SATrends Issue 61
December 2005
  • Transpiration tells tales
  • Stretching the soil sampling
  • Priorities for pest management
  • 1. Transpiration tells tales

    Transpiration efficiency (TE) is one of the three major determinants of the yield architecture, along with transpiration (T) and harvest index (HI), resulting in:

    Yield = T x TE x HI

    Where yield of a crop depends on: water the crop can capture for transpiration (T); how this water is exchanged for CO2 assimilation and biomass production through the stomata (TE); and how the biomass is converted into grain (HI).

    Transpiration efficiency (g biomass produced / kg of water transpired) is an important characteristic to improve crop productivity, especially under limited water conditions. Estimating TE in pot experiments is relatively easy because total water consumption can be measured by regular weighing of pots. Transpiration is then computed as the ratio of the biomass increase during a given period of time, divided by the total amount of water used by transpiration during the same time. Under field conditions it is difficult to obtain accurate values unless a large-scale lysimeter is installed. Also, the ratio of total moisture available in the soil divided by the grain yield is an indicator, but is less accurate, because water is also lost through evaporation.

    The carbon isotope discrimination method is used to estimate the water use efficiency of the crop in field conditions. In theory, a high TE is obtained when the CO2 concentration in the stomatal chamber is maintained at low levels, which happens either when the photosynthetic activity is high, or when the conductance of stomata is low. Enzymes involved in photosynthesis discriminate 12CO2 against its stable isotope 13CO2. However, under conditions of high photosynthetic activity or low stomatal conductance, a different proportion of 13CO2 is used in the photosynthetic process. Therefore, the differences in the proportion of 13C in the biomass can be used to indirectly estimate TE. This carbon isotope discrimination method has been applied in several crops, particularly grain legumes, to estimate the TE. A breeding program for high TE groundnut varieties is ongoing, and preliminary evidence shows that genotypic variations exist for TE in chickpea.

    Thanks to a collaboration with the Japan International Research Center for Agricultural Sciences (JIRCAS), we are investigating TE in chickpea through the carbon isotope discrimination method. In pot experiments, a significant and typical negative correlation was observed between the delta13C ratio and TE, in agreement with the theory. This opens a novel avenue for applying the technique to screen large numbers of chickpea germplasm accessions for high TE. Currently, field grown chickpea samples are being analyzed at JIRCAS, and the entire chickpea mini-core collection (211 accessions), are cultivated at ICRISAT in the field. We will evaluate the diversity on TE through leaf samples.

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    2. Stretching the soil sampling

    Soil sampling is a prerequisite for soil testing. Soil samples represent the “population” of a plot, field or a watershed, from which the nutrient status of the soil sample is determined. The most important factor in sampling is soil heterogeneity. In a relatively homogenous group of fields or plots, a small number of samples could represent the population, whereas heterogeneous fields require more samples. So far little attention has been devoted to a methodology for soil sampling on a watershed scale. ICRISAT is working towards standardizing the methodology for soil sampling in a watershed.

    The Appayapally watershed in the Mahabubnagar district, Andhra Pradesh state in India has an area of 500 ha, in which 217 farmers own land. The soils are red or red mixed types. Most of the farmers grow sorghum, castor, maize, groundnut, rice, sunflower and vegetables.

    A total of 114 soil samples were collected from the top 15 cm layer to represent the entire watershed. Each sample was a composite of 7-8 cores, randomly collected from the area represented by a crop and group of farmers. The samples were air-dried and ground fine enough to pass a 2 mm sieve before analysis for pH, electrical conductivity (EC), organic carbon (C), total nitrogen (N), available phosphorus (P, Olsen) exchangeable potassium (K), calcium (Ca), magnesium (Mg) and sodium (Na) and available sulfur (calcium chloride extractable) and DTPA extractable zinc (Zn), iron (Fe), manganese (Mn) and copper (Cu) and hot water extractable boron (B) following standard methods.

    Soil sampling tube in a
    chickpea field.
    Soil sampling tube in a chickpea field

    Using a computer-generated program, we generated sub-samples representing 5%, 10%, 15%, 20%….. up to 100% of the complete sample population. Statistical analysis was done to develop a soil sampling strategy, considering various soil chemical characteristics, that is representative and at the same time cost effective.

    The results showed that the mean or median values of pH, EC, organic C, total N, Olsen-P, exchangeable K, Ca, Mg and extractable S, Zn, Mn and Cu did not differ significantly when sub-sample size varied from 5 to 100% of the population. The uniformity may be due to lack of use of fertilizers and other inputs that create variability. The soil pH in the watershed was in the near neutral range and EC was low (no salt-related problems).

    Our results indicate that the dry lands in the Appayapally watershed are uniform in the chemical fertility parameters studied and even a small sub-sample can represent the whole population. However, such a sampling strategy may be applicable only to watersheds that are very gently sloping and where fertilizer use is very low, resulting in overall low fertility in the whole watershed.

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    3. Priorities for pest management

    Insect pests and diseases are serious constraints of production, productivity, and utilization of ICRISAT mandate crops (sorghum, pearl millet, chickpea, pigeonpea, and groundnut) in the semi-arid tropics (SAT). Crop losses due to these constraints have been estimated at over US$ 7.4 billion annually. While control of the pod borer Helicoverpa is heavily based on insecticides, chemical control of shoot and panicle feeding insects on cereals is beyond the reach of poor farmers in the SAT regions in Asia, Africa, and Latin America.

    Current sensitivities about environmental pollution are a consequence of improper synthetic pesticide use. Host-plant resistance, natural plant products, bio-pesticides, natural enemies, and agronomic practices offer a potentially viable option for integrated pest management (IPM). They are relatively safe for the non-target organisms and humans. Biotechnological tools such as marker assisted selection, genetic engineering, and wide hybridization to develop resistant crop cultivars will have a great bearing on future pest management programs. Insect and disease modeling, decision support systems, and remote sensing would contribute to scaling up and dissemination of IPM technologies.

    Shoot fly damaged crop in foreground. Shoot fly damaged crop in foreground.

    Current research projects in biotechnology, crop improvement, and natural resource management focus on the major pests such as pod borers (Helicoverpa, Maruca, and Melanagromyza), Fusarium wilt, and sterility mosaic in pigeonpea; Helicoverpa, Wilt, Ascochyta, and Botrytis gray mold in chickpea; Rossette virus, foliar diseases, Aflatoxins, and leaf miner in groundnut; Striga, grain molds, shoot fly, stem borers, midge, and head bugs in sorghum; and downy mildew, stem borer and head miner in pearl millet. IPM promotion and capacity building are significant components of research at ICRISAT. The current and future areas of research on insect pests and diseases in biotechnology, crop improvement, and crop management have been outlined below.

    • Marker-assisted selection for resistance to insect pests and diseases
    • Exploitation of wild relatives of crops for resistance to insect pests and diseases
    • Genetic engineering of crop plants for resistance to insect pests and diseases
    • Characterization and diagnosis of plant pathogens and insect pests, and bio-safety of transgenic crops to the environment
    • Host Plant Resistance, Cultural, Biological, and Chemical Approaches for Pest Management
    • Introgression of genes for resistance to insect pests and diseases into high yielding varieties and hybrid parents
    • Strategic research to improve the efficiency of genetic enhancement including identification of traits associated with resistance, and refinement of screening techniques
    • Integrate IPM components and validate their effectiveness for insect pest and disease management.

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