Use of Chlorophyll Fluorescence Assessments to Differentiate Corn Hybrid Response To Variable Water Conditions

Development of corn (Zea mays L.) germplasm tolerant to water stress will be vital to sustaining corn-based farming in the U.S. Great Plains. In a companion 4-yr field study near Shelton, NE, we found that 12 hybrids displayed differential agronomic responses to varying water levels, with tolerant hybrids yielding from 27 to 42% more than susceptible hybrids under stress while yielding similarly under no stress. The objective of this study was to determine if chlorophyll fluorescence (CF) measurements could be used to distinguish tolerant from susceptible hybrids. Leaf temperature (LT) and two CF parameters (%PSII, photosystem II quantum efficiency, and ETR, electron transport rate) were measured on three postflowering dates in 2001 using a fluorometer on a subset of original treatments involving two tolerant and susceptible hybrids grown under deficit and adequate water. Water effects were observed on only one date; LT was 2.5 C warmer and %PSII and ETR values were 25% lower for deficit vs. adequate water just after silking, signifying increased water stress and decreased photosynthesis during reproductive growth. Under stress, LTs were 2.8 C cooler and %PSII and ETR values 50% higher for tolerant vs. susceptible hybrids, while all hybrids produced similar CF values under no stress. Thus, grain yield and photosynthetic responses of hybrids to stress were similar, indicating that CF measurements can be used to distinguish tolerant from susceptible hybrids. IN COMPANION WORK (O’Neill et al., 2004), we noted the significant role that water and N fertilizer inputs have played in increasing productivity of corn grown in the Great Plains region of the USA. However, continued overuse of these inputs required to sustain current productivity poses serious environmental threats (Council for Agricultural Science and Technology, 1999). To minimize input costs and environmental impact, farmers will likely have to resort to producing corn with less irrigation water and fertilizer N in the future. This will lead to increased levels of water and N stress imposed on the crop. Development of corn hybrids tolerant to water and N stresses will be crucial to sustaining corn-based farming in the Great Plains region of the USA. Hence, future corn breeding efforts should focus on identifying physiological mechanisms that can be used to further improve tolerance of corn to these and other stresses. In the companion work we found that 12 hybrids displayed differential agronomic responses to varying levels of water and N. For example, under either limited water or N, stress tolerant hybrids yielded from 27 to 42% more than susceptible hybrids, while these same hybrids yielded similarly under adequate water and N levels. Furthermore, variation in hybrid yields under deficit water was better predicted by hybrid yields under deficit N than under adequate water conditions. Finally, variation in hybrid grain yields grown under varying water and N levels was strongly associated with hybrid variation in kernel number per unit area. Collectively, these results imply that water and N stresses produced similar adverse effects on key physiological processes, and hybrids possessing physiological mechanisms conferring ability to maximize kernel number under either water or N stress were critical to their ability to produce high grain yields. Other researchers have shown that kernel number is strongly linked to assimilate supply during the critical period around flowering (Schussler and Westgate, 1995). Tollenaar and Aguilera (1992) confirmed the role of achieving high photosynthetic rates by showing that observed differences in dry matter accumulation between old and new hybrids were due to higher photosynthetic rates after silking for newer hybrids. Others (Sanchez et al., 1983; Wolfe et al., 1988; Dwyer et al., 1992; Aguilera et al., 1999) have noted that while stress reduces photosynthesis, the degree of reduction appears to vary among genotypes. Thus, we hypothesized that corn hybrids tolerant to drought would, for example, maintain higher photosynthetic rates compared to susceptible hybrids during this critical reproductive growth period, and photosynthetic assessments during this time may offer a potential means for identifying stress tolerant germplasm. While measurements of leaf photosynthetic rates may offer promise for characterizing hybrid responses to stress, previous methods of assessing photosynthesis via gas exchange techniques have proven to be laborious and not practical in crop improvement programs (Earl and Tollenaar, 1999). Alternatively, CF techniques may serve as a more practical means for indirectly assessing leaf photosynthetic rates (Earl and Tollenaar, 1998; Adams et al., 2000; Jiang and Huang, 2000; Garty et al., 2001; Ying et al., 2002; Earl and Davis, 2003). Since each CF measurement requires only a few seconds, hundreds of measurements can be made per day with a single instrument, thus greatly improving on the USDA-ARS, Lincoln, NE. Joint contribution of USDA-ARS and Agric. Res. Div. of the Univ. of Nebraska. Published as Journal Ser. No. 15055. Mention of commercial products and organizations in this article is solely to provide specific information. It does not constitute endorsement by USDA-ARS over other products and organizations not mentioned. The USDA-ARS is an equal opportunity/affirmative action employer and all agency services are available without discrimination. Received 28 June 2005. *Corresponding author (jshanahan1@