Studies in Physiological Analysis of Yield. IX, Effect of Light Intensity on the Development of the Wheat Grain

Under average tight intensities, 57,800 (sunlight, L), 12,900 (l₁), 3900 (1₂) and 3300 (1₃) lux, during the fortnight after anthesis, the increase in the average shoot weight (excluding roots) of wheat cultivars, NP 823 and NP 824, varied as 100, 43, 24 and 10, and of cultivars, Ph C 281 and NP720, as 100, 35, 15 and 9; the corresponding increase in weight of ear varied as 100, 59, 45 and 40, in the first group and as 100, 58, 53 and 33, in the second. The increase in ear weight was relatively less affected by low light intensity than increase in shoot weight, probably because the stem-sugars were utilized in grain-filling. The stem-sugars increased only in L and decreased in 1₁, 1₂ and 1₃. The grain number and its weight were also reduced under l₁, 1₂ and l₃. Assuming that increase in shoot weight is a measure of photo-synthetic activity, it is inferred that during the fortnight after anthesis, current photosynthesis determines largely, but not entirely, the rate of growth of grain. After transfer to full sunlight, the grain weights of l₁, 1₂ and 1₃ plants amounted, at maturity, to 93, 78 and 77 % of the value of L. The actual increase in grain weight was the same in L and l₁ and slightly lower in l₂, and 1₃. It is inferred that the same amount of photosynthate was produced in all the four treatments hut was not fully utilized in 1₂ and 1₃ because of considerable reduction in grain number. Since the sugar content of the stem in l₁, I₂ and l₃ plants, on the 15th day after anthesis, was half as much as, or even lower than, in L, whereas the increase in grain weight was much less affected, it follows that the level of stem-sugars does not determine the rate of growth of grain. During the fortnight before maturity, light intensity as low as 2500 lux had little effect on grain development. It is concluded that light intensity equal to that of full sunlight may not be required during the entire course of grain development. Considering that sugars accumulate in the stem during the fortnight after anthesis, and their concentration does not control the subsequent rate of growth of the grain, it is suggested that a more efficient sink (ear) may be the means by which photosynthates can be more effectively utilized and the yield potential raised further.