Effect of glutamine on the induction of nitrate reductase

Nitrate reductase (NR. EC 1.6.6.1/2) is a substrate inducible enzyme that could be repressed by its end product glutamine or amino acids. To test this hypothesis, 6-day-old maize seedlings ( Zea mays cv. W64A × W182E) were grown hydroponically in a 1/10 strength Hougland's salt solution modified to contain no nitrogen. Previous experiments had established that after a 24-h induction with NO₃⁻ (5 mM KNO₃⁻) the level of NR activity and protein had reached a constant level. In the present experiments when glutamine (5 mM) was included together with NO₃⁻, there was a significant reduction in NR activity (34% of the control values). NR protein and NR mRNA accumulation in the root. In the shoot, on the other hand, glutamine additions had little or no effect on the levels of either NR activity (81% of control) or NR protein. Inhibition of glutamine synthetase by methionine sulfoximine (MSX) resulted in reduced levels of glutamine in both root and shoot tissues. Contrary in our prediction, however, it had no effect on NR activity and mRNA content in roots. In the shoot, on the other hand, there was a marked reduction of NR activity (34% of the control value) and NR protein, but no apparent effect on NR mRNA. When detached shoots were treated with MSX and other inhibitors of glutamine synthetase (tabtoxinine-β-lactam or phosphinothricin) the induction of NR activity by NO₃⁻ was also inhibited. Glutamine additions 15 or 50 mM to detached shoots had essentially no effect on the induction of NR activity (90% of control). These results demonstrate that the influence of glutamine and MSX on the induction of NR in maize root and shoot tissues, respectively, is very different.     

Induction and Turnover of Nitrate Reductase in Zea mays (Influence of NO3-)

Zea mays (cv W64A x W182E) was used to investigate the induction and turnover of nitrate reductase (NR). In our system, 5 or 10 mM KNO3 gave the best growth over a 6-d growing period. With these NO3- levels, NR reached steady-state levels after 24 h. For the turnover experiments, the seedlings were transferred to a NO3--free medium after a 24-h induction. Shoot NR was less sensitive to the removal of NO3- than root NR, which declined almost as soon as NO3- was removed when the seedlings were induced with 5 or 10 mM NO3-. With 1 mM NO3-, however, removal of NO3- from medium resulted in declines in both NR activity and NO3- in shoot and root. Although there was a delay in the degradation of NR protein relative to the loss of NR activity, this protein was not reactivated when NO3- was resupplied. These results indicate that NO3- regulates NR by influencing the de novo synthesis of the NR protein and not by a reversible activation-inactivation of that protein.     

Iron Mediated Nitrate Reductase Activity in Different Parts of Young Maize Seedlings

Incubation of 5-d-old maize seedlings in the half-strength Hoagland's nutrient solution containing 10 mM KNO₃ with FeCl₃ or FeSO₄ (0.5 or 2.0 mM) caused a significant increase in nitrate reductase (NR) activity and slightly increased total protein content in root, shoot and scutellum. In case of root, NADPH:NR activity was inhibited contrary to the NADH:NR activity. In spite of NR activity, nitrate uptake was inhibited from 13 to 37 % by the iron. The results presented demonstrate an isoform specific, organ specific, and to some extent salt specific responses of NR to iron.     

Glutamine transport and feedback regulation of nitrate reductase activity in barley roots leads to changes in cytosolic nitrate pools

The size of tissue amino acid pools in plants may indicate nitrogen status and provide a signal that can regulate nitrate uptake and assimilation. The effects of treating barley roots with glutamine have been examined, first to identify the transport system for the uptake of the amino acid and then to measure root NR activity and cellular pools of nitrate. Treating N replete roots with glutamine elicited a change in the cell membrane potential and the size of this response was concentration dependent. In addition, the size of the electrical change depended on the previous exposures of the root to glutamine and was lost after a few cycles of treatment. Whole root tissue pools of glutamine and phenylalanine increased when roots were incubated in a nutrient solution containing 10 mM nitrate and 1 mM glutamine. Treating roots with 1 mM glutamine increased cytosolic nitrate activity from 3 mM to 7 mM and this change peaked after 2 h of treatment. Parallel measurements of root nitrate reductase activity during treatment with 1 mM glutamine showed a decrease. These measurements provide evidence for feedback regulation on NR activity that result in changes in cytosolic nitrate activity. After 6 h in glutamine both root NR activity and cytosolic nitrate activity returned to pretreatment values, while tissue concentrations of glutamine and phenylalanine remained elevated. The data are discussed in terms of the mechanisms that are most likely to be responsible for the changes in cytosolic nitrate.     

