Tp. Saulnier et Eg. Reekie, EFFECT OF REPRODUCTION ON NITROGEN ALLOCATION AND CARBON GAIN IN OENOTHERA-BIENNIS, Journal of Ecology, 83(1), 1995, pp. 23-29
1 Reproduction in Oenothera biennis has been shown to decrease growth
in young plants, whereas reproduction in older plants temporarily incr
eases growth and has no negative effect on growth in the long term. Th
e causes of these variable effects were investigated by examining the
effect of reproduction upon photosynthetic rate, leaf area production,
chlorophyll content and nitrogen allocation in young versus old plant
s grown at low versus high nutrient availability. 2 Reproduction was c
ontrolled experimentally by gibberellic acid applications, and measure
ments were made at three developmental stages: bolting, flowering, and
capsule maturation. At each stage, measurements were also made on cor
responding vegetative plants of the same age. 3 Reproduction decreased
nitrogen allocation to roots and increased allocation to shoots. The
decrease in root allocation was greater at low nutrient availability.
Reproduction increased leaf area and, at bolting, the magnitude of thi
s increase was greater in plants grown at high nutrient availability.
Reproduction generally decreased photosynthetic rates, chlorophyll con
tent and nitrogen content of leaves. The magnitude of the decreases wa
s usually less for plants grown at high nutrient availability. Photosy
nthetic rate increased with reproduction for older plants grown at hig
h nutrient availability in the latter part of the experiment. 4 We sug
gest that differences among Oenothera biennis individuals in the effec
t of reproduction on carbon gain are related to differences in extent
of nutrient reserves. Older plants and plants grown at high nutrient a
vailability have greater nutrient reserves upon which to draw when rep
roduction is initiated. Reproduction in younger plants grown at lower
nutrient availability will rapidly deplete nutrient reserves and nutri
ents which are part of the photosynthetic apparatus (e.g. the nitrogen
within the chlorophyll molecule) will have to be mobilized to supply
reproductive structures. Reproduction in this latter case will therefo
re have more of a detrimental effect on photosynthetic rate and leaf a
rea production.