Effect of Age of Leaves upon the Rate of Photosynthesis in Some Conifers.

Information regarding the relationship between photosynthesis and the complex of internal factors encompassed by the term, age, may be of interest either from the point of view of basic science or of practical application. The literature on this subject appears to be rather meager. SINGH and LAL (2) working with different aged leaves of flax, sugar cane, and wheat found that the rates of photosynthesis were slow for young leaves, increased to a maximum for mature leaves, and then decreased with increasing age. From studies on aging effects in Chlorella cultures, WINOKUR (5) reported that the rate of photosynthesis, expressed either per unit number of cells or per unit dry w-eight, increased during the first few days of a culture and then declined with increasing age of the culture. It would appear that those conifers which maintain their leaves for several years would be especially appropriate for the study of the effects of aging on processes in the leaves. STXLFELT (3) investigated the rates of photosynthesis in spruce and pine with respect to age of needles and light intensity. He concluded for spruce that the rate of photosynthesis, when calculated per gram of fresh weight of needles, appeared to decrease with increasing age of the needles only because the weight of the needles increased with age; whereas actually the rate per unit number of leaves increased with increasing age for five years before declining. The results were most marked at about 30% of full sunlight but were quite evident both below and above this light intensity. For pine, in 30% of full sunlight, he found a similar increase in the amount of photosynthesis per unit number of leaves with increasing age up to three years, the oldest used; but in full sunlight the rate was approximately the same for leaves of all ages. From this review of literature, one may conclude that for some plants the peak photosynthetic capacity is reached at the time of maturity of the photosynthetic organ or structure, but in conifers the peak is not reached until the leaves are several years old. The purpose of the research reported in this paper was to obtain further data regarding the rates of photosynthesis in conifer leaves of varying age.