Submitted to: Crop Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/6/2005
Publication Date: 2/1/2006
Citation: Oneill, P.M., Shanahan, J.F., Schepers, J.S. 2006. Use of chlorophyll fluorescence assessments to differentiate corn hybrid response to variable water conditions. Crop Science 46:681-687. Interpretive Summary: In previous work, we noted the significant role that water and nitrogen (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. In order 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 upon 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. Finally, variation in hybrid grain yields grown under varying water and N levels was strongly correlated with hybrid variation in kernel number per unit area, indicating 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 are critical to their ability to produce high grain yields. Other researchers have in turn shown that kernel number is strongly linked to ability of tolerant hybrids to maintain higher photosynthetic rates during critical growth stages like silking. While measurements of leaf photosynthetic rates have shown promise for determining hybrid responses to stress, the traditional gas exchange techniques used to determine photosynthesis are laborious, and have not proven to be practical tool in crop improvement programs. Alternatively, the newly developed technique involving measurement of leaf chlorophyll fluorescence, may serve as a more practical means for indirectly assessing leaf photosynthetic rates. Briefly, CF is a non-invasive, highly sensitive, rapid and quantitative probe of leaf photosynthesis. 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 sampling resolution that can be achieved over gas exchange techniques. The objective of work was to determine if chlorophyll fluorescence (CF) assessments could be used to drought tolerant from susceptible corn hybrids. In this work, photosynthetic responses to deficit and adequate water levels of four of the original twelve hybrids were evaluated. Two of the hybrids were characterized by their agronomic responses as being drought-tolerant and two as drought susceptible. Photosynthetic responses were probed using a commercially available fluorometer, model PAM-2000, manufactured by Heinz Walz of Effeltrich, Germany. Leaf temperature and CF indicators were measured on three post-flowering dates in 2001 to determine if these measurements could be used to differentiate hybrid photosynthetic responses to post-flowering stress. Effects on measured variables were observed on one of three dates, with average leaf temperature 2.5 0C warmer and CF values 25% less for deficit vs. adequate water levels during the silking and reproductive growth stages, indicating water stress was increased and photosynthesis decreased for deficit water conditions. Under deficit water, leaf temperatures were 2.8 0C cooler and CF values 50% higher for tolerant vs. susceptible hybrids, while all hybrids produced similar leaf temperatur
Technical Abstract: Development of corn (Zea mays L.) germplasm tolerant to water and N stresses will be vital to sustaining corn-based farming in the USA Great Plains. In a companion 4-yr field study near Shelton, NE, we found that 12 hybrids displayed differential agronomic responses to varying levels of water and N, 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 (FPSII, photosystem II quantum efficiency, and ETR, electron transport rate) were measured on three post-flowering 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; with LT 2.5 0C warmer and FPSII and ETR values 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 0C cooler and FPSII 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.