Postpollination Phenomena in Orchid Flowers. VIII. Water and Dry Weight Relations

Pollination causes increases in FW and DW of Cymbidium ovaries and gynostemia. It also initiates FW losses in perianth segments. Auxin (NAA) applications to the stigma have the same effects except that FW of ovaries decreases. All segments of unpollinated flowers increased in DW, whereas FW variations were minimal. These weight changes reflect the aging and subsequent death and/or redifferentiation and further development of pollinated and/or unpollinated orchid flowers.


Introduction
Wilting of sepals, petals, and labella (lips); swelling of gynostemia (columns) which subsequently become green; and increases in the diameter of ovaries are among the most easily observable postpollination phenomena in orchid flowers. One intuitive explanation (HUBERT and MATON 1939;HSIANG 1951a) for these phenomena is water gains in organs which swell and water losses in those that wilt. Increases and decreases in dry-matter content of floral segments may also occur, thereby affecting FW and DW. The available evidence indicates that, after pollination, substances are mobilized from the perianth into gynostemia and ovaries (FITTING 1909a(FITTING , 1909b(FITTING , 1910SCHUMACHER 1931;SESHAGIRIAH 1941;OERTLI and KOHL 1960;HARRISON and ARDITTI 1976) .
Should the movement of water and dry matter be simultaneous, FW changes would reflect both, whereas DW variations can indicate only the latter. The relationship between water and dry matter determines the hydration of tissues and is expressed as HV (BALDOVINOS 1953;HINNAWI 1973) , which is a parameter that can provide information on whether the gains or losses of dry matter and water differ in magnitude.
Material and methods FLOWERS.--Racemes of Cymbidium 'Jungfrau' (lathhouse-grown plants, U.C.I. orchid collection), harvested after all but the two or three apical buds had opened, were placed in water for 12-16 h. Flowers were cut at the pedicel base just before the start of the experiments. Ovaries and pedicels were decontaminated by a 5-min immersion in saturated calcium hypochlorite (ARDITTI and KNAUFT 1969 (table 1).

The increases in DW and losses in HV during the first 24 h also indicate dry-matter uptake regardless of water movement. Similar uptake of sucrose, fluoride (as Na+), and red dye has been reported for Gladiolxs, Gerbera, Chrysaxthemgm, and snapdragons
The DW losses by all segments between 24 and 48 or 96 h may be the result of reduced uptake and increased utilization, or a combination of the two. Although firm evidence is not available, it appears reasonable to assume that the increased utilization is through respiration. Reports regarding respiration in orchid flowers support this assumption despite lower rates in older blossoms (HSIANG 1951b;SHEE-HAN 1954;ROSENSTOCK 1956)

. Our findings (HARRI-SON and ARDITTI 1976) showed that leakage (at least Of 32p) into the mediurn does not occur.
Life of Cymbidium flowers exceeds 1 wk. Therefore, it is not surprising that at the end of 7 days the flowers gained in DW (figs. la, 2a, 3a, 4a; table 1).

The transpiration stream (GOLDSCHMI1)T and Hu-BERMAN 1974) results in the uptake and accumulation of minerals and sugars from the medium. We have no evidence for active uptake. Increases in FW (except in the labellum) after 24 h are the result of higher water uptake brought about by elevated osmotic concentrations which have been reported in orchid flowers (HSIANG 1951a).
Reduced.subsequent water uptake, increased transpiration losses, or a combination of the two can account for the low final FW (table 1). The sizable FW gains by ovaries ( fig. 2b; table 1) are due both to water uptake and the fact that they were either submerged in medium or inside the tubes where transpiration is reduced.
Water losses occur through cuticular transpiration BOTANICAL GAZETTE [JUNE in the absence of stomata on the perianth of Cymbidium flowers (HSIANG 1951a). POLLINATED BLOWERS.-AS in control flowers, gains in DW during the-first 24 h are due to uptake of solutes from the medium. Uptake is also a contributing factor to the subsequent DW increases in gynostemia ( fig. la) and ovaries (fig. 2a). One suggestion (FITTING 1909a(FITTING , 1909b(FITTING , 1910SCHUMACHER 1931;SESHAGIRIAH 1941;GESSNER 1948;HARRISON and ARDITTI 1976) is that at least part of the increase is due to transport of substances from the perianth. Our data support this view with respect to the sepals (table 1).
The DW gains by the entire flower (14.7<70) and petals (2.7l7o) were much lower than those by the ovaries and gynostemia (42.5%). Losses in DW by all sepals (l.9%o) were minimal (table 1). These figures indicate that (1) uptake from the medium does take place, and (2) substances are transported preferentially into gynostemia and ovaries which act as sinks because of newly initiated developmental processes and increased metabolic activity. Selective transport has been reported for 32p 3a) and petals gained very slightly (table 1). Uptake from the medium, translocation patterns, and physiological differences between perianth segments can account for this apparent anomaly. In sit?z translocation of sugars into flowers is in the phloem, but distribution of substances taken up through cut peduncles or pedicels may be via the xylem (GOLD-SCHMIDT and HUBERMAN 1974). Should this be true for Cymbidi?zm flowers, the increased transpiration stream into perianth segments will bring in a higher amount of solutes. This would lead to dry-matter accumulation and gains in DW.
As can be expected in a wilting tissue, all perianth segments lose FW (figs. 3, 4; table 1). A similar decrease in the weight of cut rose flowers occurs when transpiration is greater than water uptake (MAYAK et al. 1974). Water balance is considered to be an important factor in the determination of cutflower longevity (MAYAK et al. 1974). Our results confirm this by demonstrating that pollinated or auxin-treated flowers (which senesce and die quickly) show higher FW losses than unpollinated ones (which age and die at a slower rate). The FW gains by gynostemia ( fig. lb) and ovaries (fig. 2b) result from the increased water content which is also the reason for their swelling (HUBERT and MATON 1939).
Perianth segments (figs. 3C, 4C; table 1) become drier and have a lower HV because water losses are proportionally higher than those of dry matter. The reduced HV of gynostemia ( fig. lc) and ovaries  (fig. 2c) is due to a disproportionately higher influx of water than that of dry matter.
NAA-TREATED FLOwERS. Auxins, including NAA, can mimic pollination or emasculation in orchid flowers. The overall effect is accelerated aging, sometimes, as observed here, even more than after pollination (FITTING l909a, l909b, 1910 1971a,1971b). Hence FW and DW losses in perianth segments of NAA-treated flowers may be higher than those of pollinated blossoms. The NAA application mimics pollination in respect to DW of gynostemia ( fig. la) and FW of perianth segments ( fig. 3b,4b; table 1), but not in ovaries.
Pollination initiates ovule formation in orchid ovaries, but auxin applications do not always have the same effect. Thus, NAA-treated ovaries may be senescing, and this is reflected in their F\M, DW, and HV ( fig. 2; table 1). In gynostemia these differences between pollination and NAA treatment may be due to (1) faster destruction of IAA (the auxin found in pollen), (2) the presence of other hormones in pollinia (R. MA, unpublished results), and/or (3) auxin-induced synthesis of other substances such as ethylene (ARDITTI, HOGAN, and CHADWICK 1973).
The observed FW, DW, and HV changes in Cymbidi?zm 'Jungfrau' are fully in line with our previous observations and suggestions regarding preand postpollination phenomena and aging in orchid flowers. These observations also indicate that the labellum is physiologically similar to perianth segments, thereby supporting the view that it has originated from a petal (VERMEULEN 1959;NELSON 1965NELSON , 1967.