Notas Científicas Stomatal conductance of maize under water and nitrogen deficits

The objective of this work was to evaluate the effect of drought and nitrogen (N) stresses on stomatal conductance of three maize cultivars grown in the field. The stomatal conductance of Sol da Manhã variety (BRS 4157) and Pioneer 6875 hybrid, under drought and high N, was lower than under drought and low N, which indicates drought tolerance, since these cultivars did not exhibit reduction in grain yield by drought, as observed for Amarelão variety, which flowered under more severe drought. 'Sol da Manhã' exhibited shorter anthesissilking interval under high N than under low N, an additional indication of tolerance.

Water and nitrogen (N) deficits are the most important limitations to crops production (Lea & Azevedo, 2006;Pimentel, 2006).However, most of the studies have focused on either water or N deficits separately, but only a few have addressed the complex interactions between these abiotic stresses (Bänziger et al., 2002;Moser et al., 2006).
The control of leaf stomatal conductance (g s ) is a crucial mechanism for plants, since it is essential for both CO 2 acquisition and desiccation prevention (Dodd, 2003).Studies with maize have shown that some drought tolerant genotypes reduce g s more on the onset of drought (Paterniani, 1990;Ray & Sinclair, 1997), whilst the effect of N deficit on g s includes several interactions, such as decreases (Dodd, 2003), increases or no effect (Cechin & Fumis, 2004).This study aimed to evaluate the importance of g s for tolerance to the combinations of water and N deficits in maize.
Three maize (Zea mays L.) cultivars were used: the hybrid P6875 from Pioneer, the variety Amarelão originated from the Brazilian Semiarid Region, and the variety Sol da Manhã (BRS 4157) improved for N use efficiency (Machado & Fernandes, 2001).The three cultivars were sown together on May 30 th , and the hybrid was sown again 10 days later, as an additional treatment, to allow the observation of effects of drought at the same stage for the three cultivars, and in different stages for the hybrid, which exhibited earlier flowering when compared to the other cultivars.The hybrid sown first is referred in the text as "P6875F", whereas the one sown later is referred as "P6875D".
The experiment was carried out at PESAGRO Experimental Station of Itaguaí, Rio de Janeiro, Brazil.The field used had several beds separated from each other by walls of bricks and cement, with 0.6 m depth, which allowed the placement of well-irrigated plots sideby-side with drought plots, as well as high and low N supply treatments.The plots consisted of one-row, 2.5 m long, with plants spaced 0.2 m within-row.
On July 1 st , the plots sowed first and treated with the low N level received 20 kg ha -1 N as urea.The plots treated with high N level received 50 kg ha -1 N as urea on July 1 st , and 50 kg ha -1 N on July 7 th .The plots of the second sowing date received the same N fertilizer distribution on July 9 th and 17 th .
Drought was imposed by withholding irrigation 64 days after sowing (DAS), on August 1 st , and plastic covers were used to cover the drought plots on rainy days.At 100 th DAS, the drought treatment was ended by resuming irrigation.The irrigated plots were maintained close to the field capacity using tensiometers placed at 20 cm depth in the soil.
Soil humidity (w/w) was measured weekly, during the drought period, and water potential (Ψ w ) was calculated by using previously performed pressure x humidity curves.In the soil depth from 15 to 20 cm, the soil humidity was 9.9, 8.5, 7.1, and 5.1% at 64, 71, 78 and 85 th DAS, respectively.In the soil depth from 20 to 40 cm, soil humidity was 5 and 4.8% at 85 and 100 th DAS, respectively, whereas in the soil depth from 40 to 60 cm, soil humidity was 4.7 at 100 th DAS.The Ψ w obtained in the soil depth from 15 to 20 cm were -0.19 and -0.6 MPa, at 64 and 71 th DAS, respectively.All other measured humidity was related to Ψ w values lower than -1.5 MPa.
Stomatal conductance was measured with the AP4 porometer (Delta T Company, England), in the ear leaf between 66 and 106 th DAS.Each g s interval measurement took around 1 hour and started at 9:00, 12:00 or 15:00 h.Photosynthetic photon flux density and air temperature means, at each g s interval measurement, were 874±456 µmol m -2 s -1 and 27.9±3.4o C, respectively.
Anthesis date was considered the number of days between sowing and 50% of plants exhibiting anther emergence, whereas anthesis-silking interval was the number of days between anthesis date and 50% of the plants exhibiting silk emergence.
The experimental design for grain yield, anthesis date and anthesis-silking interval traits was a factorial splitsplit plot with the water regime (well irrigated and drought) as the plots, in four blocks, the N levels as split of water regime, and maize cultivars as split of N levels.The experimental design for g s , in each day, was the same for the other traits.The three different interval measurements were considered as blocks, and only the days with significant differences were shown in Table 1.Variance analysis and the multiple comparisons (Bonferroni test) were carried out.
Plants under drought and high N supply exhibited significantly lower g s than under drought and low N supply (Table 1).This behaviour was shown mainly on 'Sol da Manhã' and indicates drought tolerance, since this maize variety also exhibited significantly lower values of anthesissilking interval under high N supply than under low N (Table 2), which is considered an important trait, when drought and N stresses are concerned (Bänziger et al., 2002).
(1) Values within a row followed by equal letters do not differ significantly (p<0.05),according to Bonferroni test.
Hybrid P6875F behaviour also indicates some level of tolerance to drought, due to N supply related to low g s since this hybrid, under drought and high N supply, exhibited low values of g s and early flowering, which could be considered as two different types of drought avoidance mechanisms.The N supply also shortened the time from emergence to tasseling in the work of Jones et al. (1986).
The hybrid P6875D began flowering when severe drought was established, but exhibited the same drought effect on anthesis-silking interval as P6875F, and such behaviour also indicates some degree of drought tolerance in this hybrid, supporting the findings of Medici et al. (2003).
The cultivar Amarelão exhibited some level of drought tolerance, since it was flowering when drought condition was more severe, and had a similar grain yield as P6875F under water and N deficits (Table 2).
The high N supply mitigated the drought effect, mainly under less severe situations, e.g., when plants flowered early during drought, as observed for P6875F.Drought effect alleviation by N application was also reported by Liu et al. (2004).The present work indicates that the maize genetic improvement for low g s and anthesis-silking interval, under N supply, could be an important approach for drought tolerance and water economy in agriculture.

Aknowledgements
To Embrapa Milho e Sorgo and Pioneer, for donation of seeds.