Life History and Energy Relationships of Hydrophyllum appendiculatum

The phenology of marked Hydrophyllum plants in five permanent 1—m² plots in Brownfield Woods near Urbana, Ill., was observed from March 1966 through August 1966 and from June 1967 through July 1968. The first—year plant which appears above the soil surface in early spring persists during the entire summer under the closed canopy with two to four erect, solid—colored, palmately shaped leaves. In autumn, mottled, palmately shaped leaves are formed which then overwinter in the form of a basal rosette. The upright, first—year leaves die back with the advent of heavy frosts. The plants break dormancy in early spring with subsequent formation of a large basal rosette of mottled, pinnately shaped leaves. As the forest canopy closes, solid—colored, palmately shaped leaves and an erect stem are formed. Floral buds soon appear. Flowering and seed set take place under a closed canopy with plant senescence occurring approximately 2 months after canopy closure. Carbohydrates are the major form of food reserves (82% of ovendry weight) in Hydrophyllum seed. At least a year is needed after seed dispersal before germination. Accumulated degree—hours and the floral bud and flowering stages are closely related in Hydrophyllum. Low temperatures are important in rosette formation in both greenhouse and field plants, but photoperiod apparently is not important if temperatures remain moderate. Plants will not flower unless they first develop a basal rosette. Although only a relatively short vernalization period is essential, longer periods of low temperature will decrease the length of time needed to flower after the plants have broken dormancy. Floral buds appeared in photoperiods ranging from less than 12 to over 13.5 hr, but longer photoperiods shortened the time interval between breaking dormancy and floral bud initiation. Hydrophyllum is adapted photosynthetically to canopy closure by producing different types of leaves for different light regimes. Plants with only solid—colored, palmately shaped leaves (first—year stage and seed—set stage) are present only under closed canopy conditions Light—compensation point of these plants is 60—80 ft—c, and light—saturation intensity is approximately 700 ft—c. their maximum net photosynthetic rates are about 2.4 mg CO₂/dm² per hour. Mottled, pinnately shaped leaves are present on second—year vegetative plants which grow under an open, spring canopy. These plants have a light—compensation point of 100 ft—c and light—saturation intensity of 2,500 ft—c. Their net photosynthetic rate at light saturation is 8.5 mg CO₂/dm² per hour. Flowering plants, which occur under a closed canopy and usually have both types of leaves, have net photosynthetic rates and light—saturation intensities at intermediate levels between the solid—colored and mottled groups of plants. All phenological stages except the seed—set stage demonstrated small but significant decreases in photosynthesis with increasing temperatures in the 15°—27°C range.