WINTER-WHEAT LEAF PHOTOSYNTHESIS, STOMATAL CONDUCTANCE, AND LEAF NITROGEN CONCENTRATION DURING REPRODUCTIVE DEVELOPMENT

Leaf N concentration (LRN), stomatal conductance (g(5)), and grain-ass imilate 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 determi ne the effects of grain development on these declines. This field stud y was conducted (i) to monitor changes in CER, LRN, and g(5),during wi nter wheat reproductive development, (ii) to examine the relationships between the rate and duration of CER and the rate and duration of gai n fill, and (iii) to determine the effects of crop N status on the dec line in LRN and CER during grain fill. Two early (Andy and Gore) and t wo medium (Northrup King Coker 9803 and Northrup Ring Coker 9835) matu ring cultivars were grown,vith irrigation and two rates of spring-appl ied 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 te rmination of grain development, and the complete loss of green Leaf ar ea 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. Ra pid reductions in LRN and CER were found only after significant grain growth occurred, with the rate of decrease being relative to the culti var ranking found for inflorescence emergence date. Averaged over cult ivars 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 ha d little effect on the loss of LRN and CER until the latter portion of grain fill, when rapid reductions in these variables occurred. Increa sing the rate of spring-applied N did not significantly delay these de creases or extend the duration of grain fill.