Growth form and seasonal variation in leaf gas exchange of Colophospermum mopane savanna trees in northwest Botswana

We investigated differences in physiological and morphological traits between the tall and short forms of mopane (Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard) trees growing near Maun, Botswana on a Kalahari sandveld overlying an impermeable calcrete duricrust. We sought to determine if differences between the two physiognomic types are attributable to the way they exploit available soil water. The tall form, which was located on deeper soil than the short form (5.5 versus 1.6 m), had a lower leaf:fine root biomass ratio (1:20 versus 1:6), but a similar leaf area index (0.9-1.0). Leaf nitrogen concentrations varied between 18 and 27 mg g(-1) and were about 20% higher in the tall form than in the short form. Maximum net assimilation rates (A sat) occurred during the rainy seasons (March-April 2000 and January-February 2001) and were similar in the tall and short forms (15-22 micromol m(-2) s(-1)) before declining to less than 10 micromol m(-2) s(-1) at the end of the rainy season in late April. As the dry season progressed, A sat, soil water content, predawn leaf water potential (Psi pd) and leaf nitrogen concentration declined rapidly. Before leaf abscission, Psi pd was more negative in the short form (-3.4 MPa) than in the tall form (-2.7 MPa) despite the greater availability of soil water beneath the short form trees. This difference appeared attributable to differences in root depth and density between the physiognomic types. Stomatal regulation of water use and carbon assimilation differed between years, with the tall form having a consistently more conservative water-use strategy as the dry season progressed than the short form.     

SRI as a methodology for raising crop and water productivity: productive adaptations in rice agronomy and irrigation water management

The System of Rice Intensification (SRI), developed in Madagascar almost 30 years ago, modifies certain practices for managing plants, soil, water, and nutrients with the effect of raising the productivity of the land, labor, and capital devoted to rice production. Certain production inputs are reduced—seeds, inorganic fertilizer, water, and fuel where water is pumped—with increased yield as a result. This paper introduces the subject of SRI, which is then addressed variously in the articles that follow. SRI is gaining interest and application in over 40 countries around the world. Its practices make soil conditions more aerobic and promote greater root growth, as well as larger, more diverse communities of beneficial soil biota. These below-ground changes support more productive phenotypes above-ground for practically all rice genotypes (cultivars) tested so far, with supportive evidence accumulating both from scientific institutions and field applications. SRI methodology remains controversial in some circles, however, because of the transformational change it introductions into traditional lowland rice production systems. This issue of PAWE brings together the results of formal research on SRI in a number of countries (Part I) and also reports on initiatives by government agencies, NGOs, universities, or the private sector, bringing knowledge of SRI to farmers in a wide range of agroecological circumstances (Part II). This introduction presents the basic principles that underlie SRI and discusses the nature of this innovation as well as considers some of the issues in contention. SRI continues to evolve and expand, being a work in progress. Its concepts and methods are being extended also to upland (rainfed) rice production, as well to other crops. Accordingly, SRI should not be regarded or evaluated in conventional terms as if it were a typical component technology. It is understood more appropriately in terms of a paradigm shift for rice production. In particular, it calls into question the long-standing belief that rice is best produced under continuously flooded conditions.     

Review of SRI modifications in rice crop and water management and research issues for making further improvements in agricultural and water productivity

Much of the focus of agricultural improvement efforts in recent decades has been on modifying crops’ genetic potential more than on improving cropping practices and production systems. Certainly, this genocentric approach has made significant contributions to food production in certain parts of the world under the banner of “the Green Revolution.” Yields have been raised substantially through varietal improvements and the increased use of inputs, including energy, agrochemicals, and delivering more water to crops through irrigation technology. In the past two decades, however, gains from this strategy have decelerated, with increasing economic and environmental costs of this input-dependent approach. Accordingly, there is reason to consider what can be accomplished by making optimizing changes in crops’ growing environments both above ground and, especially, below ground. The System of Rice Intensification (SRI) developed in Madagascar has been showing that, by modifying crop, soil, water and nutrient management, it can under most of the circumstances evaluated thus far raise of the productivity of land, water, seeds, capital, and labor used for irrigated rice production. This article summarizes and reflects on the evidence provided in the preceding articles in this special issue. It draws on the scientific evaluations and field experience from Asia, Africa, and Latin America to offer some conclusions about the methodology known as SRI. Since this methodology is still evolving, no final assessment is possible. Much more research and evaluation remain to be done, and there will be further modifications and refinements since making adaptations to local conditions is regarded as intrinsic to the methodology. Further improvements in SRI will come from both researchers and farmers, with the latter considered as partners rather than simply adopters. This is consistent with SRI’s representing a paradigm shift more than a fixed technology. The article identifies a number of areas for additional research that can probably improve factor productivity still further.