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CROP PHYSIOLOGY & METABOLISM

Nitrogen and Light Responses of Cotton Photosynthesis and Implications for Crop Growth

CSIRO Div. of Plant Industry, Australian Cotton Co-operative Research Center, Locked Bag 59, Narrabri, NSW 2390, Australia

^* Corresponding author (Steve.Milroy{at}csiro.au)

Leaf nitrogen content and light intensity affect leaf photosynthesis. Responses of cotton (Gossypium hirsutum L.) leaf photosynthesis to nitrogen and light were developed and used in a framework that scales from leaf photosynthesis to canopy radiation use efficiency (RUE). This was then used to explore the impact of nitrogen and light dynamics on RUE of cotton canopies. Photosynthetic rate and nitrogen concentration of leaves (specific leaf nitrogen, SLN) were measured from two field experiments in which the rate of nitrogen application was varied. The overall shape of the relationship between photosynthesis and SLN of individual leaves was consistent with that for other species, being described by an exponential rise to a maximum of 32.49 ± 1.08 µmol CO₂ m^-2 s^-1. Incorporating the influence of age improved the relationship, showing an impact of leaf age independent of SLN. The photosynthesis relationship developed should be appropriate for use in simulating cotton crops. The approach used to scale from leaf photosynthesis to canopy RUE was effective in capturing the variation in the RUE of cotton as observed in three field experiments. There appeared to be little impact of the ontogenetic changes in light extinction coefficient and vertical gradients of SLN within the canopy on the variation in RUE. For crop simulation purposes, this simplifies the process of modulating RUE in growth models.

Abbreviations: C_A, leaf CO₂ assimilation rate (µmol CO₂ m^-2 s^-1) • C_Adark, leaf CO₂ assimilation rate at PAR 0 µmol m^-2 s^-1 (i.e. the y-intercept of the light response curve) • C_Amax, leaf CO₂ assimilation rate measured at PAR 2000 µmol m^-2 s^-1 • DAS, days after sowing • k, canopy light extinction coefficient • LAI, leaf area index • LAI_cum, cumulative leaf area from the top of the canopy to the level of interest • LI, proportion of radiation intercepted by the crop canopy measured at midday • LI_D, proportion of light intercepted by the crop canopy over the day • PAR, photosynthetically active radiation (400–700 nm) • RUE, radiation use efficiency (g MJ^-1, glucose equivalents) • SLN, specific leaf nitrogen (g N m^-2 of leaf) • SLN_grad, vertical SLN gradient within the canopy

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