Nitrate uptake and utilization is modulated by exogenous γ-aminobutyric acid in Arabidopsis thaliana seedlings

Exogenously applied GABA modulates root growth by inhibition of root elongation when seedlings were grown in vitro on full-strength Murashige and Skoog (MS) salts, but root elongation was stimulated when seedlings were grown on 1/8 strength MS salts. When the concentration of single ions in MS salts was individually varied, the control of growth between inhibition and stimulation was found to be related to the level of nitrate (NO₃^?) in the growth medium. At NO₃^? concentrations below 40 mM (full-strength MS salts level), root growth was stimulated by the addition of GABA to the growth medium; whereas at concentrations above 40 mM NO₃^?, the addition of GABA to the growth medium inhibited root elongation. GABA promoted NO₃^? uptake at low NO₃^?, while GABA inhibited NO₃^? uptake at high NO₃^?. Activities of several enzymes involved in nitrogen and carbon metabolism including nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (NADH-GOGAT), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and phosphoenol pyruvate carboxylase (PEPCase) were regulated by GABA in the growth medium. Supplementing 1/8 strength MS medium with 50 mM GABA enhanced the activities of all of the above enzymes except ICDH activities in root tissues. However, at full-strength MS, GABA showed no inhibitory effect on the activities of these enzymes, except on GS in both root and shoot tissues, and PEPCase activity in shoot tissues. Exogenous GABA increased the amount of NR protein rather than its activation status in the tissues. This study shows that GABA affects the growth of Arabidopsis, possibly by acting as a signaling molecule, modulating the activity of enzymes involved in primary nitrogen metabolism and nitrate uptake.     

Salicylic acid-induced salinity tolerance in corn grown under NaCl stress

Effects of salicylic acid on some physiological and biochemical characteristics of maize ( Zea mays L.) seedlings under NaCl stress were studied. Pre-soaking treatments of NaCl (0, 50, 100 and 200 mM) were given to maize seeds in the presence as well as in the absence of 0.5 mM salicylic acid. Two-week-old maize seedlings exhibited significant decrease in dry weight, root length, shoot length and leaf area on 6 h exposure of 100 and 200 mM NaCl stress. Photosynthetic pigments and NR activity in leaves decreased sharply with increasing stress levels. Both proline content and lipid peroxidation (measured in terms of MDA) levels increased significantly under saline conditions. However, seedlings pretreated with 0.5 mM salicylic acid along with the salinity levels showed enhancement in growth parameters, photosynthetic pigments, NR activity while, free proline and MDA levels decreased. The results showed that salt-induced deleterious effects in maize seedlings were significantly encountered by the pretreatment of salicylic acid. It is concluded that 0.5 mM salicylic acid improves the adaptabilities of maize plants to NaCl stress.     

Patterns of proteolytic enzyme activities in different tissues of germinating corn (Zea mays L.)

Profiles of pH dependence and activities of live proteolytic enzymes, amino- and carboxypeptidase and endopeptidases active at pH 3.8, 5.4 and 7.5, with casein as substrate, were determined in crude extracts from the various organs of corn seedlings during germination and early development (30°C, dark, 8 d). With respect to the endopeptidases, caseolytic activities at pH 3.8, 5.4 and 7.5 in extracts from endosperm increased concurrently with loss of endosperm N during germination; however, the relative amounts of the pH 7.5 activity were very small. In scutellum extracts, caseolytic activities at both pH 5.4 and 7.5 increased during the initial stages of development but only the increase at pH 5.4 was concurrent with loss of scutellar N. In shoot extracts, caseolytic activities at pH 5.4 and 7.5 were very low and remained relatively constant. There was a progressive increase in shoot N with time. In root extracts, caseolytic activities at pH 5.4 and 7.5 were higher (3-fold) than in shoot extracts. The activity at pH 5.4 remained constant while the activity at pH 7.5 increased during germination. The rate of accumulation of N by the root was low after day 5. The pattern and ratio but not the amounts of the pH 5.4 and 7.5 caseolytic activities of the root were similar to those observed in senescing leaves of field-grown corn. Addition of mercaptoethanol increased (several-fold) the caseolytic activities at pH 3.8 and 5.4, especially the latter, but not the pH 7.5 activity in endosperm extracts and increased the pH 5.4 activity in extracts from scutellum (30%) and roots (30%) while the effect in shoot extracts was negligible. Carboxypeptidase activity was relatively low in young tissue (root tip, 3-d-old shoots) and increased with development of the various organs except the roots (whole) where the activity remained relatively constant. The increases in carboxypeptidase activities were concurrent with decreases in N from endosperm and scutellum; this result indicates that this enzyme in these tissues may be involved (cooperatively with endopeptidases) in the mobilization of reserve protein.Of all the enzymes tested, only carboxypeptidase activity was markedly (in excess of 50%) inhibited by phenylmethylsulfonylfluoride. Only aminopeptidase activity was found in appreciable amounts in endosperm and scutellum of dry kernels. Aminopeptidase activity was highest in organs with high metabolic activity (scutella, shoot, root tips) and decreased in plant parts undergoing rapid loss of nitrogen (endosperm, senescing leaves).Abbreviations AP aminopeptidase - CA caseolytic activity - CP carboxypeptidase - ME mercaptoethanol     

