Effects of chilling on male gametophyte development in rice.

Chilling during male gametophyte development in rice inhibits development of microspores, causing male sterility. Changes in cellular ultrastructure that have been exposed to mild chilling include microspores with poor pollen wall formation, abnormal vacuolation and hypertrophy of the tapetum and unusual starch accumulation in the plastids of the endothecium in post-meiotic anthers. Anthers observed during tetrad release also have callose (1,3-beta-glucan) wall abnormalities as shown by immunocytochemical labelling. Expression of rice anther specific monosaccharide transporter (OsMST8) is greatly affected by chilling treatment. Perturbed carbohydrate metabolism, which is particularly triggered by repressed genes OsINV4 and OsMST8 during chilling, causes unusual starch storage in the endothecium and this also contributes to other symptoms such as vacuolation and poor microspore wall formation. Premature callose breakdown apparently restricts the basic framework of the future pollen wall. Vacuolation and hypertrophy are also symptoms of osmotic imbalance triggered by the reabsorption of callose breakdown products due to absence of OsMST8 activity.

Growth and survival of seedlings of native plants in an impoverished and highly disturbed soil following inoculation with arbuscular mycorrhizal fungi.

We investigated whether arbuscular mycorrhizas influenced growth and survival of seedlings in an extremely impoverished and highly disturbed soil. Seedlings of four plants species native to the site were either inoculated with native sporocarpic arbuscular mycorrhizal (AM) fungi or fertilised prior to transplanting, and followed over 86 weeks at the site. One treatment was also irrigated with N-rich leachate from the site. In a laboratory experiment, seedlings were fertilised with excess P for 6 weeks, and location of the P store determined. Growth and survival of AM and fertilised seedlings were similar at the site. Inoculated mycorrhizal fungi and roots appeared to extend into the surrounding soil together. P concentration in leaves of all plants was extremely low. Irrigation with leachate increased growth of seedlings. In the laboratory experiment, significantly more P was stored in roots than shoots. We suggest that successful revegetation of extremely disturbed and impoverished sites requires selection of mycorrhizal fungi and plants to suit the edaphic conditions and methods of out-planting.

Carbohydrate metabolism of cactus in a desert environment.

The concentration of glucan, mucilage, soluble carbohydrates, and malic acid were determined in Opuntia bigelovii Engelm. during a 23-week period. The experiment began during the dry summer by irrigation to stimulate Crassulacean acid metabolism and was followed by 13 weeks of drought. After the 13-week drought period, the plants were irrigated throughout a 10-week period until late December. The maximum level of malic acid determined each day at dawn decreased throughout the drought period and increased after irrigation. High levels of malic acid occurring at dawn are indicative of active Crassulacean acid metabolism. Soluble carbohydrates also decreased during drought and increased after irrigation. Both glucan and mucilage increased slightly for about 9 weeks during the drought period and then began to decrease. Irrigation was accompanied by a further decrease in concentration of glucan and mucilage. Since both glucan and mucilage changed in a similar manner and since their concentrations in the tissue are correlated, it is hypothesized that both function as storage carbohydrates. Whereas glucan is the nocturnal substrate for malic acid synthesis, there are no data to support or refute a similar hypothesis for mucilage.

Dark Fixation of CO(2) by Crassulacean Plants: Evidence for a Single Carboxylation Step.

Malic acid isolated from Bryophyllum pinnatum (Lamk.) Oken (B. calycinum Salisb.), Bryophyllum tubiflorum Harv., Kalanchoë diagremontiana Hamet et Perrier and Sedum guatamalense Hemsl. after dark (14)CO(2) fixation was degraded by an in vitro NADP-malic enzyme technique. In the short term (5 to 30 seconds) the malic acid was almost exclusively labeled in the C-4 carboxyl carbon (greater than 90%). The percentage of (14)C in the C-4 carboxyl of malic acid declined slowly with time, reaching 70% in B. tubiflorum and 54% in B. pinnatum after 14 hours of exposure to (14)CO(2). It was found that malic acid-adapted Lactobacillus arabinosus may seriously underestimate the C-4 carboxyl component of label in malic acid-(14)C. The amount of substrate which the bacteria can completely metabolize was easily exceeded; there was a significant level of randomization of label even when beta-decarboxylation proceeded to completion, and in extended incubation periods, more than 25% of label was removed from malic acid-U-(14)C. The significance of these findings in relation to pathways of carbohydrate metabolism and malic acid synthesis in Crassulacean acid metabolism is discussed.