Jean-François Ledent

Effects of moderate drought conditions on green leaf number, stem height, leaf length and tuber yield of potato cultivars

Our objective was to define sensitive morphological indicators that could be used to discriminate between drought sensitive and drought tolerant potato cultivars before final tuber yield is reached. A total of six potato cultivars contrasting for drought tolerance and earliness were subjected to three levels of water supply in two field trials in 1995 and 1996. We examined which parameter among stem height, number of green leaves and leaf length (the two latter parameters were only measured on two cultivars) was most sensitive to moderate drought conditions and responded similarly to water shortage regarding tuber yield, tuber number and average tuber dry weight. Drought reduced the number of green leaves in cultivars Krostar and Desiree, respectively, up to 22 and 25% 124 days after planting during the season but this effect levelled off towards the end of the season. Drought also induced significant length reductions (29, 43 and 53%. respectively, for the leaf numbers 17. 18 and 19) but there were no differences among the two cultivars examined. Stem height was sensitive to drought and the effect was stronger on later cultivars especially when water shortage started early as in 1996. In 1996, stem height ratios were negatively related (R-2 = 0.80) to TDWS (tuber yield ratio indicative of drought tolerance) whereas they were positively related in 1995 (R-2 = 0.21). However. care is needed as the negative relationship of 1996 was associated to earliness of cultivars. Indeed, stern height was less reduced in early cultivars (13 and 14%) for Eersteling and Jaerla, respectively) than in mid-late cultivars (25 and 29% for Desiree and Nicola, respectively). In the drought treatment, no supernumerary tuber was observed across the: season contrary to the irrigated treatment. According to the year, final tuber number or average tuber dry weight was reduced by the drought treatment. Reduction in tuber number (17%,) in the drought treatment was not associated to lower tuber yields due to compensation by average tuber dry weight (6%)). Different growth strategies can be followed by the potato plant to adapt to different drought conditions without tuber yield being significantly affected. (C)! 2001 Elsevier Science B.V. All rights reserved.

Effect of water stress on growth, Na+ and K+ accumulation and water use efficiency in relation to osmotic adjustment in two populations of Atriplex halimus L.

The effect of water stress on growth, Na+ and K+ accumulation and water utilization was investigated in plants of two populations of Atriplex halimus L. originating from Kairouan (Tunisia) and Tensift (Morocco). Water deficit was applied by withholding water for 22 days. All plants remained alive until the end of the treatment although growth was strongly reduced in both populations. Water stress decreased CO2 assimilation in saturating conditions, mainly in the population obtained from Kairouan, suggesting an impact of drought on the dark phase of photosynthesis, beside a decrease in stomatal conductance which was recorded mainly in the population obtained from Tensift. The two studied populations did not differ in their water consumption, as indicated by similar soil gravimetric water content and plant transpiration. However, water use efficiency increased under stress conditions in the population from Tensift but not in the population from Kairouan. Thelatter population displayed a larger capacity for osmotic adjustment. A drought-induced specific increase in Na+ concentration was also reported in both populations. It is concluded that in A. halimus, water stress resistance estimated in terms of biomass production, could be associated with higher WUE rather than with with a greater osmotic adjustment and that sodium may assume a specific physiological function in this xerohalophytic C4 species.

Responses of early and late potato cultivars to moderate drought conditions: agronomic parameters and carbon isotope discrimination

Our main objective was to test under moderate drought conditions the hypothesis that early cultivars behave better than late cultivars under conditions of drought occurring late in the season whereas late cultivars would be less susceptible to a drought occurring early in the season. Agronomic parameters [tuber dry weight, harvest index, dry matter content, tolerance to a decrease in water supply (TDWS) and carbon isotope discrimination (Delta)] were measured in six potato genotypes (Solanum tuberosum L.) subjected in two held trials (1995 and 1996) to three levels of water supply (treatments: 1, rainfall+irrigation; 2, rainfall only; 3, rainfall reduced to stemflow during early growth). In both years, treatment 3 (lower water supply) reduced significantly tuber dry yield and harvest index in comparison with treatment 1 (higher water supply). Tuber dry matter content was increased significantly only by treatment 3 in 1996. For these parameters cultivars did not differ in their response to the lower water regimes except in the case of tuber dry matter content during one year (1996). TDWS defined as tuber dry weight of treated plots relative to the tuber dry weight of treatment I allowed more discrimination among the behaviour of cultivars in case of moderate drought than yield and Delta. Our TDWS results confirm partly the hypothesis tested: in 1996, TDWS values of mid-tate cultivars were higher than those of early cultivars in the case of early drought (treatment 3), indicating that later cultivars support better a drought stress in the early part of the season. Differences among cultivars in the responses of Delta to the different water regimes could not be shown, and there was no difference between the values of Delta observed for early and late cultivars. In our moderate drought conditions an escape mechanism of earlier cultivars could not be confirmed through the consideration of the Delta values observed. The linear regression of Delta on TDWS gave a positive slope significantly different from zero (n=24) but the R-2 values were low (R-2=0.41)

Morph differences and honeybee morph preference in the distylous species Fagopyrum esculentum Moench

The relatively low reproductive success of buckwheat (Fagopyrum esculentum Moench, Polygonaceae) is poorly understood. The question arises as to whether this distylous species is pollen or resource limited. We investigated the reproductive biology of buckwheat under controlled conditions in growth rooms and in the field in central Belgium in order to determine whether floral morph and pollination events may affect its reproductive success. In controlled conditions, flowering phenology and flower morphology of the two floral morphs did not differ. However, thrum flowers produced larger and fewer pollen grains and secreted more nectar, with a higher proportion of sucrose, than pin flowers. In the field, thrum flowers were preferentially visited by honeybees, but fewer pollen grains were deposited on their stigmas. However, numbers of pollen tubes growing in styles, seed set, and seed weight did not differ between morphs. Seed set was low under field conditions and did not increase after hand cross‐pollinations, suggesting that there was no pollen limitation. These results indicate that factors other than floral morph or pollination events were governing female fertility in buckwheat.

