Seasonal responses of photosynthetic parameters in maize and sunflower and their relationship with leaf functional traits.

Estimates of seasonal variation in photosynthetic capacity (P-c) are critical for modeling the time course of carbon fluxes. Given the time-intensive nature of calculating P-c parameters via gas exchange, it is appealing to calculate parameter variation via changes in chlorophyll (Chl) and nitrogen (N) content by assuming that P-c scales with these variables. Although seasonal changes in P-c and the relationships between N and P-c have been evaluated in forest canopies, there is limited data on seasonal parameter values in crops, nor is it clear if seasonal changes in P-c can be estimated from leaf traits under the high N fertility of managed systems. We characterized the seasonal variability of the maximum rates of carboxylation (V-cmax) and electron transport (J(max)) under well-fertilized conditions for maize (Zea mays L.) and sunflower (Helianthus annuus L.) and coupled these data with measurements of Chl, N, and leaf mass per unit area (LMA). The seasonal Chl-N relationship was significant in maize, but not in sunflower. Area-based N-V-cmax relationships were not significant for either crop. Mass-based N-V-cmax relationships were weak in sunflower, but highly significant in maize. Our results suggest that P-c can be seasonally adjusted in maize with reliable estimates of changes in LMA.