2. Material and methods
Field trials were conducted on potato crops grown on fine silt in Nodebais Belgium in 1995
and 1996, full details of which are given in De- blonde et al. 1999. Briefly, the trials comprised
six contrasting European cultivars five of which were common reported to differ in drought resis-
tance and earliness. We used two early cultivars Eersteling, sensitive; and Jaerla, tolerant, two
mid-early cultivars Krostar Eersteling, sensitive, both years; Claustar, intermediate, in 1995; and
Bintje, tolerant, in 1996 and two mid-late culti- vars Nicola and De´sire´e, both tolerant. A strip-
plot design with two factors cultivar and water status and four replicates was used in both years.
Plot size was 3 × 9 m and three water supply treatments were applied irrigated, rainfed and
drought. Row distance was 75 cm, and the dis- tance between plants was 32 cm. Field trials were
planted on the 15th and the 12th of April in 1995 and 1996, respectively. The irrigated treatment
received water from rainfall plus a limited amount of irrigation 37 and 35 mm, respectively, in 1995
and 1996. The rainfed treatment received water from rainfall only. The drought treatment was
imposed by placing a strong plastic sheet on the soil surface of the plots from 50 emergence to
8 weeks after thereby excluding 85 mm 1995 and 57 mm 1996 of rainfall. The irrigated, rain-
fed and drought correspond to treatments 1, 2 and 3 in Deblonde et al. 1999.
3. Measurements
The number of green leaves was measured at 108, 124 and 157 days after planting in 1996 on
three consecutive plants in each plot for Krostar and De´sire´e. When a leaf could not be considered
as completely green, visual estimates of green leaf area were made on a scale of 25, 50, 75 and 100.
A yellow leaf was counted as fully senescent. Only leaves on the main stem were recorded.
Leaf lengths, from the 14th to the 19th leaf on the main axis, were measured in 1996 on two
cultivars Krostar and De´sire´e 133 days after planting. According to Vos and Biemond 1992
leaf numbers 10 – 18 on the main stem correspond to the leaves of larger size.
Stem height distance from the soil surface up to the insertion of the petiole of ‘uppermost’ leaf
was measured twice in 1995 101 and 115 days after planting and once in 1996 109 days after
planting on all cultivars. We define the ‘upper- most’ leaf as the leaf attached on the top of the
uppermost internode characterised by a length superior to 3 mm. Stems were upheld vertically
during the measurements. Statistical analysis was based on F-test and contrast tested according to
Scheffe’s test.
Tuber dry weight was determined on two plants lifted fortnightly from the central ridges in each
plot with the aid of a spade. The first harvest started 60 and 67 days after planting in 1995 and
1996, respectively. Thus a total of 14 early culti- vars or 16 late cultivars plants altogether were
harvested to assess the evolution of tuber weight. Four plants per plot were lifted at final harvest.
Tubers were separated from the rest of the plant and weighed in the field. The number of tubers
per plot was determined tubers smaller than 10 mm were not considered. Other details of tuber
sampling and harvest are given in Deblonde et al. 1999.
We investigated if a relationship could be estab- lished between stem height and final tuber dry
weight across cultivars.
4. Results
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