Hypogeal

Abstract: Despite the importance of seedling establishment in plant biology, there is no consensus on what constitutes a 'seedling'. Here we examined aspects of early plant development that could be used to mark the transition from the seedling to the postseedling phase. Using a hypogeal species (pea, Pisum sativum) and an epigeal species (sunflower, Helianthus annuus), we investigated whether the utilization of cotyledon-stored mineral nutrients coincides with any changes in relative growth rate (RGR). We also examined how the timing of cotyledon removal at different points during early development affected subsequent growth. For both species, the timing of RGR(max), the exhaustion of cotyledon reserves, and the attainment of independence from cotyledons all roughly coincided (though exhaustion of seed reserves was not observed in the epigeal species because the cotyledons absorbed external nutrients). We conclude that because the point of attainment of RGR(max) is a distinctly identifiable event, it is a more reliable marker for defining the end of the seedling stage than either the exhaustion of mineral reserves, or the cessation of dependence on cotyledon reserves.

Abstract: To investigate the existence of coordinated sets of seedling traits adapted to contrasting establishment conditions, we examined evolutionary convergence in seedling traits for 299 French Guianan woody plant species and the stress response in a shadehouse of species representing seed size gradients within five major cotyledon morphology types. The French Guianan woody plant community has larger seeds than other tropical forest communities and the largest proportion of hypogeal cotyledon type (59.2%) reported for tropical forests. Yet the community includes many species with intermediate size seeds that produce seedlings with different cotyledonal morphologies. A split-plot factorial design with two light levels (0.8% and 16.1% PAR) and four damage treatments (control, seed damage, leaf damage, stem damage) was used in the shadehouse experiment. Although larger-seeded species had higher survival and slower growth, these patterns were better explained by cotyledon type than by seed mass. Even larger-seeded species with foliar cotyledons grew faster than species with reserve-type cotyledons, and survival after stem grazing was five times higher in seedlings with hypogeal cotyledons than with epigeal cotyledons. Thus, to predict seedling performance using seed size, seedling morphology must also be considered.

Abstract: Cotyledons of 11 species were studied at a number of stages of germination. The hypogeal pea and runner bean cotyledons did not expand, lost weight, and survived for a relatively short time only. They also produced little chlorophyll on exposure to light, possessed no stomata, and had a very low capacity for 14C02 fixation. The epigeal french bean had cotyledons that were basically of the hypogeal type. Although both white and blue lupin cotyledons showed a pro gressive weight loss, they underwent limited expansion and were more persistent than either pea or bean. They also produced considerable amounts of chlorophyll, had stomata on both upper and lower surfaces, and fixed restricted quantities of 14C02. The cotyledons of the other epigeal species studied showed varying degrees of expansion, up to almost fifty-fold in cucumber, and generally maintained or increased in total dry weight for at least a restricted period. Stomata occurred on both upper and lower surfaces, extensive chlorophyll production took place, and 14COa fixation values were high. Expansion was determined by increase in cell size, and not in cell number except in the case of cucumber where both factors were involved. In species where cotyledon cells were large initially little or no expansion occurred, whereas initial cell size was small in cotyledons which expanded to a large extent. Epigeal cotyledons with a high expansion factor possessed other characteristics which made them adapted for photosynthesis, whereas epigeal species with lower cotyledon expansion together with hypogeal species were less well adapted. This was not unexpected in the case of pea and runner bean, but led to the conclusion that french bean cotyledons are ' accidentally epigeal' in that they showed virtually no adaptation to an aerial existence. The different capacities of the cotyledons studied suggests that they have differing roles in the control of seedling growth.

Abstract: The growth of young plants of the epigeal species Phaseolus vulgaris and Glycine max is compared with that of the hypogeal species Pisum sativum and Vicia faba, with particular reference to synchronization between the exhuastion of seed reserves of N and the availability of fixed N. It is argued that the N stress symptoms which occur when these two processes are not synchronized are more common and obvious in Phaseolus or Glycine than in Pisum or Vicia. This is primarily because in these species (a) the first fixed N is used for nodule growth rather than being exported to the shoot system and (b) the first foliage leaves have a much greater area and contain a larger proportion of N reserves from the seed. It is further suggested that Phaseolus and Glycine may show the greater response to nitrogen fertilizer applied at sowing since (a) most of the applied nitrate is passed directly to the shoots (rather than being reduced in the roots as in Pisum or Vicia) and (b) in addition to being used for growth (following reduction), it may also be used prior to reduction as part of the osmotic force driving cell expansion.