Growth and Floral Development of Five Species of Range Grasses in Central Oklahoma

Abstract

1. Growth and floral development of little bluestem (Andropogon scoparius), big bluestem (A. furcatus), switch grass (Panicum virgatum), Indian grass (Sorghastrum nutans), and side-oats grama (Bouteloua curtipendula) under prairie conditions were investigated on the farm of C. H. Taylor (lat. 35⚬9' N.) near Norman, Oklahoma, during the growing season of 1948. Three plots-one on a ridge-top, one on a southeast slope, and one on a northwest slope-were fenced to exclude cattle. In addition to field observations on the condition of the five species, one culm was collected from each of ten different plants of each species (except little bluestem) each week for dissection and microscopic examination to determine whether inflorescences were being formed. For little bluestem, one culm was collected from each of five plants on each site each week to determine whether there were differences in growth and floral development on the various sites. Fairly complete microclimatic data were obtained for each plot. 2. All species started growth within a period of slightly over a week. Little bluestem and side-oats grama were earliest, showing the first signs of growth about March 30. 3. The number of leaves with blades expanded-hereafter referred to as expanded leaves-on a given culm was found to be a fairly accurate index of the time of floral initiation in all species up to the end of the major period of inflorescence initiation for each species. This number-counting the first leaf with a blade as the first-was seven or eight in little bluestem, nine to eleven in big bluestem, five or six in switch grass, three or four in Indian grass, and five or six in side-oats grama. In all except side-oats grama, there was considerable evidence that any culm which does not have the number of expanded leaves specified for the species by a certain date does not initiate an inflorescence. Under such conditions the growing point apparently remains vegetative and continues to initiate leaves. Such a criterion, if thoroughly worked out for the various species in a given locality, could be of considerable practical value to anyone who wishes to use a particular plot of grassland for both grazing and seed production. 4. Side-oats grama was the only species which showed practically no internodal elongation before floral initiation. Indian grass showed the greatest amount of such elongation. 5. Little bluestem did not show any very definite variations among the different topographic sites. The plants were shorter on the average on the southeast slope, but the pattern of elongation was exactly the same on all plots. Flowering data for little bluestem were essentially the same on all plots also. 6. Floral initiation proceeded at the greatest rate during the long days of late spring and early summer in all species. Flowering occurred chiefly during the long days of early summer in side-oats grama, but in the other species it did not begin until the daylength had decreased a substantial amount below that at the time of floral initiation. Considering the fact that all these grasses have previously been shown to be photoperiodically sensitive, using flowering as a criterion, the present data suggest a long- or intermediate-day flowering status for the plants of side-oats grama at Norman and an intermediate- or short-day flowering status for those of the other species investigated. They also suggest either that, with the exception of side-oats grama, the optimum photoperiod for floral initiation is different from that for flowering in the particular plants observed or that inflorescence initiation can occur over a greater range of photoperiods than it does under these field conditions. 7. The time between inflorescence initiation and exsertion varied with each species, being approximately as follows in each case: 43 days in little bluestem, 50 days in big bluestem, 35 days in switch grass, 64 days in Indian grass, and only 15 days in side-oats grama. The time between the start of exsertion in a given culm and its flowering (anthesis) was less than 1 week in little bluestem, big bluestem, and Indian grass; but it was about 4 weeks in switch grass and 2 weeks in side-oats grama. The total time between inflorescence initiation and flowering, therefore, was just slightly less than 2 months in little bluestem and big bluestem, slightly over 2 months in switch grass and Indian grass, and about I month in side-oats grama. An investigation as to the effects of various constant daylengths on the above periods should be of value in explaining the observed variations.