Plant Population and Planting Date Effects On Drought-Tolerant Corn Hybrid Physiology.

Seed companies are promoting the use of drought-tolerant corn (Zea mays L.) hybrids in Ohio to manage risks associated with in-season precipitation variability.  Delays in planting due to wet field conditions can contribute to greater moisture stress during pollination, and drought-tolerant hybrids may exhibit more yield stability from year to year in late-planted conditions.  However, the mechanisms behind drought tolerance are not well understood.  Higher plant population can contribute to greater stress levels, but drought-tolerant hybrids may exhibit a greater yield response to increasing plant population as compared to susceptible hybrids.  A field study was conducted in 2013 at Hoytville, South Charleston, and Wooster, OH to evaluate the physiological response of drought-tolerant hybrids to susceptible hybrids at five populations (59,000, 74,000, 89,000, 104,000, and 124,000 plants ha^-1) and two planting dates (early May and early June).  Four DuPont Pioneer® brand hybrids were examined: two AquaMax^TM drought-tolerant hybrids and two drought-susceptible hybrids.  Physiological measurements, including leaf greenness (using a SPAD meter), leaf water potential, photosynthesis, transpiration, stomatal conductance, chlorophyll fluorescence, and biomass were collected at V10, R2, and R5.  Preliminary results indicate that as plant population increased, SPAD values decreased for all hybrids.  However, the drought-tolerant hybrids consistently exhibited lower SPAD readings at V10 and R2 than the susceptible hybrids.  While leaf water potential, stomatal conductance, and transpiration did not differ between hybrids at V10 and R2 at most sites, photosynthesis and the light-adapted quantum yield was greater in the drought-tolerant hybrid at multiple locations at V10 and at two sites at R2 (P<0.1).  These trends were observed in both the early and the late planting date.  These results suggest drought-tolerant hybrids may be more efficiently utilizing absorbed photons within the photosystem, and may be using water more efficiently to fix carbohydrates.