Higher Leaf Elongation Rate Research Articles

Identification of Expansin Genes as Promoting or Repressing Factors for Leaf Elongation in Tall Fescue.

Expansins are cell-wall loosening proteins involved in plant cell expansion and elongation. Objectives of this study were to identify expansins related to leaf elongation in a perennial grass species and determine the relationship between the expression of expansin genes and leaf elongation. A total of 20 expansin genes were identified in tall fescue (Festuca arundinacea), out of which 9 genes belonged to the EXPA- and 11 to the EXPB subfamily. Two genotypes ('TF007' and 'TF116') with different growth rates were used to determine the correlation between expansins and leaf growth. Among the 20 expansins, 16 were differentially expressed in the leaf growth zone in 'TF007' and 'TF116'. The further analysis of gene expression in different leaf segments of 'TF007' and 'TF116' revealed that the expression level of FaEXPB16 was positively correlated with leaf elongation rate, and 'TF007' had a higher leaf elongation rate than 'TF116' due to the greater expression level of FaEXPB16. FaEXPA7 exhibited significantly higher expression level in leaves of the rapid-growing genotypes than the slow-growing genotypes, suggesting that FaEXPA7 acts as a positive regulator for leaf elongation. FaEXPA7 also exhibited its highest expression level in the cell division zone located in the leaf base. FaEXPB3, FaEXPB4-2, and FaEXPB11-2 showed a negative correlation with the leaf elongation rate in 'TF007' and 'TF116' and were highly expressed in leaves of the slow-growing genotypes. As promoting or repressing factors for leaf growth, these five expansins could be used as candidate genes in developing the rapid or slow-growing perennial grass species. This article is protected by copyright. All rights reserved.

Chloris gayana Kunth under different defoliation regimes. Morphogenesis, sward structure and leaf area index

AbstractSubtropical pastures are an important alternative to increase forage yields to fulfil cattle nutritional requirements. Despite the increasing expansion of these pastures in the semiarid subtropical region of Argentina, there is very little information about their responses to grazing management. The aim of this study was to evaluate the effect of different defoliation regimes on morphogenesis, sward structure and leaf area index of one of the most expanded forage species in this region, Chloris gayana Kunth. A combination of two defoliation frequencies (300 and 500 GDD) and two defoliation intensities (1 and 3 green stubble leaves) was compared by a controlled experiment that comprised 1,500 GDD. Defoliation frequency significantly affected leaf elongation rate (LER) and leaf area index (LAI). Under the high defoliation frequency, LER and LAI resulted almost half than under low defoliation frequency (0.34 ± 0.08 vs. 0.67 ± 0.08 mm·tiller−1·GDD−1; 8.31 ± 2.27 m2/m2 vs. 13.27 ± 1.59 m2/m2, at 300 or 500 GDD respectively), regardless of the intensity. Defoliation frequency or intensity did not affect leaf appearance rate, leaf lifespan, leaf size, number of green leaves per tiller nor tiller density at the end of the experiment. We conclude that to maintain high LER and LAI in Chloris gayana Kunth cv. Épica INTA‐Pemán pastures, defoliation frequency could be of 500 GDD. Since leaf lifespan was 415 ± 110 GDD, under this defoliation frequency, a maximum accumulation of green leaf tissues with very little dead tissues may be achieved.

Breeding perennial grasses for forage usage: An experimental assessment of trait changes in diploid perennial ryegrass ( Lolium perenne L.) cultivars released in the last four decades

