Characterization of traits contributing to gains in maize genetic improvement under drought and heat stress

Abstract

Drought and heat stress are important limitations to maize growth and sustainable agriculture worldwide. These two environmental factors influence physiological processes, crop growth, crop development and yield processes. The study was carried to (i) to assess the genetic gain in yield and other secondary traits in CIMMYT hybrids released from 1999 to 2016 for their tolerance to drought stress (ii) to assess the genetic gain in yield and other secondary traits in CIMMYT hybrids released from 1999 to 2016 for their tolerance to heat stress (iii) to examine the transpiration response to a drying soil of different maize hybrids under varying VPD regimes. Genetic gain within the International Maize and Wheat Improvement Centre (CIMMYT) breeding program were estimated using time series of maize (Zea mays L.) hybrids. The experiments were planted using a 15*15 alpha (0.1) lattice design. A total of 72 of the best-performing hybrids from local trials from 1999 to 2016 were selected to form an era panel and evaluated in 2 trials in Zimbabwe – (Chiredzi). Treatments included heat stress and managed drought stress. Genetic gain was estimated as the slope of the regression of grain yield on the year of hybrid release. Grain yield was significantly different for the hybrids at P ≤ 0.01. The hybrids significantly varied for traits Anthesis date, Anthesis-silking interval, Plant height, Ear height, Senescence 1 at P ≤ 0.001. No significant differences were recorded for Ears/plant and Senescence 2. Heritability estimates under heat stress and managed drought stress conditions ranged from medium to moderately high indicating that the secondary traits can be directly selected for, to improve the hybrids. The same CIMMMYT hybrids were also grown in a pot experiment at CIMMYT Zimbabwe, Harare station to observe how they manage transpiration under varied Vapor pressure deficit regimes and also varying water deficit conditions. The experiment was laid out in completely randomized deign. 75 hybrids were planted with 3 replications. Understanding response of transpiration in maize hybrids contrasting for drought tolerance under different VPD levels and varying moisture depletion is needed as it enables identification of hybrids that tolerate moisture stress and elevated temperatures. Newly released hybrids exhibited high ability to conserve water under limited water conditions as compared to the old hybrids. The experiment was done when the ambient air had mean of 15.75% Relative Humidity and 28.73℃. In both (Vapor pressure deficit and soil drying) experiments hybrids CZH1261, CZH132119 and CZH15013 consistently appeared in the best 10 ranked hybrids that conserve water, whilst hybrids CZH01008, CZH15212 and CZH15467 consistently appeared in the least ranking group for not being able to conserve water. Key Words: Transpiration - Vapor pressure deficit - Drought stress - Heat stress - Genetic gain