Jr. Frederick, WINTER-WHEAT LEAF PHOTOSYNTHESIS, STOMATAL CONDUCTANCE, AND LEAF NITROGEN CONCENTRATION DURING REPRODUCTIVE DEVELOPMENT, Crop science, 37(6), 1997, pp. 1819-1826
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.