Breeding to improve perennial grasses for forage usage aims to increase the whole-plant aerial biomass. However, the ability to increase the whole-plant yield by breeding may be questionable in grasses which are expected to reach an optimal leaf area index during grass stand growth rather rapidly. We carried out a field experiment aiming to compare herbage yield and other performances of seven natural populations and 21 cultivars of diploid perennial ryegrass registered on European National lists in the last 40 years. Cultivars were sampled in the spike emergence earliness range used for pasture usage, i.e. in the semi-late and late earliness range according to the French classification into spike emergence earliness groups. Forage performances of populations and cultivars were tested in four locations in monthly cut dense sward plots. Morphological and phenological traits were recorded in two spaced-plant experiments, and seed yield was assessed in two locations in standard seed production conditions. Trait regression on cultivar registration year pointed out that total dry-matter yield of cultivars increased by +3.2% per decade. However, dry-matter yield was not improved in the most favourable period for grass growth, i.e. in spring. On the other hand, it was improved in summer (+2.8% per decade) and autumn (+7.4% per decade) when the aerial biomass production is on average rather small. Breeders succeeded in significantly reducing aftermath heading. Reduction in aftermath heading is likely to have contributed to the noted increase in summer and autumn dry-matter yields, and to a moderate improvement in herbage feeding value. Breeding was also efficient in improving rust resistance and in bringing forward the start of spring growth. For monthly cut swards, we evidenced a clear association of leaf and lamina lengths with spring and summer dry-matter yields. Breeding for long leaves, or for high leaf elongation rate, should contribute to improve the cumulated intercepted radiation during re-growths, and consequently could be a way of improving the spring dry-matter yield. Seed yield did not improve with breeding for forage performances. However, we did not notice any negative association between the seed yield and traits related with forage usage. Direct selection pressures on seed yield criteria applied at the core of the breeding process should enable to improve the seed yield without any negative impact on forage performances.

Phenotypic and QTL analyses of herbage production-related traits in perennial ryegrass (Lolium perenne L.)

Phenotypic and genetic evaluation of morphological traits associated with herbage biomass production was undertaken in a perennial ryegrass (Lolium perenne L.) biparental F1 mapping population (n = 200) with parent plants from cultivars ‘Grasslands Impact’ and ‘Grasslands Samson’. Morphological traits measured on three clonal replicates of the parental genotypes and 200 F1 progeny in a glasshouse in two separate trials (autumn and spring) included: dry weight (DW), leaf elongation rate (LER), initial tiller number (TNs), final tiller number (TNe), site filling (Fs), tiller weight (TW), leaf lamina length, leaf tip and ligule appearance rates (ALf, ALg) and leaf elongation duration (LED). Principal component analysis of patterns of trait association identified negative correlation between TNs or TNe, and TW as the primary basis for morphological difference and indicated that either high LER or long LED could reduce TN. Plants with higher LER tended to have increased DW. Quantitative trait loci (QTL) were detected on all seven linkage groups (LG) of a perennial ryegrass linkage map for all but three traits. A total of 61 QTL were identified, many of which clustered at 15 shared genome locations. Significant genotype by environment effects were encountered, evidenced both by variation between experiments in genotype rankings and by a general lack of commonality for QTL for the same traits in the different experiments. Only five QTL, for ALf, ALg and TN, were conserved between autumn and spring trials. A QTL for TN and DW on LG6 is a strong candidate for application of MAS in future plant improvement work and was found to be co-linear with QTL for equivalent traits reported on chromosome 2 in rice.

Morphogenetic and structural characteristics of xaraes palisadegrass submitted to cutting heights

The objective of this work was to evaluate the morphogenetic and structural characteristics of xaraes palisadegrass submitted to three cutting heights (15, 20 and 25 cm) when the sward reached 95% of light interception during regrowth. The experiment was carried out according to a complete randomized block design with three replications. The sward surface height under the cutting condition of 95% of light interception was very homogeneous with values around 30 cm. The increase in cutting height reduced the leaf elongation rate and the number of live leaves but increased stem elongation rate. As the cutting criterion was the same, before the intense competition for light, the variation in the cutting severity had a small influence on the morphogenetic and structural characteristics of this grass. In the summer, due to the better growth conditions, the plants presented a higher tissue renewal, which was characterized by a higher leaf elongation rate, reduced leaf lifespan and leaf senescence rate when compared to the other season of the year. Under intermittent defoliation conditions, the regrowth of xaraes palisadegrass can be interrupted at a sward height of 30 cm (frequency) keeping a residual height of 15 cm.

Plant morphology differences in two perennial ryegrass cultivars

‘Grasslands Samson’ and ‘Grasslands Impact’ are cultivars of perennial ryegrass (Lolium perenne L.) widely used in New Zealand. This study investigated differences in vegetative morphology of these cultivars. Seeds were germinated, and 50 seedlings per cultivar assessed for 10 different morphological traits in a greenhouse in spring, up to the 10th leaf stage of growth. The cultivars differed most in mean leaf width, leaf‐sheath length, leaf appearance interval, and tiller number per plant. ‘Grasslands Samson’ had broader leaves, shorter leaf‐sheaths, a shorter leaf appearance interval, and fewer tillers per plant on average than ‘Grasslands Impact’, but with overlap between cultivars in all cases. Principal component (PC) analysis of morphological data yielded four PCs judged to contain biological information. PCs 1 and 2 together explained 63% of data variation and identified individual plants of either cultivar with greater leaf length arising from higher leaf elongation rate or leaf elongation duration, respectively. PCs 3 and 4 discriminated between cultivars. Future studies of this type would benefit from the inclusion of a tiller weight measurement not included here. PC analysis of multiple morphological traits appears to provide a useful tool for interpreting alternate forage grass growth strategies.

