leaf lengths of different leaf numbers within a cultivar showed that leaves inserted higher had
shorter lengths than leaves of lower ranks whatever the water regime. This reduction of lengths of the
upper leaves was higher in the case of the drought treatment significant leaf number by treatment
interactions; F-test. This was observed in cultivar Krostar as well as in cultivar De´sire´e.
4
.
5
. Number of green lea6es The number of green leaves present on the main
stem decreased through dates of measurements from 108 days after planting to 157 days after
planting; Table 3. Cultivar Krostar showed a significantly higher number of green leaves than
De´sire´e only for one date 108 days after planting; P B 0.01, afterwards differences between the two
cultivars were not significant. The drought treat- ment reduced significantly the number of green
leaves at two dates 108 and 124 days after planting; P B 0.01 as did also the rainfed treatment for one
date 124 days after planting in comparison with the irrigated treatment. However, at the end of the
season, differences between the three treatments levelled off with a tendency of the drought treat-
ment to show a higher number of green leaves. This was especially so in cultivar Krostar. Cultivars
behaved similarly across treatments cultivar by treatment interactions were not significant.
4
.
6
. Tuber number The statistical analysis of tuber number mea-
sured at final harvest and the corresponding aver- age tuber dry weight are shown in Table 4. Cultivar
differences were observed for both parameters both years. Treatments differences were observed in
1995 for tuber number and for average tuber dry weight in 1996. In 1996, the average tuber number
was significantly P B 0.05 lower than in 1995 14.1 instead of 15.3, analysis over years whereas the
average tuber dry weight was significantly P B 0.01 higher 20.5 instead of 17.2 g. The higher
average tuber dry weight compensated largely the tuber number reduction observed that year. Fi-
nally, the average total dry weights per plant were similar 264.1 and 288.4 g per plant in 1995 and
1996, respectively. The statistical analysis indi- cated for both parameters that one or several
cultivars behaved differently across years. More- over, the effects of treatments on tuber number
were also different between years. Differences in the fore-season may account for these results as indi-
cated above.
5. Discussion
Stem height was sensitive to water supply, the drought treatment significantly reducing height. In
1996, the effect was less on earlier cultivars
Table 3 Number of green leaves present on the stem excluding leaves on lateral branches for two cultivars and three water regimes in 1996
a
DAP 124
108 157
Krostar De´sire´e
De´sire´e Krostar
De´sire´e Krostar
Treatment 12.5
9.1
a
11.1 Irrigated
9.3
a
4.9 5.5
a
Rainfed 9.3
11.0 8.5
a
4.9
a
5.3 7.8
b
7.5
b
8.7 7.0
b
Drought 5.8
10.7 5.6
a
Cultivar NS
NS NS
Treatment NS
NS NS
Cult.×treat. S.E.
b
0.47 1.06
0.25
a
Treatments followed by different letters are significantly different Scheffe’s test at the level a = 0.05.
b
Standard error of cultivar×treatment interactions.
P .M
.K .
Deblonde ,
J .F
. Ledent
Europ .
J .
Agronomy
14 2001
31 –
41
Table 4 Tuber number per plant and average tuber dry weight g m
− 2
measured at final harvest average of 16 plants in 1995 and 1996 for the three water treatments 1996
1995 Cultivars
Average tuber dry weight Tuber number
Tuber number Average tuber dry weight
Irrigated Rainfed
Drought Irrigated
Rainfed Drought
Irrigated Rainfed
Drought Irrigated
Rainfed Drought
12.0 16.2
14.1 14.7
16.3 15.1
14.2 13.4
12.9 16.9
19.7 16.6
Eersteling 22.9
18.5 22.7
6.8 5.9
6.0 42.1
38.0 34.0
Jaerla 11.2
10.8 9.2
25.2 –
– –
– –
17.7 –
17.0 Claustar
13.1 13.0
8.6 –
16.6 16.8
18.1 17.9
14.2 Bintje
13.2 –
– –
– –
13.1 19.8
16.9 15.6
16.7 15.2
12.9 15.6
Krostar 14.2
15.4 20.3
20.0 14.2
14.5 15.1
21.1 17.3
20.4 12.5
12.4 11.4
Nicola 18.6
19.3 16.1
25.0 11.1
13.8 11.9
25.4 20.4
21.4 19.9
23.6 De´sire´e
12.1 13.1
10.6 Mean
18.9a 15.4a
15.3a 14.1a
14.5a 21.8a
19.1b 17.9b
15.9a 12.8b
17.8a 16.4a
Analysis within years Average tuber dry weight
Average tuber dry weight Tuber number
Tuber number C
a
NS NS
T NS
NS C×T
NS 1.52
S.E.
