Physiological Causes of Differences in Grain Yield between Varieties of Barley

In a field experiment on barley at Rothamsted with the highmean yield of 49 cwt. of grain per acre, the varieties Proctorand Herta produced 10—15 per cent, more grain than Plumage-Archeron plots that received no nitrogenous fertilizer. When nitrogenwas applied the difference was increased to about 30 per cent.,because the higher nitrogen supply caused the Plumage-Archercrop to lodge and did not increase its yield, while Proctorand Herta remained standing. The three varieties did not differ in leaf-area index nor innet assimilation rate before ear emergence, so that all hadthe same total dry weight. After ear emergence, the leaf-areaindices of Proctor and Plumage-Archer were nearly equal, butthat of Herta was smaller. Assuming that the photosyntheticefficiency of the leaves continued to be the same in all varieties,the higher grain yields of Proctor and Herta cannot be attributedto greater production of dry matter by the leaves, either beforeor after ear emergence. A pot experiment on plants with shadedears confirmed that the dry matter contributed to grain yieldby unit leaf area was nearly equal in all the varieties. The higher grain yield of Proctor and Herta than of Plumage-Archermust therefore have come from additional photosynthesis in partsof the plant other than the leaves, i.e. in the ears themselves.An attempt to demonstrate this directly in a pot experiment,by comparing the grain yields of plants with shaded or withunshaded ears, was unsuccessful because the varieties behaveddifferently in pots; Proctor and Herta produced only about 6per cent, more grain yield than Plumage-Archer, and though thedecrease in grain yield by shading the ears was slightly greaterfor Proctor and Herta, the differences were not significant. The sum of ear sizes (estimated from length and breadth measurements)per m.1 in the field experiment was greater for Proctor andHerta than for Plumage-Archer. Also the distribution of drymatter between developing ears and shoots apparently differedwith variety, so that at ear emergence the dry weight of earsper m.² was greater in the two higher yielding varieties. Theincreased amount of photosynthetic tissue in the ears of Proctorand Herta, as measured by size or weight, may not wholly explaintheir greater dry-matter production; ears of Herta may alsohave a higher photosynthetic efficiency. No differences in nutrient uptake that could account for thevarietal differences in grain yield were found. Plumage-Archerabsorbed more potassium, and Herta less phosphorus than theother varieties. About a quarter of the final content of nitrogen,and a third of the phosphorus, was absorbed after ear emergence,but the potassium content was nearly maximal at ear emergenceand later decreased. The pot experiment showed that, on the average of all varieties,26 per cent. of the dry matter in the grain at harvest originatedfrom photosynthesis in the ears, including 10 per cent, fromthe awns; 59 per cent, came from photosynthesis in the flag-leaflamina and sheath and peduncle, and 15 per cent, from partsof the shoot below the flag leaf.