1. Introduction
Compared to other crops the potato crop is considered drought sensitive Salter and Goode,
1967; van Loon, 1981. Drought stress influences the development and growth of potato shoots,
roots and tubers. Drought stress also induces reduced leaf area and, in the long term, stem
height and ground coverage are lower Ojala et al., 1990. The effects of drought on a plant
depends on the timing, duration and severity of the stress Jefferies, 1995. Cavagnaro et al. 1971
showed that drought stress at the beginning of the tuberisation stage induced a longer period of tu-
ber formation but decreased tuber number, growth and yield. This effect on tuber number has
been confirmed by several authors MacKerron and Jefferies, 1986; Haverkort et al., 1991. Stud-
ies on the effect of drought on leaf senescence are scarce. In pot experiments, leaf senescence was
found to be somewhat delayed by nitrogen Vos and Biemond, 1992. Higher nitrogen supply in-
creased by 3 weeks the life spans of leaves in comparison with the control. In field trials, the
longevity of the canopy was found to be more strongly influenced by the continuation of new
leaf production than by differences in leaf longevity between cultivars, or levels of nitrogen
supply Firman et al., 1995. Individual leaf longevity can be influenced by cultivar and by
nitrogen but this seems mainly due to the effect on shading. Indeed, the interception of light by
the upper leaves alters the quality of the light penetrating to deeper levels Thomas and Stod-
dart, 1980. The photosynthetic activity of the lower leaves lasted longer in the absence of nitro-
gen than in the case of nitrogen application Fir- man and Allen, 1988. Thus, the light regime
experienced by the leaf has more effect on the duration of the photosynthetic capacity than leaf
age. The increase in tuber dry matter weight due to nitrogen application was achieved through ef-
fects on leaf area duration rather than on effi- ciency of conversion of incident radiation Firman
and Allen, 1988. However in the case of drought appearing around the middle of the season the
efficiency of conversion of incident radiation can be significantly reduced without affecting the cu-
mulated intercepted radiation Deblonde and Ledent, 2000.
Deblonde et al. 1999 found that, in the condi- tions of their experiments carbon isotope dis-
crimination D accounted for more or less 40 of the variation across the drought treatments of
tuber yield and drought tolerance index TDWS tolerance to a decrease in water supply defined as
tuber dry weight of the drought treatment relative to the tuber dry weight of the irrigated treatment.
D could therefore be very useful for multitraits breeding. As tuber yield cannot be assessed prop-
erly in the early stages of the election process Weber, 1984; Clarke et al., 1991, our main ob-
jective was to find morphological parameters or secondary characters which are correlated with
tuber yield. Indeed, 12 morphological parameters integrate during their growth the effects of
droughts. Measurements of the morphological parameters of plant parts which are growing dur-
ing a drought stress might show differential re- sponses of cultivars which in turn could be related
to tuber yield performance. In moderate drought conditions as encountered some years in sandy-
loamy soils in Belgium, our hypothesis was that cultivars which show less growth reduction for a
determined morphological parameter might also show less tuber yield reduction. In more severe
drought conditions, such an hypothesis might not hold, as for reaching the highest tuber yields it
might be more adequate for crops to save water until the next rain event. However, in the latter
case, problems with the quality of tubers second growth, growth cracks,… are expected to be
more frequent, a point which cannot be neglected.
This study examines whether stem height on six cultivars, number of remaining green leaves
and leaf length the latter two parameters were only measured on two cultivars are sensitive to
moderate drought conditions and related to TDWS. The knowledge of such relations would
allow insight on the behaviour of the whole plant when submitted to moderate water shortage and
possibly define sensitive indicators that could be used to discriminate between drought sensitive
and drought tolerant cultivars before final tuber yield is reached. The effect on tuber number and
average tuber dry weight was also investigated.
2. Material and methods
Kategori