Response of upland rice varieties to drought stress: II. Screening rice varieties by means of variable moisture regimes along a toposequence

The effects of variable soil moisture regimes along a toposequence on root development, plant water status, and grain yield were investigated for ten rice varieties: (1) 63-83, (2) IB 43, (3) OS 6, (4) IB 6, (5) IR 1529-680-3, (6) C 22, (7) IRAT-13, (8) TOS 4680, (9) IET 1444 and (10) SE 302 G. The depth of water table below the soil surface in three regions along the toposequence was 100 (M1), 30 (M2) and 15 cm (M3). Soil moisture regime did not affect total root dry weight significantly. However, depth of root penetration did vary for the different moisture regimes and varieties investigated. The angle of root penetration was also affected by soil moisture regime. Roots of tall varieties (63-83, IB 6 and IRAT 13) penetrated deeper into the soil profile than short varieties. The pattern of soil moisture depletion was similar to that of root density profile. Relative water stress, expressed as a ratio of leaf water deficit under the M1 moisture regime to that under M3, increased exponentially with decrease in root density at a depth of 25 cm. The mean grain yield was 2.4, 1.9 and 1.4 t/ha for moisture regimes M3, M2 and M1, respectively. Although the grain yield of ten varieties exhibited normal distribution under an adequate soil moisture regime (M1), the distribution pattern changed under the adverse regime. Variety IB 6 and other tall varieties maintained stable yields under the adverse moisture regime. Grain yield was linearly related to root density at a depth of 25 cm

Pruning effect on nitrogen nutrient release in the root zone of Albizia lebbeck and Leucaena leucocephala

The effect of pruning on the dynamics of N release in the root zone of Albizia lebbeck and Leucaena leucocephala was studied using potted soil and minilysimeters with presterilized sand-medium supplied with Nfree nutrient solution. Plants were pruned twice at 11 and 13 months and leachates were collected weekly for 16 weeks starting from first pruning, and analysed for mineral N content. Removal of plant shoots reduced nodule and root biomass by some 30–38% and halved nodule N yields, while total N yields did not differ between pruned and unpruned plants. The dynamics of N nutrient in the rhizosphere was also affected by pruning, irrespective of the growth medium. In soil culture, unpruned plants of both species maintained greater levels of total N in their rhizosphere compared to those that were pruned. In sand culture, nitrate-N was by 66–84% the predominant N form in soil leachate across the two sequential prunings. Over the 16 weeks following these prunings of A. lebbeck and L. leucocephala, cumulative mineral N in both forms was significantly higher in the root zone of unpruned plants, and was consistently greater under rhizobial inoculation. Less mineral N was released into the root zone of each species during the period after the second pruning than after the first one. L. leucocephala released significantly more N than A. lebbeck over the sampling period, but net N release beneath both species was lower than 1%, indicating that tree legumes do not release sizeable proportions of their N into root zone, and that pruning reduces their N release still further

Perspective d'utilisation des termitières dans l'amélioration de la fertilité des sols tropicaux : cas d'une expérimentation en pots de végétation

Perspective of termitary mounds utilization in improvement of tropical soil fertility : case of a pot vegetation experiment. The fertilizing power offive termitary mounds of Yakonde soil serie (Y2) in Yangambi has been tested by rice growing in vegetation pots and compared with those of host soil 's and complex manure's. The obtained data have shown that termitary soils are more fertile than host soil, butless than complex manure. Among five forms of termitary mounds tested, "semi-arboricole" form proved to be efficient. Then, utilization of different forms of termitary soils as fertilizing materials in tropical regions appears possible, because of their wealth in nutrient elements for plants, abundance in tropical forests and easy get by tropical peasant

Water deficit and nitrogen nutrition of crops. A review

Among the environmental factors that can be modified by farmers, water and nitrogen are the main ones controlling plant growth. Irrigation and fertilizer application overcome this effect, if adequately used. Agriculture thus consumes about 85% of the total fresh water used worldwide. While only 18% of the world’s cultivated areas are devoted to irrigated agriculture, this total surface represents more than 45% of total agricultural production. These data highlight the importance of irrigated agriculture in a framework where the growing population demands greater food production. In addition, tighter water restrictions and competition with other sectors of society is increasing pressure to diminish the share of fresh water for irrigation, thus resulting in the decrease in water diverted for agriculture.The effect of water and nutrient application on yield has led to the overuse of these practices in the last decades. This misuse of irrigation and fertilizers is no longer sustainable, given the economic and environmental costs. Sustainable agriculture requires a correct balance between the agronomic, economic and environmental aspects of nutrient management. The major advances shown in this review are the following: (1) the measurement of the intensity of drought and N deficiency is a prerequisite for quantitative assessment of crop needs and management of both irrigation and fertilizer application. The N concentration of leaves exposed to direct irradiance allows both a reliable and high-resolution measurement of the status and the assessment of N nutrition at the plant level. (2) Two experiments on sunflower and on tall fescue are used to relate the changes in time and irrigation intensity to the crop N status, and to introduce the complex relationships between N demand and supply in crops. (3) Effects of water deficits on N demand are reviewed, pointing out the high sensitivity of N-rich organs versus the relative lesser sensitivity of organs that are poorer in N compounds. (4) The generally equal sensitivities of nitrifying and denitrifying microbes are likely to explain many conflicting results on the impact of water deficits on soil mineral N availability for crops. (5) The transpiration stream largely determines the availability of mineral N in the rhizosphere. This makes our poor estimate of root densities a major obstacle to any precise assessment of N availability in fertilized crops. (6) The mineral N fluxes in the xylem are generally reduced under water deficit and assimilation is generally known to be more sensitive to water scarcity. (7) High osmotic pressures are maintained during grain filling, which enables the plant to recycle large amounts of previously assimilated N. Its part in the total grain N yield is therefore generally higher under water deficits. (8) Most crop models currently used in agronomy use N and water efficiently but exhibit different views on their interaction