Influences of some internal and external conditions on the induction of nitrate reductase in rice seedlings

Induction of nitrate reductase (NR) in 7-day-old rice seedlingswas depressed when endosperms were removed. NR activity in theseedlings from which endosperms were removed (deendospermedseedlings), reached a maximum 6 hr after supplying NaNO₃ andthen gradually decreased. That in intact seedlings continuedto increase for 12 hr, and then decreased fairly rapidly. Sucrose(30 mM concentration) supplied exogenously to deendospermedseedlings raised NR activity to the level of the intact seedlings.Macronutrients added exogenously did not show such an effect. NR activity in deendospermed seedlings placed in the dark wasextremely low. However, in the presence of exogenous sucrose,the activity was raised to the same level as that in the lightin the absence of exogenous sucrose. This suggests that sucrosesubstitutes for light in the induction of NR in deendospermedseedlings. Protein inhibitors suppressed NR induction when theplants were fed continuously with nitrate solution containingthe inhibitor. In cases where the plant roots were immersedin inhibitor solutions for 2 hr before transfer to nitrate solution,only chloramphenicol promoted and the others inhibited NR induction.NR induction was also suppressed by respiratory inhibitors,of which sodium azide was very potent. (Received August 25, 1976; )     

大麦氮敏感基因型苗期对氮饥饿的生理响应

以大田试验获得的大麦氮敏感基因型BI-45为材料,利用溶液培养方法,测定了苗期株高、根长、叶绿素含量、含氮量、谷氨酰胺合成酶和硝酸还原酶活性,以及与氮代谢相关的基因（GSI-GSl-2、GSI-3、GS2、Narl、NRT2．J、NRT2-2、NRT2-3和NRT2-4）的表达。结果表明：相对于正常供氮,氮饥饿胁迫下,BI-45根和叶中的氮素利用率提高,含氮量降低,叶绿素含量减少,根冠比增加;叶片中的谷氨酰胺合成酶活性和硝酸还原酶的活性高于根,但是,与叶中的相比,根中的谷氨酰胺合成酶活性升高及硝酸还原酶活性降低的差异性更显著;与正常供氮相比,氮饥饿处理下,根中基因傩家族,基Narl和硝酸盐转运蛋白基因NRT2家族的相对表达量皆达到显著性差异,其中GSl-I、GSl-2和NRT2-2在苗期大麦氮饥饿处理下表现尤为突出,并且在6h都有上调表达。     

Consumption of wheat seed reserves during germination and early growth as affected by soil water potential

Seed size and weight are important criteria for determining seedling vigour and stand establishment. Evolution of seed dry weight of wheat (Triticum aestivum L.) during germination and early growth was examined because poor stands are often associated with the depletion and exhaustion of seed reserves. Two laboratory experiments were conducted on filter paper and in soil at three water potentials using wheat seeds. Seed, root, and shoot dry weights were recorded at approximately one-day intervals. Coleoptile and first leaf lengths were also measured at all sampling periods. Wheat seedlings grown on filter paper in the dark grew to a length of 90 to 100 mm with 50% of the initial seed weight remaining after eight days when the experiment was terminated. In soil, wheat seedlings grew 15 mm with 25% of the initial seed weight remaining. Seed reserves were depleted more quickly when the soil was wet because seedlings grew more quickly. There were significant and similar negative relationships between seed weight and coleoptile length of wheat seedlings grown on filter paper and in soil. There was no effect of soil water potential on the relationship between seed weight and shoot length. Therefore, it was concluded that poor wheat stands are not likely to occur due to depletion of seed reserves under field conditions without mechanical obstacles.     

