The effect of growth stage and re-growth on the nutritional and ensilage characteristics of two new sorghum hybrids, BMR-101 and Silobuster, and one commercial variety, FS-5, was examined in this study. Varieties were sampled during the summer at the early heading (EH) stage and were harvested at the soft dough (SD) stage. Additional irrigation enabled autumn re-growth and a second harvest. Plants of FS-5 and BMR-101 were resistant to lodging at EH. However, BMR-101 and Silobuster suffered from high lodging at the SD stage of the summer harvest. Dry matter (DM) content of FS-5 and BMR-101 at EH was below 250 g kg −1 . DM yields of the varieties were similar at the summer harvest and higher than their respective re-growth cuts. Ensilage DM losses were moderate and similar across varieties. Hemicellulose of SD plants was partly solubilised and most of the water-soluble carbohydrate fermented, yielding lactic acid, ethanol and volatile fatty acids (VFA), and a pH < 4. In vitro DM digestibility of varieties was similar in summer silages, but lower in the respective re-growth silages of FS-5 and BMR-101, reflecting the higher content of neutral detergent fibre and lignin in the re-growth silages. The summer plus re-growth cumulative yields of digestible DM of the respective FS-5, Silobuster and BMR-101 silages were 14.7, 16.6 and 14.5 t ha −1 . The commercial variety, FS-5, may have some advantage over BMR-101 and Silobuster owing to its relative resistance to lodging in addition to its high yield and good ensilage properties.
Photoinhibition of photosynthesis was induced in attached leaves of kiwifruit grown in natural light not exceeding a photon flux density (PFD) of 300 μmol·m(-2)·s(-1), by exposing them to a PFD of 1500 μmol·m(-2)·s(-1). The temperature was held constant, between 5 and 35° C, during the exposure to high light. The kinetics of photoinhibition were measured by chlorophyll fluorescence at 77K and the photon yield of photosynthetic O2 evolution. Photoinhibition occurred at all temperatures but was greatest at low temperatures. Photoinhibition followed pseudo first-order kinetics, as determined by the variable fluorescence (F v) and photon yield, with the long-term steady-state of photoinhibition strongly dependent on temperature wheareas the observed rate constant was only weakly temperature-dependent. Temperature had little effect on the decrease in the maximum fluorescence (F m) but the increase in the instantaneous fluorescence (F o) was significantly affected by low temperatures in particular. These changes in fluorescence indicate that kiwifruit leaves have some capacity to dissipate excessive excitation energy by increasing the rate constant for non-radiative (thermal) energy dissipation although temperature apparently had little effect on this. Direct photoinhibitory damage to the photosystem II reaction centres was evident by the increases in F o and extreme, irreversible damage occurred at the lower temperatures. This indicates that kiwifruit leaves were most susceptible to photoinhibition at low temperatures because direct damage to the reaction centres was greatest at these temperatures. The results also imply that mechanisms to dissipate excess energy were inadequate to afford any protection from photoinhibition over a wide temperature range in these shade-grown leaves.
The concentration of organic acids, organic nitrogen (N), nitrate (NO 3 ), and total cations increased in annual ryegrass [Lolium multiflorum Lam.) with salinity and N concentration in the growth medium. Increasing salinity and N in the growth medium induced changes in the level of key enzymes of N assimilation and organic acids: nitrate reductase (NR, EC 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31), and glutamine synthetase (GS, EC 6.3.1.2). Plants grown in pots filled with sand were irrigated with nutrient solutions with an electroconductivity of 2 or 11.2 dS m -1 and N applied as ammonium nitrate (NH 4 NO 3 ), sodium nitrate (NaNO 3 ), or ammonium 707 Journal of Plant Nutrition 1998.21:707-723. 708 SAGI ET AL.applied as ammonium nitrate (NH 4 NO 3 ), sodium nitrate (NaNO 3 ), or ammonium (NH 4 ) as ammonium sulfate [(NH 4 ) 2 SO 4 ] at concentrations of 0.5, 4.5 or 9.0 mM. Nitrate reducíase, PEPc, and GS increased with salinity and N level. Shoot NR was highest in the presence of NH 4 NO 3 irrespective of salinity level, while root NR activity responded best to NO 3 . Enhancement of PEPc activity in both shoots and roots was highest with NH 4 NO 3 and lowest with NH 4 . Nitrogen source had no significant effect on GS activity in shoots or roots of ryegrass. Shoot NR activity increased with NO 3 concentration in the tissue, as calculated from regression coefficients. The PEPc activity correlated positively with total cations and NO 3 concentrations in the plants, irrespective of the salinity level, suggesting that the increase in total cations and NO 3 induced by salinity may have triggered the changes in enzyme activities. The concentration of organic acids in both shoots and roots correlated positively with PEPc activity irrespectively of the salinity level. The PEPc activity was higher in roots than in shoots, while organic acid concentration was higher in shoots. These results suggest that a significant part of the organic acids produced in the roots were used as carbon skeleton for transamination reactions. The increased activity of NR, PEPc, and GS in roots may constitute part of an adaptation strategy of the plant to increasing salinity in the medium. These enzymes have an important role in the metabolism of amino acids and the synthesis of organic N in annual ryegrass irrigated with saline water, and boosting them with suitable N fertilizers could increase the nutritional value and protein content of the crop.
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