The effect of light on the growth and reproduction of Floerkea proserpinacoides

Floerkea proserpinacoides (Limnanthaceae) is a spring ephemeral annual spec ies that grows in deciduous forests throughout eastern North America. Seeds germinate from late November to December, although the first leaf emerges only from late March to early April. Growth begins in early April at the on set of favourable temperatures, following snowmelt, and continues through m id-June. Senescence coincides with increasing air temperature and decreasin g light level as a result of canopy closure, In this paper, we present the results of a growth chamber study designed to determine the effect of light level on growth, biomass allocation and reproduction of F. proserpinacoide s. The study consists of two parts: in a first experiment, plants were grow n at fire constant photosynthetic photon fluence rates (PPFR: 90, 180, 360, 540 or 900 mu mol m(-2) s(-1)), and in a second experiment, PPFR was reduc ed from 900 mu mol m(-2) s(-1) to 180 mu mol m(-2) s(-1) after 0, 14, 21, 2 8 or 35 d of growth. Relative humidity, temperature, nutrient and water sup ply were kept constant in a hydroponic sand culture experiment. Total bioma ss, leaf mass and leaf area increased with increasing PPFR up to 540 mu mol m(-2) s(-1). Plants grown at the highest (900 mu mol m(-2) s(-1)) and the lowest (90 mu mol m(-2) s(-1)) PPFR had a substantially lower biomass by th e end of the 35-d growth period than plants grown at intermediate PPFRs (36 0 or 540 mu mol m(-2) s(-1)). Despite differences in total biomass, there w ere no significant differences in seed production among treatments. The mea n relative growth rate (RGR) increased with increasing light levels between 90-540 mu mol m(-2) s(-1), and it was reduced at 900 mu mol m(-2) s(-1). H owever, differences in RGR were not significant among treatments. Specific leaf area did not vary consistently as a function of light level, whereas l eaf area ratio and leaf mass ratio tended to increase with increasing PPFR, reaching maximum values at 360-540 mu mol m(-2) s(-1). However, none of th ese growth variables differed significantly across the range of PPFR levels . The transfer of plants to lower PPFR had no significant effect on any of the growth components. Biomass production for the species appeared to be op timized at PPFR of 360-540 mu mol m(-2) s(-1). Growth might be restricted b y an insufficient supply of photosynthates at low PPFR and by photoinhibito ry processes at higher PPFRs.