Growth of individual tillers and tillering rate of Lolium perenne and Bromus stamineus subjected to two defoliation frequencies in winter in Argentina

AbstractThe aims of the experiment were to (i) test whether the higher leaf elongation rate per tiller (LERT) of Bromus stamineus D. than Lolium perenne L. at moderately low temperatures was maintained at high defoliation frequencies and (ii) explore responses in tiller dynamics during the onset of the cool season in the south‐east of the humid Pampas region in Argentina. The following treatments were applied: defoliation frequency at the 3·0‐leaf stage (i.e. one‐leaf lifespan), which is considered optimal, and higher defoliation frequency at the 1·5‐leaf stage (i.e. half‐of‐a‐leaf lifespan). The higher defoliation frequency reduced leaf elongation rate in both the species but it did not affect the leaf appearance rate. This confirms previous studies on several C3 grasses, suggesting a similar pattern of response. Changes in tiller size are proposed as a possible mechanism to explain such long‐term defoliation effects on leaf elongation rates. Responses in tiller production depended on the species considered. The higher defoliation frequency caused a reduction in site‐filling which led to lower tiller production rates in L. perenne but not in B. stamineus. Thus, B. stamineus maintained the advantage in LERT over L. perenne and its tiller production was not affected when defoliated at frequencies higher than those considered optimal.

Características morfogênicas do capim-mombaça (Panicum maximum Jacq. cv. Mombaça) adubado com fontes de fósforo, sob pastejo

O experimento foi realizado no período de dezembro de 2002 a abril de 2003, com o objetivo de avaliar, em capim-mombaça, o efeito de duas fontes de fósforo (superfosfato simples + superfosfato triplo e termofosfato magnesiano YoorinÒ), em função dos dias (24) e da época de avaliação (verão e outono), sobre intervalo (IAPF) e taxas de aparecimento (TAPF), alongamento (TALF) e total de folhas senescentes (TFS), altura da primeira (HPLIG) e da última lígula (HULIG) e número de folhas expandidas (NFEX), em expansão (NFE), total de folhas (NTF) e total de folhas vivas (NTFV). O delineamento experimental utilizado foi em blocos ao acaso com três repetições. Entre as variáveis avaliadas, somente a taxa de alongamento foliar foi maior para os perfilhos adubados com o SFS+SFT. Diferenças foram encontradas para a época de avaliação, em que o verão apresentou maiores TALF, HPLIG e HULIG. Os dias de avaliação realizados nos períodos do verão e outono apresentaram efeito para HPLIG, NFEX e NFE, NTF, NTFV, TFS. Para a interação dias × época, somente para o NTFV e TFS houve correlações lineares positivas. As fontes de fósforo não influenciaram a morfogênese do capim-mombaça, exceto o alongamento foliar, no qual o tratamento SFS + SFT - fósforo solúvel promoveu maior aumento que Yoorin ®. No verão, as plantas apresentaram maior crescimento dos perfílhos e de sua taxa de alongamento foliar que no outono.

Are source and sink strengths genetically linked in maize plants subjected to water deficit? A QTL study of the responses of leaf growth and of Anthesis-Silking Interval to water deficit

Leaf growth and Anthesis-Silking Interval (ASI) are the main determinants of source and sink strengths of maize via their relations with light interception and yield, respectively. They depend on the abilities of leaves and silks to expand under fluctuating environmental conditions, so the possibility is raised that they may have a partly common genetic determinism. This possibility was tested in a mapping population which segregates for ASI. Maximum leaf elongation rate per unit thermal time (parameter a) and the slopes of its responses to evaporative demand and soil water status (parameters b and c) were measured in greenhouse and growth chamber experiments, in two series of 120 recombinant inbred lines (RILs) studied in 2004 and 2005 with 33 RILs in common both years. ASI was measured in three and five fields under well-watered conditions and water deficit, respectively. For each RIL, the maximum elongation rate per unit thermal time was reproducible over several experiments in well-watered plants. It was accounted for by five QTLs, among which three co-localized with QTLs of ASI of well-watered plants. The alleles conferring high leaf elongation rate conferred a low ASI (high silk elongation rate). The responses of leaf elongation rate to evaporative demand and to predawn leaf water potential were linear, allowing each RIL to be characterized by the slopes of these response curves. These slopes had three QTLs in common with ASI of plants under water deficit. The allele for leaf growth maintenance was, in all cases, that for shorter ASI (maintained silk elongation rate). By contrast, other regions influencing ASI had no influence on leaf growth. These results may have profound consequences for modelling the genotype x environment interaction and for designing drought-tolerant ideotypes.