ECOP: an economic model to assess the willow short rotation coppice global profitability in a case of small scale gasification pathway in Belgium

This paper presents a software package developed to assess the economic profitability of an original way to produce renewable energy: the small scale gasification of willow cultivated as short rotation coppice (SRC) in Belgium. The theoretical bases of the model (process hypotheses and economic indicators) are firstly presented together with the most relevant characteristics of the energy production route (SRC management and wood production, storage and conversion). A reference case is then defined which combines the most influencing parameters (reference interest rate, rotation length, subsidies, harvest mode, SRC yield, power of the electricity generator and annual production of electricity). A sensitivity analysis on these parameters highlighted that the project profitability, from the net present value point of view, is very sensitive to the reference interest rate, to the subsidies (of the conversion unit but probably also of any other kind of subsidies), to the SRC yield and to the generator power, all other parameters remaining constant. The rotation length has only a low influence, at least in the range of classic values (3 to 7 years). To harvest the wood in stems (with delayed chipping) seems also to be the most interesting option

Maize ear temperature

An experimental study of ear temperature was made in Morocco (June, 1997) and in Belgium (September, 1998). Measurements were made at mid-length of ear in the centre of cob and at different points in the middle of grains. The results showed that temperature in the middle of the grains: depends significantly on the position of grain around the ear axis, especially in Morocco, between 13 and 16 h (UT). The result obtained shoed that, because of the considerable ear thermal inertia and non-isotropic direction of the direct solar radiation, the average temperatures in the middle of the grain and in the centre of the cob were similar. The difference between grain and air temperature was shown to be clearly more affected by solar radiation than by wind speed. The protection of ear against direct solar radiation led to an average temperature reduction of about 2.1 degreesC in ear protected compared with normal ears. Also, the important role played by the canopy on ear temperature was underlined. A temperature difference of up to 8.5 degreesC was observed between the temperature of ears exposed to direct radiation and other (normal) ears. The effects of the differences observed between grain temperatures on both final weight grain and kernel growth rate were calculated using CERES-Maize functions. These effects were shown unlikely to affect drastically grain yield

Development and validation of model of heat diffusion in maize ear

To calculate the variations of ear temperature, a two-dimensional numerical model of heat transfer within the different parts of the ear was developed and tested. It is based on a heat conduction equation. The ear being considered as a cylinder form with a variable section consisting of three concentric layers: cob, kernels and husk. The heat transfer between two adjoining layers was assumed to be purely conductive and at the external boundary of the ear, free convective heat transfer was assumed. Temperature response curves obtained from cooling and heating experiments were compared with those obtained by the model developed and a simpler model treating the ear as a single cylindrical layer. The average difference between the observed and calculated temperatures was 0.4 and 1 degrees C, respectively for our model and the simple model (ear temperature varied between 5 and 25 degrees C). The delay (lag) of response of ear temperature to air temperature was clearly larger in the deeper layers (center of cob and grains) than just under the husk. The proposed model was used to determine the overall thermal properties of maize ears. Conductivity k*, heat capacity c* and thermal diffusivity a* of each layer within the ear were determined using the model fitting method i.e. their values were obtained fitting a model of heat diffusion in the ear by the least squares method. The values of k* and c* obtained for the kernel layer were respectively significantly higher and lower than those for husk layer

Thermal properties of maize ears and sugar beet roots

Abstract Thermal characteristics of plant tissues affect temperatures in the physiologically active zones. The buffering of variations of temperatures through better thermal characteristics could be favourable to growth and yield. Experimental values of conductivity (k), diffusivity (a) and its components are presented for harvested sugar beets, and maize ears, and the literature on the subject is surveyed. The absence of marked differences between maize and beets despite the large difference in their water content underlines the importance of the geometrical, and anatomical, properties of the tissues for heat exchange. The kernel layer in maize appears especially prone to cooling whereas the husk layer gives some protection.

Use of Haun's scale for monitoring development of wheat from flag leaf emergence to end of stem elongation

Hauns scale of foliar stages allows a continuous numerical expression of plant development until the complete emergence of the flag leaf. Our objective was to test in a contrasting set of cultivars (common and spelt wheats), locations (Belgium and Argentina). and water status the adequacy of an extension of Hauns scale to stages beyond flag leaf emergence based on observation on the flag sheath and peduncle. The four main post-foliar stages were identified as: 0, corresponding to flag sheath elongation; 1, to flag sheath increase in diameter (booting); 2, to ear emergence; and 3, to the extrusion of the peduncle above the flag leaf ligular zone. The extended scale allowed the monitoring of development during the 3-4 weeks following flag leaf emergence and was sensitive enough to reveal a differences at daily intervals. Plots of post-foliar stages versus-thermal time were roughly linear, little affected by treatments (cultivar, water status, location) or by discontinuities due to the initial and final value of the decimal part of the numerical codes in the intervals between mean stages.

The Effect of the Direction of Gravity On the Leaf Angle in Wheat (triticum-aestivum)

Summary Wheat seedlings were grown upside down or on a vertical wheel rotating slowly. Leaf angle in the first leaves changed considerably with time independently of the direction of gravity. Differences from the controls were mostly due to elastic deformation. Leaf angle changes appeared to be basically of a nastic nature.