多裂骆驼蓬中生物碱类物质对小麦种子萌发和幼苗生长的影响

室内培养和盆栽试验结果表明,多裂骆驼蓬生物碱提取液浸种的小麦萌发种子呼吸速率、淀粉酶和蛋白酶活性受抑,种子萌发率下降并随处理浓度提高而降低。幼苗根系活性和叶绿素含量提高,叶片硝酸还原酶（NR）、超氧化物歧化酶（SOD）和过氧化物酶（POD）活性增强,过氧化氢酶（CAT）活性下降;幼苗根体积,株高、根系和地上部干重增加。多裂骆驼蓬生物碱提取液浸种对小麦种子萌发和幼苗生长作用的大小顺序为：总生物碱〉水溶性生物碱〉脂溶性生物碱。     

NaCl stress effects on enzymes involved in nitrogen assimilation pathway in tomato "Lycopersicon esculentum" seedlings

Tomato plants (Lycopersicon esculentum Mill, cv. Chibli F1) grown for 10 days on control medium were exposed to differing concentrations of NaCl (0, 25, 50, and 100mM). Increasing salinity led to a decrease of dry weight (DW) production and protein contents in the leaves and roots. Conversely, the root to shoot (R/S) DW ratio was increased by salinity. Na(+) and Cl(-) accumulation were correlated with a decline of K(+) and NO(3)(-) in the leaves and roots. Under salinity, the activities of nitrate reductase (NR, EC 1.6.6.1) and glutamine synthetase (GS, EC 6.3.1.2) were repressed in the leaves, while they were enhanced in the roots. Nitrite reductase (NiR, EC 1.7.7.1) activity was decreased in both the leaves and roots. Deaminating activity of glutamate dehydrogenase (GDH, EC 1.4.1.2) was inhibited, whereas the aminating function was significantly stimulated by salinity in the leaves and roots. At a high salt concentration, the nicotinamide adenine dinucleotide reduced (NADH)-GDH activity was stimulated concomitantly with the increasing NH(4)(+) contents and proteolysis activity in the leaves and roots. With respect to salt stress, the distinct sensitivity of the enzymes involved in nitrogen assimilation is discussed.     

Effect of Osmotic Stress on Ion Transport Processes and Phospholipid Composition of Wheat (Triticum aestivum L.) Mitochondria

The effect of osmotic stress on wheat (Triticum aestivum L.) mitochondrial activity and phospholipid composition was investigated. Preliminary growth measurements showed that osmotic stress (−0.25 or −0.5 megapascal external water potential) inhibited the rate of shoot dry matter accumulation while root dry matter accumulation was less sensitive. We have determined that differences in sensitivity to osmotic stress existed between tissues at the mitochondrial level. Mitochondria isolated from roots or shoots of stressed seedlings showed respiratory control and ADP/O ratios similar to control seedlings which indicates that stressed mitochondria were well coupled. However, under passive swelling conditions in a KCl reaction mixture, the rate and extent of valinomycin-induced swelling of shoot mitochondria were increased by osmotic stress while root mitochondria were largely unaffected. Active ion transport studies showed efflux transport by stressed-shoot mitochondria to be partially inhibited since mitochondrial contraction required the addition of N-ethylmaleimide or nigericin. Efflux ion transport by root mitochondria was not inhibited by osmotic stress which indicates that stress-induced changes in ion transport were largely limited to shoot mitochondria. Characterization of mitochondrial fatty acid and phospholipid composition showed an increase in the percentage of phosphatidylcholine in stressed shoot mitochondria compared to the control. Mitochondrial fatty acid composition was not markedly altered by stress. No significant changes in either the phospholipid or fatty acid composition of stressed root mitochondria were observed. Hence, these results suggest that a tissue-specific response to osmotic stress exists at the mitochondrial level.     