Morfogênese de Lolium multiflorum Lam. e Paspalum urvillei steud sob níveis de adubação de fósforo e potássio

O estudo foi conduzido em área de várzea, com o objetivo de avaliar a resposta morfogênica de Lolium multiflorum e Paspalum urvillei, submetidos a níveis de adubação (50, 100 e 150% da recomendação) de fósforo (P) e potássio (K). Foram utilizados afilhos marcados para avaliar as taxas de alongamento e aparecimento de lâminas foliares e o filocrono. Os níveis de adubação afetaram (P<0,044) a taxa de alongamento de lâminas foliares de , no período de 03/08 a 17/08, com variação de 0,374 a 0,436mm GD-1 afilho-1, do menor ao maior nível de adubação. O intervalo para surgimento de folhas foi de 11,7 dias e o filocrono de 156 GD, na média dos tratamentos e períodos. Em P. urvillei, os níveis de adubação não afetaram a taxa de alongamento de lâminas foliares, sendo na média dos períodos e tratamentos de 0,375mm GD-1 afilho-1. Houve efeito dos níveis de adubação no aparecimento de folhas no período de 19/10 a 09/11, quando o nível de 100% da adubação, apresentou a maior taxa de alongamento de lâminas foliares, diferindo (P<0,003) dos demais. O filocrono foi maior (250 GD) no período de 04/01 a 24/01, nos demais (19/10 a 09/11 e 02/02 a 23/02) foi semelhante (166 e 175 GD, respectivamente). Os maiores níveis de adubação de P e K influenciam as taxas de alongamento em e de aparecimento de folhas em P. urvillei. Os valores de filocrono das espécies são semelhantes, demonstrando a potencialidade de crescimento e produção de forragem da espécie nativa principalmente.

Differences in Tillering of Long‐ and Short‐Leaved Perennial Ryegrass Genetic Lines under Full Light and Shade Treatments

There is uncertainty among plant breeders as to which characteristics to select for to optimize grass growth dynamics. The objective was to study the relationship between leaf length and tillering in perennial ryegrass (Lolium perenne L.). Two long‐leaved and two short‐leaved genetic lines were grown in simulated shade and near‐full sunlight (control) environments. The genetic lines were New Zealand cultivars Ellett (long‐leaved) and Grasslands Ruanui (short‐leaved), both early flowering. The other two were late‐flowering divergent selections ‘LL’ (long‐leaved) and ‘SL’ (short‐leaved). Differences between genetic lines in leaf length were attributable mainly to higher leaf elongation rate (LER) in the two long‐leaved genetic lines, and leaf elongation duration (LED) did not differ significantly between genetic lines. Grasslands Ruanui had a higher tiller number per plant than Ellett in both light environments, explained by higher site filling but similar leaf appearance rate (AL). In contrast, LL had a higher tiller number per plant than SL, arising from a higher AL in LL. This difference decreased during the experiment under the control treatment because SL tended to have a higher site filling ratio than LL. However, in the shade treatment, differences in tiller number between LL and SL were more mediated by AL than site filling. Therefore, selection for high LER and long lamina length, even though associated with reduced site filling in all treatments, did not necessarily result in reduced tiller number per plant.