b
1.16 1.43
1.03 Analysis over years
Average tuber dry weight Tuber number
Y C
NS T
NS NS
NS C×T
Y×C NS
Y×T NS
NS Y×C×T
a
C, cultivar; T, treatment; Y, year.
b
S.E., standard error of cultivar×treatment interactions.
reduction than on later cultivars. Indeed the reduction was 13 and 14 for Eersteling and
Jaerla, respectively, and 25 and 29 for De´sire´e and Nicola, respectively. This may be due to the
shorter vegetative cycle of the early cultivars and thus to the shorter duration of the possible
limitation
by water
shortage. This
escape mechanism was not observed in 1995. Indeed,
smaller stem height reductions were not observed in earlier cultivars that year because of the water
stored under the plastic sheet and the later start of the drought conditions. The cultivar by drought
interactions were mainly due to the cultivars of the two extreme earliness groups. Within an
earliness group, such an interaction was not found probably because of a lack of cultivars contrasted
for drought tolerance. Future studies should concentrate
on that
aspect to
ensure the
usefulness of
such parameter
in breeding
programmes. The relationship between stem height ratios and
TDWS drought tolerance index was positive in 1995 and negative in 1996. Stem height ratios
accounted for 80 and 20 of the variation of TDWS in the two cases, respectively. In 1996, the
determination coefficient increased up to 90 when cultivar De´sire´e was omitted. We believe
that the timing of the drought stress and the absence of terminal drought were the main factors
explaining the inconsistent relationships over years between these two criteria. The earlier
drought of 1996 had more effects on stem height than in 1995 but the stress was relieved 14 days
earlier in 1996. This allowed more recuperation in tuber dry weight production for later cultivars at
the end of the season as all necessary nutrients had not been used or washed off earlier and more
especially
nitrogen was
still available.
The negative relationship found in 1996 between stem
height ratios and TDWS was strongly related to the earliness of cultivars as early and late cultivars
were grouped at the two extremities of the regression line Fig. 2. This was not the case with
the positive relationship observed in 1995. Further research should focus on one group of earliness
with more cultivars to ascertain the existence of such either positive or negative relationship
between stem height ratios and TDWS and avoid the effects of differences in earliness. The high
determination coefficients emphasise the potential interest of stem height measurements for drought
tolerance assessment in early stages of the breeding process.
R
2
corresponding to the relationship between stem height and tuber dry weight showed the
limits of the use of stem height for prediction of yield behaviour in drought conditions. Indeed, in
the drought treatment, stem height was only related to tuber yield when drought was applied
very early in the season. In 1995, the absence of a relationship between height and tuber weight in
the case of the drought was more surprising but might be explained by the difference between the
season
starts: stem
growth in
the wetter
fore-season of 1995 was faster than in 1996 and once the water shortage affected the plants
around the middle of the 1995 season, near maximal stem heights had nearly been reached by
all except the later cultivars. Tuber yield however was affected by the drought treatment both years
and this was more marked in 1995 since the strength and the length of the middle season
water
shortage was
higher that
year see
Deblonde et al. 1999 for temperature and rainfall data. Tuber yield production of the
sensitive mid-early cultivar Krostar was more strongly affected than for the other cultivars. In
the case of tolerant cultivar De´sire´e, drought affected more strongly stem height than tuber
yield. Table 1 also shows that when drought was earlier as in 1996 stem heights reductions were
larger in later than in earlier cultivars. For tuber yield however the reverse was observed indicating
the tolerant mid-late cultivars could reduce strongly stem height without affecting markedly
final tuber yield. Identical results were obtained when the tuber weight measured at the same date
as stem height measurements was considered data not shown. In this case the possible bias
through the effects or after-effects of rewatering, which occurred some time before final tuber
harvest and which could have favoured more late cultivars than the early cultivars, was avoided. In
irrigated treatment, plants were near the optimum water conditions and stem height differences be-
tween cultivars should mainly be due to genetic differences. So, similar yield levels could be ob-
tained with different stem heights as was shown in 1996 and to a lesser extent in 1995 and no rela-
tionship was observed between the two characters. In drought conditions, a significant relationship
between stem height and tuber dry weight was observed especially when drought appeared early
in the season and lasted long enough to affect also tuber production. Stem growth was more affected
when drought appeared early.
