Canopy development, leaf traits and yield in high-altitude Andean maize under contrasting plant densities in Argentina

In highlands, the increase in altitude results in a drastic decrease in temperature (T) that delays phenological development of maize, decreasing light interception during the cycle. This could be partially overcome by increasing plant density, but information is scarce for designing specific management options. The objective of this work was to describe changes in canopy development, photosynthetic performance, biomass and yield of maize grown at contrasting plant densities (5.7 plants m(-2), locally used, and 8.7 plants m(-2), 50% higher). Three experiments were carried out in two high-altitude environments within the Argentinean Andean region, Hornillos (HOR, 2380 masl, 2019-20 and 2020-21) and El Rosal (ERO, 3350 masl, 2019-20), and complementary data were obtained from samplings in 8 farmer's fields (from 2400 to 3400 masl, 2022-23). In the experiments, mean T during the first 150 days of the cycle was 33% lower at ERO, which implied 39 extra days but 25% shorter thermal time to achieve silking. The higher plant density significantly increased leaf area index and light interception at ERO, whereas at HOR, this was only evident during the second season. At the leaf level, plants grown at ERO had thicker leaves with higher chlorophyll (+36%) and nitrogen (40%) content. Photosynthetic electron transport rate at full irradiance was +20% higher at ERO but significantly varied throughout the day with lowest values in the morning, which was not observed at HOR and was not related to light intensity or stomatal conductance. At HOR, the increase in plant density did not improve light interception, nor yield in 2019-20 (with average yields of 6356 kg ha(-1)) but it did improve both in 2020-21 when generally lower yields were attained (4821 kg ha(-1)). Across farmer's fields, increasing densities consistently reduced yield per plant (r(2) = 0.57***) but improved yield per area basis, which was maximised at 10 pl m(-2) as a result of a steady increase in kernel number m(-2) (up to 15 pl m(-2)). Thus, in these high-altitude environments, increasing plant density beyond recommended (6 pl m(-2)) is a promising approach for improving yield, with major penalties of supra-optimum densities being related to kernel weight. Further work is needed to explore the effect of different factors limiting kernel growth, over plant density responses.