Recent advances in breeding for drought tolerance in maize

Abstract

Drought is an important cause of instability of national maize grain yields and of the food supply and economy of small-scale maize-based farming systems in the tropics. Water shortages affect maize yields throughout the crop cycle, but most severely at flowering and to a lesser degree at establishment. Maize is often grown in environments thought to be better suited to sorghum and millet, yet because farmers in these areas persist with maize, researchers have an obligation to help stabilize maize production. Recurrent selection, which focused mainly on increasing tolerance to drought during flowering and grainfilling, has successfully increased grain yield over a wide range of moisture regimes. Gains ranged from 80-108 (mean 94) kg ha⁻¹ yr⁻¹ when selecting among full-sib (FS) families to 73–144 (mean 111) kg ha⁻¹ yr⁻¹ when selecting among S, families, and were observed when selections were evaluated at a drought-induced yield level of 1.5–2.4 ton ha⁻¹. This represents annual gains in severely droughted environments of >5%. Under well-watered conditions where yields ranged from 5.6-8.0tonha⁻¹, gains from the same two selection schemes ranged from 38–108 (mean 73)kgha⁻¹ yr⁻¹ and from 27–89 (mean 59)kgha⁻¹ yr⁻¹, respectively. Gains in yield were mainly due to increased numbers of grains per plant, and were associated with a reduced anthesis-silking interval (ASI) and an increased harvest index. These changes are consistent with increased assimilate partitioning to the ear at flowering, a phenomenon that will need to be addressed by maize models that focus on genotypic responses to stress. Little change was observed in plant water status or foliar senescence rates. Drought-tolerant varieties have shown similar gains when evaluated under low N conditions, suggesting that partitioning of N to the ear has also been improved. The challenge now is to develop drought-tolerant hybrids. Conventionally improved populations have provided a lower frequency of drought-tolerant hybrids than their counterparts with a history of improvement for drought tolerance. Molecular marker-based linkage maps of ASI, an easily observed indicator of drought tolerance at flowering, suggest that this trait could be efficiently transferred to elite inbred lines using marker-assisted backcrossing techniques. Yields of drought-tolerant varieties, lines and hybrids, when grown under well-watered conditions, have shown clearly that drought tolerance does not come at the cost of yield potential. The key to efficient improvement for drought-tolerance lies in the choice of elite adapted germplasm, use of carefully managed drought stress, and selection based on a minimum data set comprising anthesis date, ASI, ears per plant and shelled grain yields. This results in stabilized and improved yield for the maize component of maize-based cropping systems exposed to mid-season and terminal drought in highly variable rainfall environments.