Produção de matéria seca e fluxo de tecidos de Cynodon spp. cv. Tifton 85 em resposta a doses de nitrogênio

The objective of this study was to determine dry matter production (DMP) and tissue flow of Tifton 85 bermudagrass ( Cynodon spp .) under the effect of four doses of nitrogen (NO-O, N1-45, N2-90, N3-135 mg/kg soil) in the form of nitrate of ammonium in a randomized design, with five replications. Nitrogen increased DMP of leaves and stems (p 0.05). The DMP of leaves presented quadratic behavior in response to the nutrient up to the dose of 90 mg/kg. With regard to the tissue flow, the highest doses of N caused higher leaf elongation rate, higher leaf emergence rate and larger number of live leaves per tiller (p 0.05). For maximum dry matter yield of Tifton 85 bermudagrass, higher doses than the used in this experiment would be necessary, while for stems the maximum yield was reached with 133,5 mg N/kg soil. The 135 mg/kg N dose had greater influence on the tissue flow of the plant. The rate of leaf elongation and number of live leaves per tiller were better correlated with the dry matter yield. A low rate of senescence is of basic importance for the maintenance of a high number of live leaves per tiller. Tifton 85 bermudagrass crude protein provided higher values when 135 mg N/kg soil was used.

Control of Leaf Growth and its Role in Determining Variation in Plant Growth Rate from an Ecological Perspective

Abstract: Plants vary widely in their relative growth rate (RGR), be it dependent on environmental conditions or due to their genetic background. In a comparison of the RGR of grasses growing under different environmental conditions, variation in RGR tends to correlate with that in the leaf elongation rate (LER). When different species or genotypes thereof are compared under identical growing conditions, variation in LER may or may not correlate with that in RGR, depending on the comparison. However, since RGR is described by an exponential equation, whereas LER is mainly a linear process, we conclude that any correlation between RGR and LER must be fortuitous. That is, exponential growth must be due to increases with time in plant traits such as 1) leaf dry mass per unit leaf length invested per unit time, and/or 2)E̊, i.e., the total LER of all the growing leaves at one point in time. The latter can be achieved as follows: 1) each subsequent leaf has a higher LER than the preceding one; 2) leaves appear at an increasing rate; 3) the duration of the process of leaf elongation increases for subsequent leaves. In this review, we only explore possible factors that account for changes in E̊ with time, in different genotypes and under different environmental conditions. Inherent variation in LER of individual leaves and variation due to environmental factors may reflect variation in the rate of cell division and/or in cell elongation.

Role of Leaf Appearance Rate and the Coleoptile Tiller in Regulating Production

Forage production in tall fescue (Festuca arundinacea Schreb.) depends on elongation of existing leaves and production of new leaves and tillers. Genetic selection for high leaf elongation rate (LER), however, results in plants with limited tillering capacity. Our objective was to examine how the development of individual leaves and tillers affects total tiller production in populations of tall fescue selected for high or low LER. Seedlings were grown for 32 to 70 d in controlled environment chambers (500 μmol m−2 s−1 PPFD, 14‐h photoperiod). The low LER (high tillering) population produced twice as many coleoptile tillers as the high LER population, and coleoptile tillers and their progeny accounted for 80% of the difference between populations in tillers per plant after four tiller production cycles. With the exception of the coleoptile tiller, no difference in site usage (number of tillers appearing per axillary bud produced) was observed between populations. Site usage was greater than 0.80 in both populations during the first 30 to 35 days after planting (DAP), but decreased to ≈0.20 by 70 DAP. Both populations had similar leaf and tiller phyllochrons until 30 to 35 DAP. Thereafter, rate of leaf and tiller appearance slowed significantly, beginning first in the high LER population. Thus, tillering differences between populations were initially due to differences in coleoptile tiller production, but after 30 to 35 DAP differences in leaf appearance rate also contributed to differences in potential number of tillers per plant.

Diurnal Growth of Tall Fescue Leaf Blades

The spatial distributions of net deposition rates of water soluble carbohydrate-free dry matter (WSC-free DM) and WSC were evaluated within and above the elongation zone of tall fescue (Festuca arundinacea Schreb.) leaf blades during light and darkness. Imported DM used for WSC-free DM synthesis during darkness (67% of the total in experiment I and 59% in experiment II) was greater than during light (47% in both experiments), suggesting that the 65% higher leaf elongation rate during darkness was accompanied by higher rates of synthesis of cellular structural components. Deposition rates of WSC in the basal and central part of the elongation zone (0-20 mm from the ligule) were similar during light and darkness, but above 20 millimeters WSC deposition occurred during light and WSC loss occurred during darkness. WSC deposition and loss throughout the elongation zone and the recently expanded tissue were mostly due to net synthesis and degradation of fructan. Fructan was predominantly low molecular weight and contributed about 50% of the total osmotic partial pressure of WSC. In the most actively growing region, where fructan synthesis was most rapid, no diurnal change occurred in molecular weight distribution of fructan. WSC solute concentrations were diluted in the most actively growing tissue during darkness because net monosaccharide and fructan deposition were unaltered and sucrose deposition was decreased, but growth-associated water deposition was increased by 77%. Net rates of fructan synthesis and degradation were not related to tissue sucrose concentration, but appeared to respond to the balance between assimilate import and assimilate use in synthesis of cellular structural components (i.e. WSC-free DM) and deposition of monosaccharides. Fructan synthesized in tissue during most active elongation was degraded when the respective tissue reached the distal limit of the elongation zone where assimilate import in darkness was insufficient to maintain synthetic processes associated with further differentiation of cells.