Leaf lengths in the two cultivars measured in 1996 were significantly reduced by drought from
the 17th leaf number upwards 29, 43 and 53 reductions for the leaves 17, 18 and 19, respec-
tively whereas lengths of leaves of lower number were not significantly affected. The absence of
cultivar differences and cultivar by treatment in- teractions for leaf length might indicate that leaf
length was not a suitable discriminant parameter for water stress tolerance but care is needed as
this was based on two cultivars and 1 year only.
The number of green leaves present on the main stem was reduced up to 22 and 25 in cultivars
Krostar and De´sire´e, respectively, 124 days after planting by our drought treatment during the
season whereas towards the end of the season plots affected by drought had a tendency to show
similar or even higher numbers of green leaves than in the irrigated treatment. This confirms the
higher ground cover values observed in 1996 in the drought treatment at the end of the season
Deblonde and Ledent, 2000 and shows the ca- pacity of potato plants to compensate at the end
of the season for radiation which was not inter- cepted during the early part of the season. The
available nitrogen which was not consumed dur- ing the drought stress due to the dry conditions
became again available when rainfall resumed which in turn may have favoured the maintenance
of green leaves at the end of the season. In the 1996 field trial and for cultivar Krostar only,
leaves on lateral branches appeared at the basis of the stem at the end of the season visual observa-
tion and participated in the maintenance of ground cover. Future studies on this aspect
should be considered.
All three morphological parameters were sensi- tive to drought and participated to the reduction
of intercepted radiation at the crop level. The cultivar × treatment interaction observed for in-
tercepted radiation Deblonde and Ledent, 2000 might be partly related to the significant culti-
var × treatment interactions found for stem height in 1996 no interactions was observed in 1995 for
stem height. There was however no significant interaction for the number of green leaves and
leaf lengths, but this was measured on only two cultivars in 1996 which does not allow to draw a
clear conclusion. In 1996, a stronger reduction in stem height was observed in later cultivars, than
in earlier cultivars. However, it was essentially in the early cultivars Eersteling, Jaerla and Krostar
that intercepted radiation was reduced Deblonde and Ledent, 2000. Strong reductions of inter-
cepted radiation seem therefore not to be neces- sarily related to strong stem height reductions.
Further research should concentrate on leaf length and leaf senescence.
Number of tubers at final harvest showed that significant reductions in tuber number observed
across years on average − 1.2 tuber per plant were not always associated with tuber dry weight
reductions since they were accompanied by a sig- nificant increase of the average tuber dry weight
+ 3.3 g dry weight per tuber. The effect of drought on tuber yield components was not simi-
lar across years: in 1995 tuber number but not average tuber dry weight was significantly reduced
− 2.6 tubers per plant, whereas the reverse was true the following year + 3.9 g dry weight per
tuber. These results suggest two differing strate- gies under drought conditions but final tuber dry
weights were similar showing the adaptation ca- pacity of potato plants to different drought condi-
tions. The effect of drought on tuber number of the early cultivars can be explained by the condi-
tions in the fore-season. In the case of the mid- late cultivars besides the effect of the fore-season
the intensity of the water shortage in the middle of the season also influenced final tuber number
stronger effect in 1995 because of the stronger drought during the mid-season. The average tu-
ber dry weight depended on the number of tubers initiated but also on the intensity of the drought
experienced by the crop until the end of the tuber bulking period.
6. Conclusion
