Harvest index and remobilization of 13 elements during wheat grain filling: Experiences from ozone experiments in China and Sweden

Wheat efficiently remobilize and allocate a large fraction of aboveground biomass and nutrients into their grains during maturation. This senescence process has been streamlined through crop breeding, which lead to increasing harvest index (HI) for biomass. With field data from two ozone exposure experiments, we derived HI for 13 elements, both nutrients and non-essential, to determine how efficiently they are allocated into the wheat grain in two different agro-ecological environments (Sweden and China) and under different ozone exposure regimes. Element HI ranged from 10 to 90 %, with highest rates for P, N and Zn (90 %, 80 % and 70 %, respectively), while HI was low for the non-mobile elements Ba, Sr and Ca (<10 %). HI for biomass was about 50 %, and the non-essential and toxic element Cd was in the same range (similar to 40 %). Overall element HI was very similar in Chinese and Swedish wheat cultivars. This was also the case when comparing the two Chinese genotypes. We conclude that element HI for wheat crops are highly element specific and not strongly dependent on site or cultivar. Ozone exposure significantly reduced HI for both macmnutrients (Ca, K, Mg, N, P) and micronutrients (Cu, Mn, Mo, Zn), but also for Cd, while there was no ozone effect on the total aboveground pool for any element except P and Ba. Consequently, the reduction in grain element yield induced by elevated ozone, observed in previous studies, can be explained by lower remobilization rates rather than reduced total uptake. Our results provide new insights of nutrient use efficiency in wheat crops in general and under ozone exposure, which can be implemented in crop modelling and also useful for breeding strategies aiming to improve the nutritional value of food crops.