Inheritance of nitrite reductase and regulation of nitrate reductase, nitrite reductase, and glutamine synthetase isozymes

Banding patterns of nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS) from leaves of diploid barley (Hordeum vulgare), tetraploid wheat (Triticum durum), hexaploid wheat (Triticum aestivum), and tetraploid wild oats (Avena barbata) were compared following starch gel electrophoresis. Two NR isozymes, which appeared to be under different regulatory control, were observed in each of the three species. The activity of the more slowly migrating nitrate reductase isozyme (NR1) was induced by NO3- in green seedlings and cycloheximide inhibited induction. However, the activity of the faster NR isozyme (NR2) was unaffected by addition of KNO3, and it was not affected by treatments of cycloheximide or chloramphenicol. Only a single isozyme of nitrite reductase was detected in surveys of three tetraploid and 18 hexaploid wheat, and 48 barley accessions; however, three isozymes associated with different ecotypes were detected in the wild oats. Inheritance patterns showed that two of the wild oat isozymes were governed by a single Mendelian locus with two codominant alleles; however, no variation was detected for the third isozyme. Treatment of excised barely and wild oat seedlings with cycloheximide and chloramphenicol showed that induction of NiR activity was greatly inhibited by cycloheximide, but only slightly by chloramphenicol. Only a single GS isozyme was detected in extracts of green leaves of wheat, barley, and wild oat seedlings. No electrophoretic variation was observed within or among any of these three species. Thus, this enzyme appears to be the most structurally conserved of the three enzymes.     

缺氮和复氮对菘蓝幼苗生长及氮代谢的影响

对基质育苗后水培的菘蓝进行缺氮与复氮处理,分析其生长情况及氮代谢产物含量的变化,探讨缺氮和复氮对菘蓝幼苗生长及氮代谢的影响,以提高菘蓝产量和品质以及栽培过程中的氮素利用效率。结果显示:(1)正常供氮条件下,菘蓝幼苗的叶绿素含量、谷氨酰胺合成酶(GS)活性、硝态氮含量、靛玉红含量为最高,而其株高、主根直径、根的鲜重与干重、叶的鲜重与干重、根系活力均最小。(2)缺氮处理增加了菘蓝幼苗的主根直径和根干重,提高其根系活力和硝酸还原酶(NR)活性,促进游离氨基酸在叶中的积累;但降低了GS的活性,也降低了叶中硝态氮、可溶性蛋白、靛玉红及根中游离氨基酸的含量;缺氮对叶中靛蓝的含量无明显影响。(3)复氮处理增加了菘蓝幼苗的株高、主根长、根鲜重、叶鲜重、叶干重,提高了其根系活力,降低了NR和GS的活性;与对照相比,复氮降低了叶中硝态氮含量,提高了叶中可溶性蛋白、靛蓝及根中游离氨基酸的含量,但对叶中游离氨基酸和靛玉红含量影响较小。研究表明,缺氮后再复氮有利于菘蓝幼苗叶的生长,同时有利于增加其叶内靛蓝含量,从而提高其产量和品质。     

Modification of chromium (VI) phytotoxicity by exogenous gibberellic acid application in <Emphasis Type="Italic">Pisum sativum</Emphasis> (L.) seedlings

Effects of exogenous gibberellic acid (GA; 10 and 100 μM) application on growth, protein and nitrogen contents, ammonium (NH₄ ⁺) content, enzymes of nitrogen assimilation and antioxidant system in pea seedlings were investigated under chromium (VI) phytotoxicity (Cr VI; 50, 100 and 250 μM). Exposure of pea seedlings to Cr and 100 μM GA resulted in decreased seed germination, fresh and dry weight and length of root and shoot, and protein and nitrogen contents compared to control. Compared to control, Cr and 100 μM GA led to the significant alteration in nitrogen assimilation in pea. These treatments decreased root and shoot nitrate reductase (NR), glutamine synthetase (GS) and glutamine 2-oxoglutarate aminotransferase (GOGAT) activities (except 50 μM Cr alone for GOGAT) while glutamate dehydrogenase (GDH) activity and NH₄ ⁺ content increased. Compared to control, the root and shoot activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) increased (except APX activity at 250 μM Cr + 100 μM GA) while catalase (CAT), glutathione reductase (GR) and dehydroascorbate reductase (DHAR) activities were decreased (except GR at 100 μM GA alone) following exposure of Cr and 100 μM GA. Total ascorbate and total glutathione in root and shoot decreased by the treatments of Cr and 100 μM GA while their levels were increased by the application of 10 μM GA compared to Cr treatments alone. It has been reported that application of 10 μM GA together with Cr alleviated inhibited levels of growth, nitrogen assimilation and antioxidant system compared to Cr treatments alone. This study showed that application of 10 μM GA counteracts some of the adverse effects of Cr phytotoxicity with the increased levels of antioxidants and sustained activities of enzymes of nitrogen assimilation; however, 100 μM GA showed apparently reverse effect under Cr phytotoxicity.     

