No abstract

No abstract

Algae of higher intertidal regions tend to be tolerant of extended periods of desiccation, while many lower tidal or subtidal species do not withstand even mild water loss. (Tidal regions can be characterized as high (regularly immersed at high tide and exposed at low tide), low (emergence only during minus tides (lower than mean low tide)), or subtidal (never exposed at low tide and extending to the maximum depth at which net photosynthesis can occur).) The ecological necessity for tolerance in frequently emerged species is obvious, but the physiological basis of it is not well understood. Changes of photosynthetic partial reactions upon desiccation and rehydration of tolerant and sensitive algae were studied by measurements of chlorophyll fluorescence induction kinetics (Kautsky effect). With progressive decrease in water content the gradual disappearance of the characteristic fluorescence transients was observed in both tolerant and sensitive species. The water content ranges where typical changes occurred were species dependent. Rehydration in tolerant plants resulted in rapid recovery from severe desiccation; there was no such recovery in sensitive plants when water content was decreased below a critical value. Analysis of the fluorescence changes upon desiccation and rehydration suggests: (1) electron transport between photosystem II and photosystem I, as well as H2O splitting are the partial reactions sensitive to desiccation; (2) in the resistant Porphyra sanjuanensis, intersystem electron transport is blocked at around 25% water content; (3) further desiccation leads to loss of water-splitting activity and eventually to the complete loss of variable fluorescence photosystem II reaction centers; and (4) on rehydration intersystem electron transport begins almost immediately while recovery of H2O splitting requires several minutes.

The leaf, petiole, stem and root anatomy of an aseptically cultured red raspberry clone (Rubus idaeus L.) was studied before and 5 weeks after transfer to soil under controlled environmental conditions. Tissues persistent from culture showed little or no change with time in soil; they grew minimally and slight secondary wall deposition occurred. New organs formed in successive weeks after transplantation showed a graded increase in potential size and development. Some features, such as collenchyma formation, rapidly returned to control levels; this was seen in new leaves expanding in the first week after transplantation. Other features, such as sclerenchyma formation, did not occur in leaves expanding during the first 2 weeks after transplantation, even when these were a month or more in age. Some sclerenchyma was seen in leaves expanding in the third week after transplantation, increasing in later-formed leaves. Increasing the light intensity of transplant accelerated the return to control-type organ size and appearance. During acclimatization transitional forms of leaves, petioles, stems and roots develop that ranged anatomically from culture-to control-type. This trend is analagous to the normal developmental sequence of organ formation as it affects the potential for development of successily formed organs.

In a variety of plants, the induction kinetics of chlorophyll fluorescence vary substantially depending on whether measured on the upper or lower side of the same leaf. The responses are comparable to those of plants grown under sun and shade conditions. Leaf morphology appears not to be the primary cause of the differences since inversion of the leaves can lead to reversed fluorescence responses. Fluorescence induction was analyzed in control and inverted leaves, and in one case, in chloroplasts from sun and shade leaves. It is concluded from the data that the major differences between the chloroplasts of the upper and lower leaf side reflect ionic and thylakoidmembrane conformational factors, rather than structural differences. Mg(2+) flux probably plays a significant role in the adjustment of the thylakoid membrane to high or low light conditions.

No abstract

Red light and gibberellic acid were about equally effective in promoting germination of Grand Rapids lettuce (Lactuca sativa L.) seeds. With initial far red light treatment more than 80% remained dormant in subsequent dark storage. After 2 days of dark storage, red light effectively promoted germination, while gibberellic acid action was weak. With between 2 and 10 days of dark storage, gibberellic acid had little effect, while promotion by red light decreased slowly and finally disappeared. After 10 days of dark storage, both gibberellic acid and red light were required for germination. The dark storage treatment interferes with phytochrome-independent germination processes and cannot be overcome by added gibberellic acid. However, storage may also decrease the effectiveness of endogenous gibberellins. Phytochrome-dependent germination seems to require only low levels of endogenous gibberellin activity or the addition of gibberellic acid. Gibberellins and red light appear to act on germination by regulation of sequential sites of a branchedlooped pathway.A recurring problem in light-mediated seed germination studies centers on interactions of BY and gibberellins in promoting germination. Some considerations have included R stimulation of endogenous gibberellin production or activity (11,14). Ikuma and Thimann (8,9) We have examined aspects of gibberellin-phytochrome interactions and conclude that these agents exert their influence on the germination of Grand Rapids lettuce seeds by regulation of sequential sites on a branched-looped germination pathway. MATERIALS AND METHODSAll experiments were carried out at 20 C with three replicates of 33 Grand Rapids lettuce seeds (Lactuca sativa L.), 1970 harvest, obtained from Buckerfields Ltd., Vancouver, B. C. These seeds have been stored at -20 C since their acquisition and normally exhibit strong light sensitivity. In some experiments, seeds were given an initial 1-min irradiation with FR (730 nm, 60,000 ergs cm-2 sec') at about 0.5 hr after the onset of imbibition; in others no initial light treatment was given. Seeds were, in some cases, irradiated with R (660 nm, 60,000 ergs cm-2 sec') either initially, immediately after FR, or some multiple of 2 days after the onset of imbibition. When 0.5 mm GA (Sigma, St. Louis, Mo.) was given, it was either present from the onset of imbibition or seeds were transferred from distilled water to fresh GA solution at some multiple of 48 hr after the beginning of imbibition. For clarity all time intervals from day 0 till the final treatments with R, GA, or both will be referred to as dark storage, and the post-treatment interval (usually 48 hr) until germination was scored, as the germination test. After each 2-day DS interval, any seeds observed to have germinated were removed and discarded; subsequent germination percentages are those of the remaining seeds. Inspection of Figures 1, 2

Rooting and acclimatization procedures for micropropagated conifers are reviewed, with emphasis on their effects on root quality and plantlet performance in the nursery and field. Major influences on root production include auxin concentration and mode of application, shoot quality, donor age, clone and temperature. The development of a fibrous, well-branched root system has been a problem that may be solved by using rooting substrates that are better-aerated than agar. Further development of the root system may be enhanced by early air-pruning and ectomycorrhizal associations. During acclimatization, high humidity is required for conifers. However, conifers have an advantage over non-coniferous plantlets with respect to water loss because of a better development of the needle cuticles prior to transfer to in vivo conditions. In greenhouse and field comparisons with seedlings, plantlets were similar in survival and growth rate, but root systems were less fibrous. Also, features of early maturation have been observed for plantlets, the cause of which is uncertain. Pertinent research with rooted cuttings and seedlings of conifers has been cited to gain a better understanding of the factors involved in root production and development.