Boron deficiency in wheat

Abstract

Boron deficiency in soils has been reported to affect more and more crops, especially so in the warmer environments. Two reasons come to mind: Either the problem has not been recognized in the past and the symptoms have not been attributed to other causes, or Intensification of cropping over the last few decades has accelerated depletion of existing soil boron. Probably both of these playa role. In a timely manner, the Australian Centre for International Agricultural Research (ACIAR) has supported research in boron deficiency of grain legumes at Chiang Mai University (CMU) in Thailand. More recently, the Australian International Development Assistance Bureau (AIDAB) has provided funds to the regional office ofthe International Maize and Wheat Improvement Center (CIMMYT) in Southeast Asia to cooperate with CMU as an expert institution and national research institutions in South and Southeast Asia to gain some more knowledge about the same problem in wheat. Part of this project was a small boron workshop conducted at Chiang Mai University, Feb. 17-19, 1992, the proceedings of which make up this Wheat Special Report. The various papers report a wide range of findings and knowledge currently available in South and Southeast Asia. Some results of the AIDAB-funded research are also included. Generally, micronutrient deficiency research requires special care as compared to toxicity work or macronutrient experimentation (Loneragan, page 1) because contamination occurs easily during the experimental process and tiny amounts can blur results. Problems also arise frequently due to the masking effect of other nutrient deficiencies in poor soils. The major symptom of boron deficiency in wheat is grain set failure. Wheat growing in low boron soils may develop malformed anthers and pollen grain; the external boron supply may also limit the germination of healthy pollen (Cheng and Rerkasem, page 5). Prediction of grain set failure from boron analysis of wheat tissues is complicated (Rerkasem and Lordkaew, page 9) because 1) boron is immobile inside the plant, and 2) grain set fails if boron availability is interrupted during a few crucial days at pollen development or fertilization. On the other hand, chemical analysis ofsoil or plant tissue boron does not require highly sophisticated equipment and can be done in many laborato~ies (Netsangtip and Lordkaew, page 15). Once boron deficiency is established through chemical analysis as the most probable factor causing sterility in a given environment, then other more simple methods can be employed for surveys or varietal screening. These methods could include boron probe nurseries (Rerkasem, page 21), measurements of anthers and pollen (Cheng et aI., page 32) or grain set counting as the simplest and most rapid method (Appendix 2, page 126). How widespread is the problem? Various soils are prone to boron deficiency (KhatriChhetri and Ghimire, page 34) and reports are available from many countries worldwide (see bibliography in Appendix 1, page 113). But despite numerous anecdotal observations of severe sterility problems in different years by researchers from national programs, nobody reports yield loss estimates on a national or even provincial level. This does not mean losses are minimal, but given the year to year changes of water status, which in turn controls boron availability, varietal differences, macronutrient fertilizer contamination with boron, and temperature at flowering plus the difficulty of determining boron as the cause of sterility make loss estimates of a sizeable area a formidable task. Most probably, minor sterility often goes unrecognized. Ifit is severe, it can also be attributed to drought, cold, or waterlogging (Subedi, page 57). It is unclear whether waterlogging alone can cause sterility (Misra et aI., page 65) or whether it is only an indirect factor reducing the availability and/or uptake of boron. Superimposed boron fertilizer experiments (Yang, page 72) may show one way to obtain better yield loss estimates, but due to the above mentioned interactions, fertilizer experiments are not always conclusive (Subedi). Due to all these problems, there is currently a lack of awareness and diagnosis as well as inadequate research for solutions. Varietal differences are reported by many authors from various soils based on experiments in farmers' fields (Yang), experiment stations (Tandon and Naqvi, page 76; Misra et aI.; and Subedi) or under controlled conditions in sand culture (Jamjod et aI., page 79; numerous additional references can be found in Appendix 1). First estimates of inheritance and heritability (Jamjod et aI., page 86) as well as mode of gene action (Jamjod et aI., page 83) are available. Nevertheless genetic knowledge is still far from what is known about boron toxicity (Paull et aI., page 90) although some inferences can be made. In the same way, physiological research of boron toxicity in wheat adds to the present knowledge of physiological processes in boron-deficient wheat plants (Nable, page 98). Much remains to be researched for a better understanding of the problems as well as for the most practical solutions. The workshop participants listed areas in which they would want to continue research on their own or in cooperation with the current project and beyond (See minutes of final discussion, page 110).