Potassium efficiency of safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.)

The oil crop safflower may have a certain production potential under low-input conditions (organic farming, developing countries), where the putatively low nutrient requirement is highly welcomed. However, current knowledge regarding the nutrient use efficiency of safflower as compared to similar oil crops is limited. It was thus the aim of this study to determine the potassium (K) use efficiency of safflower (Carthamus tinctorius L.) as compared to sunflower (Helianthus annuus L.). Safflower and sunflower were cultivated with increasing K supply in a mixture of equal volumes of sand, nutrient-poor limed soil, and perlite in 5 L Mitscherlich pots. Both species responded strongly to increasing K supply with respect to growth and yield. Safflower out-yielded sunflower at low K supply, while at high K level, the opposite was observed. Both species accumulated similar amounts of K in shoots at low K supply. Only at extremely low K supply, safflower took up more K than sunflower. However, achene yield of sunflower exceeded that of safflower at optimal and high K supply. Safflower utilized absorbed K more efficiently than sunflower to produce achene yield at suboptimal K supply in terms of both efficiency ratio and utilization index. The efficiency of a crop to use supplied or accumulated K for dry-matter and achene production was interpreted in terms of Michaelis-Menten kinetics, specifically addressing the shape of the yield response curve. Indeed, the efficiency of safflower to use K for growth and yield, analogue to a low K-m in enzyme kinetics, was higher than in sunflower, while the K supply or K accumulation required to initiate yield formation in safflower was significantly lower. Similarly, safflower had a lower external K requirement for achene yield than sunflower at low and optimal K supplies. It can be concluded that safflower represents a low-input crop and outperforms sunflower on soils low in available K. The data analysis also reveals that using just one efficiency indicator is usually not sufficient to adequately describe the K efficiency of the crop under consideration.