 1994; Westgate and Boyer, 1986; Zinselmeier et al., 1999; Zinselmeier et al., 2002).
Low moisture stress during the reproductive stage reduces sink strength and kernel development. Setter et al. (2001) reported that moisture stress at pre-pollination reduced accumulation of carbohydrates in apical and basal florets. Vasal et al.
(1997) reported that assimilates were preferentially distributed to the tassel resulting in poor seed set. Setter et al. (2001) reported that water deficit increased abscisic acid (ABA) concentration in the reproductive tissues. According to Boyer (1992), high ABA level inhibited endosperm cell division and reduced seed set. Thus ABA, probably plays a critical role in controlling drought tolerance. Zinselmeier et al. (1999) reported that water deficit resulted in abortion and few kernels. Previously, Zinselmeier et al. (1995) had reported that moisture stress inhibited ovary growth, decreased levels of reducing sugars, depleted starch and inhibited the activities of acid invertase, which maintains the reproductive sink strength and facilitates early kernel development.
1.9.3 Sources and Gene Action Conditioning Drought Tolerance
Generally, the best sources of drought stress tolerance should have exceptionally high agronomic performance and large genetic variance for other important traits.
According to Vasal et al. (1997), inbred parents should be preferred, because heritability increases with inbreeding levels. Vasal et al. (1997) reported that inbreeding of segregating populations resulted in high frequency of inbred lines with long anthesis to silking interval (ASI) under drought. Secondary traits such as the ASI are correlated with grain yield under drought stress, and are easy to measure (Bolanos and Edmeades, 1996; Banziger et al., 2000). In another study, Betran et al.
(1996) reported that there was a low correlation (0.40) between the grain yield of inbred parents and grain yield of their testcrosses under drought conditions. Thus, the crosses between drought tolerant inbred lines still have to be tested for tolerance to drought.
A survey of literature showed that there is little research on gene action conditioning drought stress tolerance in Southern African maize germplasm. Betran et al. (2003a) reported predominance of additive gene action in controlling grain yield in tropical germplasm under drought stress. However, in another study, Betran et al. (2003b) reported significant non-additive effects for grain yield under drought conditions.
These results were consistent with Guei and Wassom (1992) who also reported predominance of additive gene action in controlling flowering traits, while dominance was more important for grain yield and number of ears per plant under drought stress. Studies of quantitative trait loci also confirmed the importance of both additive and dominance action in conditioning yield and the associated flowering traits (Agrama and Moussa, 1996). This suggests that breeders should utilise selection strategies such as reciprocal recurrent selection and hybridisation that employ both additive and dominance gene action in improving maize for drought tolerance.
1.9.4 Selection for Drought Stress Tolerance
Progress in improving maize for drought stress tolerance, especially in Southern Africa has been slow. This has been partly attributed to the large G x E interaction in the highly variable production environments of small-scale farmers. Rainfall amount and timing are highly variable such that it is difficult to predict the occurrence and the severity of drought stress in these environments (Pingali and Pandey, 2001). In addition breeding is made complicated by the low heritability for grain yield under the drought conditions (Bolanos et al., 1993; Byrne et al., 1995). However, research has indicated that grain yield can be improved under drought improved by selecting for the highly heritable secondary traits like the ASI and number of ears per plant. These traits have been confirned to be highly correlated with grainyield under drought stress conditions (Bolanos and Edmeades, 1993a; Chapman and Edmeades, 1999; Pingali and Pandey, 2001; Tollenaar et al.; 1992).
Selection for yield under managed stress results in better breeding progress than selecting under non-stress conditions. Edmeades et al. (1999) reported yield gains of 3.8 to 12.6% per cycle in tropical populations following simultaneous selection under well-watered and managed drought stress at flowering. Byrne et al. (1995) reported higher progress under managed drought stress (1.68%) than in multi-location trials (1.06%) in Tuxpeno Sequia and Tuxpeno confirming previous reports by Bolanos and Edmeades (1993b). However, Byrne et al. (1995) reported that selection under water stress only might result in cultivars with lower yield under favourable conditions.
Boyer (1992) suggested multilocation testing at varying water regimes to avoid selecting cultivars that incur a yield penalty under favourable conditions. According to Boyer (1992), multilocation testing can identify cultivars with the following combinations: a) “high yielding in both optimum and water-deficit; b) high yielding in optimum, but low yielding under water deficit; and c) low yielding in optimum, but high yielding under water deficit conditions.” As a result Boyer (1992) suggested that selections should be made under drought conditions followed by testing in multilocation trials. Byrne et al. (1995) also suggested a similar two step approach with the first aimed at reducing ASI by evaluating lines at a few sites under managed drought stress at flowering and then select for specific adaptation and high yield potential in multilocation trials. An integrated approach that begins with screening under managed drought stress before multilocation testing is thus suggested.