Winter wheat leaf photosynthesis, stomatal conductance, and leaf nitrogen concentration during reproductive development

Leaf N concentration (LRN), stomatal conductance (g(5)), and grain-assimilate demand each affect the leaf CO2-exchange rate (CER) of winter wheat (Triticum aestivum L.) during grain fill. Although leaf CER, g(5) end LRN decrease during this time,little research has been conducted to examine whether these decreases occur simultaneously or to determine the effects of grain development on these declines. This field study was conducted (i) to monitor changes in CER, LRN, and g(5),during winter wheat reproductive development, (ii) to examine the relationships between the rate and duration of CER and the rate and duration of gain fill, and (iii) to determine the effects of crop N status on the decline in LRN and CER during grain fill. Two early (Andy and Gore) and two medium (Northrup King Coker 9803 and Northrup Ring Coker 9835) maturing cultivars were grown,vith irrigation and two rates of spring-applied N (56 and 112 kg N ha(-1)) in 1994 and 1995. Cultivar ranking was similar for the date of inflorescence emergence, the initiation and termination of grain development, and the complete loss of green Leaf area and flag-leaf photosynthetic activity. For all cultivars, Bag-leaf CER and g began to decrease near the beginning of grain fill, whereas LRN and Leaf area index (LAI) began to decrease prior to gain fill. Rapid reductions in LRN and CER were found only after significant grain growth occurred, with the rate of decrease being relative to the cultivar ranking found for inflorescence emergence date. Averaged over cultivars and years, increasing the rate of spring-applied N increased LAI and LRN near inflorescence emergence by 37 and 24%, respectively, but only delayed the complete loss of green leaf area by 2 to 3 d and had little effect on the duration of grain fill. Stage of grain growth had little effect on the loss of LRN and CER until the latter portion of grain fill, when rapid reductions in these variables occurred. Increasing the rate of spring-applied N did not significantly delay these decreases or extend the duration of grain fill.