Carbohydrate Metabolism in Leaf Meristems of Tall Fescue

Our objective was to examine alterations in carbohydrate status of leaf meristems that are associated with nitrogen-induced changes in leaf elongation rates of tall fescue (Festuca arundinacea Schreb.). Dark respiration rates, concentrations of nonstructural carbohydrates, and soluble proteins were measured in leaf intercalary meristems and adjacent segments of elongating leaves. The two genotypes used differed by 43% in leaf elongation rate. Application of high nitrogen (336 kilograms per hectare) resulted in 140% higher leaf elongation rate when compared to plants receiving low nitrogen (22 kilograms per hectare). Leaf meristems of plants receiving high and low nitrogen had dark respiration rates of 5.4 and 2.9 microliters O(2) consumed per milligram structural dry weight per hour, respectively. Concentrations of soluble proteins were lower while concentrations of fructan tended to be slightly higher in leaf meristems of low-nitrogen plants when compared to high-nitrogen plants. Concentrations of reducing sugars, nonreducing sugars, and takadiastase-soluble carbohydrate of leaf meristems were not affected by nitrogen treatment. Total nonstructural carbohydrates of leaf meristems averaged 44 and 39% of dry weight for low- and high-nitrogen plants, respectively. Within the leaf meristem, approximately 74 and 34% of the pool of total nonstructural carbohydrate could be consumed per day in high- and low-nitrogen plants, respectively, assuming no carbohydrate import to the meristem occurred. Plants were able to maintain high concentrations of nonstructural carbohydrates in leaf meristems despite a 3-fold range in leaf elongation rates, suggesting that carbohydrate synthesis and transport to leaf intercalary meristems may not limit leaf growth of these genotypes.

Diurnal Leaf Elongation of Contrasting Tall Fescue Genotypes1

Leaf elongation rate (LER) of tall rescue (Festuca arundinacea Schreb.) leaves is associated with tillering rate and forage yield. We performed two experiments. Objectives of Exp. I were to determine if differences in LER among genotypes were due to differential growth during the photoperiod and dark period, and of Exp. II to determine the relationship between leaf water potential and LER. Plants were grown in a controlled environment chamber with a 14‐hour photoperiod and a constant temperature of 21 or 23 C measured at the leaf intercalary meristem. Potentiometers were used to measure leaf elongation of genotypes selected for high yield per tiller (HYT, rapid LER) and low yield per tiller (LYT, slow LER). In Exp. II, stomatal diffusive resistance, leaf water potential, and LER of these genotypes were measured.Mean LER for the HYT genotype (0.9 mm hour‐1) throughout two 48‐hour periods was greater than that for the LYT genotype (0.6 mm hour‐1). Mean LER of both genotypes was significantly higher during the dark periods than during the photoperiods, averaging respectively 1.1 and 0.7 mm hour‐1 for the HYT genotype, and 0.7 and 0.5 mm hour‐1 for the LYT genotype. The 67% increase in LER during the dark period over that during the photoperiod for the HYT genotype was significantly greater than the 33% increase for the LYT genotype. Brief, but substantial increases and decreases in LER coincided with the time the lights went off and on. This response was associated closely with times of increases and decreases in stomatal diffusive resistance, and subsequent changes in leaf water potential. Leaf water potential was higher for the HYT than for the LYT genotype and was associated with LER of both genotypes. In Exp. I, LER averaged 1.3 and 0.8 mm hour‐1 for the HYT and LYT genotypes, respectively, during a 41‐hour dark period. These rates were similar to those maintained during the normal 10‐hour dark periods, and we concluded that daily photosynthate production was not needed to maintain high LER in the dark. Thus, the difference in LER between genotypes was due, in part, to a genotype by leaf water potential interaction which allowed the HYT genotype to more effectively utilize the dark period.