UNIVERSITE DE BOURGOGNE THÈSE Yongbo LIU - Université de ...

while ensuring that the same ratio of CP to NC existed for the eight plants in the center and
for the 16 pl ants at the border. This allowed us to compare performance at the two positions
(Figure 3.4). C enter pl ants e xperienced hi gher c ompetition w hile bor der pl ants experienced
lower competition. Houseflies, Musca domestica, were continuously provided inside the cages
to facilitate random pollination.
The 2009 experiment The experiment was replicated in April 2009, but this time five
or six B. juncea seeds were directly sown in each spot within the cage. After emergence, one
seedling per spot was randomly selected and the others were removed. In addition, we clipped
one out of three successive leaves instead of one out of two, because greenhouse experiments
indicated that this would reduce the plant damage to an acceptable intensity (see Liu et al.,
2009).
Measurements The num ber of l eaves o n t he s tem a nd t he da te when t he f irst f lower
opened w ere r ecorded for each p lant. W hen t he f irst co mpletely m ature s iliques w ere
observed (i.e. golden brown in color) the siliques were counted on every plant and then all
plants were cut at the base and left to dry at room temperature. The plants were threshed to
collect the seeds. Stems, leaves, and silique material (except seeds) were dried in an oven at
80°C f or 48 h, a nd f inal dr y weight w as m easured as p lant w eight. T he s eed n umber w as
counted and the seed weight per plant was measured. The number of seeds per silique was
calculated (seed number/ silique number). Biomass was calculated as plant weight plus seed
weight. A resource allocation i ndex ( reproductive i ndex) w as c alculated as t he r atio of t he
plant w eight t o t he s eed w eight. V iability of up t o 100 s eeds pe r pl ant w as t ested b y
germinating on m oist pa per i n a growth chamber ( 15°C dur ing 8h i n t he da rk, and 25°C
during 16h unde r artificial light). Germinated seedlings were checked once every three days
for a period of three weeks, and then trays were sprayed with 0.4 % gibberellic acid and put in
continuous darkness to break seed dormancy. The number of viable seeds was calculated as
the seed number multiplied by the germination rate.
Statistical analysis The mean value for all CP or NC plants in the center and at the border
of each plot was used for the statistic analysis of all measured traits. For seed weight, total
biomass, and seed number, a per-plot analysis was carried out using the sum of the data from
all C P o r N C p lants in the c enter o r all a t th e border. Bartlett's te st was u sed to te st f or
homogeneity of variance, and as a result the data were log transformed to ensure a N ormal
104