T-3

To facilitate mechanized tillage, transplanting and later management in a solar greenhouse, we evaluated the effects of four row spacing treatments: T₁ (large row spacing 150 cm; small row spacing 30 cm; plant spa-cing 44 cm; width of walkway 110 cm), T₂ (large row spacing 140 cm; small row spacing 40 cm; plant spacing 44 cm; width of walkway 100 cm), T₃ (large row spacing 130 cm; small row spacing 50 cm; plant spacing 44 cm; width of walkway 90 cm), and T₄ (large row spacing 120 cm; small row spacing 60 cm; plant spacing 44 cm; width of walkway 80 cm). A control treatment (CK) was included, with large row spacing 90 cm, small row spa-cing 70 cm, plant spacing 50 cm, and width of walkway 50 cm. We examined the effects of different row spacing configurations on photosynthetic efficiency and yield of eggplant in solar greenhouses, aiming to screen out the sui-table row spacing setting for mechanization suitability. The results showed that across T₁-T₄ treatments, leaf angle of middle leaves, chlorophyll a and chlorophyll b contents, net photosynthetic rate, dry matter accumulation, and yield first increased and then decreased with the increases of small row spacing. All these parameters, except for the middle leaf angle, peaked in treatment T₃. The petiole angle first decreased and then increased, reaching its lowest point in treatment T₂, while the canopy transmittance of direct solar radiation gradually increased. Yield was positively correlated with the indexes other than the leaf angle, and dry matter accumulation was highly correlated with the yield. At 90 days after transplanting (during the peak fruiting period), the chlorophyll a content in leaves was increased by 19.2%, 24.9%, 27.9%, and 17.2% in treatments T₁ through T₄, respectively. Chlorophyll b content was enhanced by 14.4%, 15.8%, 27.0%, and 0.9%. Net photosynthetic rate was improved by 4.7%, 15.5%, 17.0%, and 5.6%, while dry matter accumulation was increased by 1.9%, 4.9%, 4.9%, and 2.9%. Yield enhancements were 0.4%, 6.1%, 11.3%, and 10.9% for T₁ to T₄, respectively. For all these metrics, treatment T₃ exhibited the strongest effects. In addition, the walkway between the borders under this treatment was wider, which was convenient for agricultural machinery operation. In all, the T₃ treatment was not only beneficial for achieving mechanized cultivation of eggplants in solar greenhouses, but also can increase eggplant yield, making it the most suitable row spacing setting.

Mammalian thyroid status is governed by thyroid secretion of L-thyroxine (T₄) as a prohormone that is monodeiodinated in peripheral tissues to bioactive T₃ (3,5,3'-triiodo-l-thyronine). T₄ secretion is controlled by the hypothalamic-pituitary-thyroid (HPT) axis (central control) whereas T₃ availability to target cells depends mainly on mechanisms in extrathyroidal tissues such as cellular transport and deiodination (peripheral control). Does this model apply to poikilothermic teleost fish which in contrast to homeothermic mammals may show major surges in plasma T₄ due to season, feeding, reproductive state or stressors? We have evaluated the contributions of central and peripheral mechanisms to fish thyroid status in light of recent discoveries employing both traditional endocrine approaches and more modern molecular biological techniques, focusing primarily on salmonid species which may undergo a unique thyroid-implicated premigratory parr-smolt transition (PST), and which as tetraploids may express multiple paralogs of regulatory peptides. Most teleost research has focused on peripheral control by the three classic deiodinases (D1, D2 and D3). In salmonids they determine systemic (D1, D2) and tissue-specific (D2) T₃ generation from T₄ and the equally critical T₄ and T₃ degradations (D1, D3). Tetraploid salmonids may express up to four paralogs for a given deiodinase, providing the potential for species-specific or tissue-specific T₃ production, curtailment of T₃ action, or iodine recapture. Critical as they appear, salmonid deiodinases do not function in isolation but in concert with, and dependence on, TH plasma transport, cell-membrane translocation, hepatic conjugation, biliary excretion and gastrointestinal metabolism. Two rainbow trout properties are particularly distinct from the mammalian model: i) T₃, but not T₄, exchanges rapidly between plasma and erythrocytes permitting plasma T₃ stability despite marked acute changes in plasma T₄ and ii) in contrast to ingested T₄, which is unavailable from food due to complete gastrointestinal deiodination, ingested T₃ contributes to the plasma T₃ pool. Thus the teleost liver, poised at the confluence of exogenous and endogenous T₃ sources, may play a strategic role through its TH biliary excretion, deiodination and other pathways in regulating systemic T₃ availability involved in anabolic/catabolic balance and somatic growth. A major consequence of ingested T₄ degradation is the exclusive delegation of T₄ availability to the HPT axis. Since mammalian TSH consistently stimulates teleost T₄ secretion a mammal-like HPT central control model has been assumed. However, teleost HPT function differs from that of homeotherms in both its hypothalamic control and response to external stimuli. T₄ secretion could be regulated mainly by T₄ negative feedback with the HPT axis playing a subsidiary role of merely ensuring adequate T₄ substrate for regulated peripheral deiodination to proceed. However, this does not account for the notable surges in salmonid plasma T₄ and implies resetting of the HPT 'thyrostat'. Thus the role of central TSH control in the regulation of plasma T₄ changes remains unclear, awaiting further characterization of endogenous TSH secretion. Furthermore, discoveries of TSH-subunit and TSH-receptor expression in piscine peripheral tissues such as the CNS, liver, and gonad require reassessment of TSH function with a focus not only on its traditional endocrine actions but also on its potential as a paracrine regulator of TH action in peripheral tissues. In conclusion, while there are many similarities in thyroid regulation between mammals and salmonids there are also key differences. These likely stem from the evolution of homeothermy, the constraints of terrestrial iodine availability and a plasticity in salmonid peripheral and central control resulting from tetraploidy.