Safflower yield, chlorophyll content, photosynthesis, and water use efficiency response to nitrogen fertilization under rainfed conditions
Introduction
Safflower is an oilseed crop which is grown throughout the semiarid region of the temperate climates in many areas of the world for use as vegetable and industrial oils, spices, and birdfeed (Weiss, 2000, Johnston et al., 2002). However, these regions are dominated by winter wheat (Triticum aestivum L. and Triticum turgidum sub. durum) and the acceptance and production of another crop requires that both have an important agronomic benefit to the cropping system and also improves the farmers’ economic position (Johnston et al., 2002). Given that most oilseed crops have an indeterminate growth habit, adaptation is influenced by tolerance to high temperature and drought stress and by crop management such as nitrogen (N) fertilization to take advantage of optimum environmental conditions for flowering and seed fill (Kaffka and Kearney, 1998, Weiss, 2000).
The importance of oil crops such as safflower has increased in recent years, especially with the interest in the production of biofuels. Safflower is a crop species which is well adapted to dry land conditions (Kaffka and Kearney, 1998, Weiss, 2000). Moreover, safflower is a deep-rooted annual crop which has the ability to meet its water requirements by exploring a larger volume of soil than most crops (Knowles, 1958). N fertilization management is important to optimize production and oil content of safflower (Kaffka and Kearney, 1998, Bassil et al., 2002). However, research on the effect of N fertilization on safflower yield, and physiology under rainfed conditions is limited.
Nitrogen fertilization of safflower resulted in different responses as observed by Nasr et al. (1978). Those authors found that 75 kg N ha−1 was adequate for optimum seed, oil, and protein yield. While higher N rates (up to 600 kg N ha−1) did not improve yield and yield components. Also, Gilbert and Tucker (1967) reported that up to 168 kg N ha−1 increased yield by increasing the number of heads per plant. Jones and Tucker (1968) found that high rates of N were effective in increasing seed yield when irrigation was included. Haby et al. (1982) found that 275 kg of N ha−1 from applied fertilizer, and expected mineralization from organic matter, was sufficient for higher yields of irrigated safflower. However, in other studies there was no yield increase with N fertilization (Strasil and Vorlicek, 2002). In California it is recommended 110–170 kg N ha−1 in irrigated safflower (Werkhoven et al., 1968, Kaffka and Kearney, 1998). The discrepancy found in the fertilizer N requirement for safflower may be related to the residual soil N, different cultivars that were used, different climatic conditions, and the effect of irrigation on crop growth and response to N fertilization. Varying N fertilizer recommendations have been proposed for irrigated safflower but data are unavailable for the N requirements of safflower for high yields under rainfed conditions.
Nitrogen is one of the most important nutrients for crop production because it affects dry matter production by influencing leaf area development and maintenance as well as photosynthetic efficiency. Under N deficiency, growth is inhibited and shoot–root ratio is decreased (Steer and Harrigan, 1986). In addition, N deficiency reduces the radiation interception, radiation use efficiency, dry matter partitioning to reproductive organs, leaf area index, protein content of the plant, and the seed (Marschner, 1995). Moreover, N deficiency delays both vegetative and reproductive phenological development, reduces leaf emergence rate, yield, and yield components such as the number of heads per plant, the number of seeds per head, the single seed weight, and the number of seeds per plant (Gilbert and Tucker, 1967, Jones and Tucker, 1968, Steer and Harrigan, 1986).
Nitrogen level affects the physiology of crop plants, however, there are no reports, to our knowledge, on the effect of N on photosynthesis of safflower. High N fertilization can increase photosynthesis and plant growth on several crops (Evans, 1989, Huber et al., 1989, Cechin and Fumis, 2004). Up to 75% of leaf N is found in chloroplasts and most of it is part of ribulose bisphosphate carboxylase (Rubisco) alone. Under low N level the lower photosynthesis is often attributed to the reduction in chlorophyll content and Rubisco activity (Evans and Terashima, 1987, Fredeen et al., 1991). In some species (wheat (Triticum aestivum L.), beans (Phaseulus vulgaris L.)) stomatal conductance (gs) is increased with N (Shimshi, 1970, Shangguan, 1997). In others such as cotton (Gossypium hirsutum L.) there was a negative response (Radin et al., 1988). Sugiharto et al. (1990) found positive correlation between the photosynthetic capacity of leaves and leaf nitrogen concentration.
Water use efficiency (WUE) indicates the performance of a crop which is grown under any environmental constraint (Howell, 2001). Crop management practices such as N fertilization can influence both the amount of water extracted by a crop and crop growth, and consequently can affect WUE. Optimal N enhances rooting depth as well as total root mass, hence, alleviating drought effects where deep sub-soil moisture is present provided there is water deficiency in the soil and there are no physical constraints to rooting. However, the effects of N on crop water use are expected to vary with the availability of soil moisture. In addition, it is not known how N fertilization can affect WUE by safflower under rainfed conditions. Under rainfed and dry conditions N fertilization is not as effective in increasing crop yields as in irrigated crops because the plants cannot take up N from the soil (Marschner, 1995, Fageria and Baligar, 2005).
The effect of N fertilization on safflower has not been extensively studied especially under rainfed conditions. In addition, there is not enough information about the effect of the application of different levels of N on yield, yield components, seed quality characteristics, chlorophyll content, gas exchange characteristics, and instantaneous water use efficiency (WUE). The main hypothesis that was tested was whether N fertilization can affect yield, yield components, chlorophyll content, gas exchange characteristics, and instantaneous WUE of safflower under rainfed conditions.
Section snippets
Experimental site and experimental design
The experiment was carried out at the experimental farm of the Aristotle University of Thessaloniki, Greece during the 2003–2004 and 2004–2005 growing seasons. The farm is located in Northern Greece (22°59′6.17″E, 40°32′9.32″N). Two different hybrids were used CW9048 and CW9050, which are high yielding hybrids with high oil content. The soil type where the experiment took place was a calcareous sandy loam (Entisols, Orthents, Typic Xerorthent) with wheat (Triticum turgidum subsp. durum L.) as
Results and discussion
The weather conditions during the growing season in 2004 and 2005 were quite different (Table 2). Above normal temperature and rainfall occurred in the 2004 growing season, whereas normal temperature and below normal rainfall occurred in 2005. Therefore, there were differences in the response of the two hybrids to N fertilization during the 2 years of the study which was probably because of the different weather conditions.
The soil type was sandy loam (Sl) and it contained all the necessary
Conclusion
Nitrogen is one of the most important nutrients needed for plant growth and development. We studied whether N level can affect yield, yield components, seed quality characteristics, chlorophyll content, photosynthesis, and WUE under rainfed conditions. N fertilization increased seed yield by an average of 19% and the yield components (such as the number of heads per plant, seed weight per plant, seed weight per head) were increased with N application compared with the control. Seed oil content
Acknowledgements
The authors are grateful to Mr. Arthur Hill (Safftech, USA) for providing safflower seeds for the experiments. We would like to thank two anonymous reviewers and the Editor Dr. Ray for their critical review and constructive suggestions which resulted in a great improvement of the quality of the paper. Also the authors are grateful to Professors N. Fotiadis and A. Gagianas, Faculty of Agriculture, Aristotle University of Thessaloniki, for their critical review of the manuscript.
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