The Hydrolysis of Endosperm Protein in Zea mays

Degradation of the major storage proteins in maize endosperm, zein and glutelin, begins during the 2nd day of germination. The protein most abundant in the mature endosperm is degraded most rapidly. The patterns of protein loss are essentially similar in germinating seeds and excised endosperms. Cycloheximide, added at the beginning of the incubation period, prevents the development of α-amylase and protease activities and the disappearance of starch and protein reserves. Late additions (70 hours) of cycloheximide still inhibit the increase in hydrolase activity but have no effect on the hydrolysis of storage reserves. The results indicate that the hydrolytic enzymes are synthesized de novo in the maize endosperm.     

Elevated CO2 alters tissue balance of nitrogen metabolism and downregulates nitrogen assimilation and signalling gene expression in wheat seedlings receiving high nitrate supply

Tissue and canopy-level evidence suggests that elevated carbon dioxide (EC) inhibits shoot nitrate assimilation in plants and thereby affects nitrogen (N) and protein content of the economic produce. It is speculated that species or genotypes relying more on root nitrate assimilation can adapt better under EC due to the improved/steady supply of reductants required for nitrate assimilation. A study was conducted to examine the effect of EC on N assimilation and associated gene expression in wheat seedlings. Wheat genotypes, BT-Schomburgk (BTS) with comparatively high leaf nitrate reductase (NR) activity and Gluyas Early (GE) with high root NR activity were grown in hydroponic culture for 30 days with two different nitrate levels (0.05 mM and 5 mM) in the climate controlled growth chambers maintained at either ambient (400 ± 10 μmol mol⁻¹) or EC (700 ± 10 μmol mol⁻¹) conditions. Exposure to EC downregulated the activity of enzyme NR and glutamate synthase (GOGAT) in leaf tissues, whereas in roots, activities of both the enzymes were upregulated by exposure to EC. In addition, EC downregulated N assimilation and signalling gene expression under high N availability. Root N assimilation was less affected in comparison with shoot N assimilation; thereby, the proportion of root contribution towards total assimilation was higher. The results suggest that EC could alter and re-programme N assimilation and signalling in wheat seedlings. The genotype and tissue-specific effects of EC on N assimilation also warrants the need for identification of suitable genotypes and revision of fertiliser regime for tapping the beneficial effects of EC conditions.

     

Allometric relationships between seed mass and seedling characteristics reveal trade-offs for neotropical gap-dependent species

A seed size–seed number trade-off exists because smaller seeds are produced in greater number but have a lower probability of establishment. This reduced establishment success of smaller-seeded species may be determined by biophysical constraints imposed by scaling rules. Root and shoot diameter, root growth extension rate (R _GER) and shoot length at death for dark-grown seedlings are predicted to scale with the cube root of seed embryo and endosperm mass (m). We confirmed this expectation for ten neotropical gap-dependent tree species with an embryo and endosperm dry mass >1 mg. However, for nine smaller seeded species (m < 1 mg) with photoblastic germination, root and shoot diameters were larger than expected, and consequently, R _GER was slower than expected. The maximum shoot thrust of seedlings from seeds with masses ≥1 mg was comparable to the estimated force required to displace overlying litter, supporting the hypothesis that photoblastic behaviour only occurs in seeds with insufficient shoot thrust to displace overlying leaves. Using the model soil water, energy and transpiration to predict soil drying in small and large gaps, we showed that: (1) gaps that receive a significant amount of direct sunlight will dry more quickly than small gaps that do not, (2) compared to the wet-season, soil that is already dry at depth (i.e. the dry-season) will dry faster after rainfall (this drying would most likely kill seedlings from small seeds) and (3) even during the wet-season, dry periods of a few days in large gaps can kill shallow-rooted seedlings. We conclude that the smaller the seed, the more vulnerable its seedling would be to both covering by litter and soil drying because it can only emerge from shallow depths and has a slow R _GER. Consequently, we suggest that these allometrically related factors contribute to the reduced establishment success of smaller-seeded species that underpins the seed size–seed number trade-off.