Corn Growth Response to Temperature and Photoperiod II. Leaf‐Initiation and Leaf‐Appearance Rates1

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Abstract
The prediction of evapotranspiration and canopy photosynthesis, for example in crop simulation modeling and in remote sensing applications, requires a quantitative description of the influence of environmental factors such as temperature and photoperiod on leaf area development. The objectives of this study were to define the temperature response curves and the responses to photoperiod for two components of leaf area development in corn (Zea mays L.), namely leaf‐initiation rate and leaf‐appearance rate. Plants of two corn hybrids, W346 and XL45, were grown in controlled environments under 17 day/night temperature regimes ranging from 16/6 to 38/33°C (12 h photoperiod) and under three photoperiod regimes (12, 14, and 16 h) at two selected temperatures (constant 18 and 28°C). Leaf‐initiation rates, determined from frequent dissections, were constant from seedling emergence until tassel initiation in all treatments. Similarly the appearance rate of the first 12 leaves was constant from seedling emergence onward, but after leaf 12, appearance rate increased, presumably as a consequence of rapid stem elongation and because the upper stem leaves were smaller and required less time to expand. Both cultivars had similar leaf‐initiation and leaf‐appearance rates under each day/night temperature regime. Data from the day/night temperature regimes were used to derive rate of leaf‐initiation (leaf primordia/day) vs. temperature and rate of leaf‐appearance (leaves/day) vs. temperature response curves which could be described by third and fourth‐degree polynomial equations, respectively. Previously published values for leaf‐appearance rates have varied by up to three or four‐fold at any particular temperature, primarily because of different definitions that have been used for appearance rate. Values for the rate of change in leaf appearance rate with temperature within the 16 to 26°C range, however, were more similar ranging from 0.21 to 0.30 leaves day−1°C−1 and were similar to a value of 0.23 leaves day−1°C−1 obtained in this study. Both leaf‐initiation and leaf‐appearance rates increased with an increase in photoperiod, particularly under constant 18°C conditions. Cultivar W346 appeared to be more responsive to photoperiod than XL45.