Historical Changes in Harvest Index and Crop Nitrogen Accumulation

Plant harvest index, the ratio of grain weight to total plant weight, is an important trait associated with the dramatic increases in crop yields that have occurred in the twentieth century. Harvest index reflects the partitioning of photosynthate between the grain and the vegetative plant and improvements in harvest index emphasize the importance of carbon allocation in grain production. The objective of this review is to examine from an historical perspective some of the changes that have occurred in crop harvest index and to consider the importance of crop nitrogen accumulations asociated with changing this trait. In modern times, harvest index has generally increased. Priort o the twentieth century,t here is evidencet hat plants elections also resulted in changes in harvest index. One factor that may have influenced these changes was the relative value of grain, compared with straw. Historically, straw production was a high priority, making low harvest index a desirable trait. Another factor wast he level of nitrogena vailabilef or the productionof highg rain yields. Accumulation of high levels of nitrogen is essential for high grain yields, and thus, high levels of nitrogen are commonly associated with crops having high harvest indices. Under conditions where nitrogen is limiting, a low harvest index crop is advantageous. Limited nitrogen can be partitioned into the low nitrogen concentration vegetative tissue, which results in high total production of plant mass. However, increasing grain yield and crop harvest index with high nitrogen grain requires a concomitant increase in crop nitrogen accumulation.