Photosynthesis in Two Contrasting Mexican Landraces of Maize in Response to Water Stress and High Temperature Under Controlled Conditions.

The objective of this study was to determine the effect of water stress and high temperature on gas exchange in two contrasting Mexican landraces of maize under greenhouse conditions. The genotypes studied were C-3014 (tolerant) and C-3015 (susceptible), previously classified as such under field conditions.  Seeds were planted in plastic bags containing 5 kg of dry soil.  Treatments were begun at the 4^th ligulated leaf stage and included: 1) Normal temperature + normal watering (NT+NW); 2) Normal temperature + water stress (NT+WS); 3) High temperature + normal watering (HT+NW); and 4) High temperature + water stress (HT+WS). The soil for plants that were well watered was maintained at or above 80% of field capacity throughout the experiment, while for treatment for drought; irrigation was applied only after the soil reached the permanent wilting point.  Plants treated with normal temperature were maintained at 28/20 °C while high temperature plants were maintained at 44/28 °C during the day/night cycle respectively. The experimental layout was a randomized complete block with five replicates under a 2³ factorial where the factors were genotype, temperature, and irrigation.  A LICOR 6400 portable photosynthesis system was used to measure CO₂ fixation rate (A), stomatal conductance (g_s), transpiration (E), and leaf temperature (Lt).  Results of analysis of variance showed significant differences due to genotype, temperature and water availability, but interactions between treatment factors were significant only for A.  Water stress and high temperature alone reduced A by day 4 and 7 respectively after beginning treatment, concomitant with reductions in E and g_s. Water stress and high temperature combined, reduced gradually the A by day 6. The A in the susceptible genotype was completely inhibited by day 8. We concluded that injuries of water stress, high temperature, or the combined stress, a manifested by declines in A, g_s and E could cause permanent physiological damage for maize genotypes, particularly for the susceptible genotype. The adverse impact was more severe with simultaneous water and heat stresses.