DE “LA GOTA DE LECHE” A LA CUNA MATERNAL “JOSEFINA PINILLOS DE LARCO”

In well-aerated agricultural soils, NO3-is the main form of N in the soil solution taken

up by crops (von Wiren et al. 1997). Because of its high mobility in the soil, movement to the roots and replenishing of the rhizosphere is mainly by mass flow regulated by plant transpiration, with diffusion making a smaller contribution (Miller and Cramer 2004). Uptake by the roots is mostly active via proton coupled symporters (2H+: 1NO3-) and has a relatively low energy demand calculated to represent about

10% of the total N nutritional costs (Miller and Cramer 2004; Raven 1985). At very low endogenous NO3-concentrations (<200 μM) uptake is by a saturable high affinity

transport system (HATS) in root hairs (von Wiren et al. 1997). At higher concentrations (>200 μM) uptake is by non-saturable and inducible low affinity transport system, also concentrated in root hairs (von Wiren et al. 1997). Because of the effectiveness of the HATS plants can achieve optimum growth at very low concentrations of NO3-in the soil solution. For example, 90% of optimum growth

could be achieved in some plant species at NO3-concentrations in the soil solution of

only 14 μM NO3--N (Clement et al. 1978). However, at low levels of N availability,

plants typically respond with increased assimilate partitioning to the roots. In kiwifruit the peak of root growth overlaps with that of fruit dry matter accumulation during Stage 2 of fruit development, which suggests that under conditions of low N availability fruit growth might be reduced. Although girdling during Stage 2 could counter this effect it is unclear what the effects of sustained low N availability (e.g., if N was omitted from annual fertiliser inputs) in combination with girdling would have on vine health.

Nitrate is chemically very reactive and its reduction and assimilation bears a high energy cost and may compete for energy resources with reduction of CO2in fruiting

plants or in low light situations, leading to reduced carbohydrate synthesis (Marschner 2002). Within the plant NO3-is considered to be the primary signal molecule involved

in N assimilation and can induce multiple gene responses in tissues within minutes of exposure (Crawford 1995). For example, in Arabidopsisshoots 183 genes were

identified as responding within 20 minutes of exposure to NO3-, while in tomato roots

over 1200 genes responded to NO3-exposure within 96 hours (Wang et al. 2001).

Nitrate uptake increases the synthesis of organic acids, decreases starch synthesis, changes plant hormone levels, and alters shoot:root allocation and root morphology (Stitt 1999). Nitrate also leads to wide ranging and rapid changes in enzyme

transcription involved in carbon and N metabolism (Stitt 1999). The capacity of NO3-

to increase plant water uptake has been noted in a wide range of species (McIntyre 1997; Cardenas-Navarro et al. 1999). This might involve the increased activation or induction of aquaporins by NO3-(Guo et al. 2007; Wang et al. 2001) and its role as an

osmoticum lowering the Ȍp (McIntyre 1997).

There are few published reports of NO3-levels in kiwifruit berries. In two commercial

orchards NO3-concentration on a dry weight basis in fruit ranged from 60 to 80 ppm

(Pickston et al. 1980). The levels of NO3-found were high compared to other fruit

included in the survey such as, citrus, grapes, and peaches, although no details of N fertiliser inputs to the studied orchards were included (Pickston et al. 1980). Walton and De Jong (1990) reported NO3-levels in ‘Hayward’ fruit of 140 ppm (dry weight)

52 days after flowering but by harvest no NO3-was detected. Nitrate accumulation in

other fruits increases with increasing N availability and NO3-uptake (Chairidchai

2000; Menary and Jones 1972; Hoff and Wilcox 1970), and it is therefore reasonable to expect this would also be the case in kiwifruit. Although in respect to human food safety, levels of NO3-are unlikely to accumulate to toxic levels in fruit, as they can in

some vegetables where concentrations >2500 ppm can be found (Blom-Zandstra 1989). Nevertheless, considering the activity of NO3-in initiating wide ranging

physiological responses and effects, even small amounts in developing fruit could have an effect on fruit quality.

Most plant tissues, including fruit, possess the capacity for NO3-reduction, although

NO3-in excess of the tissue’s reduction capacity can be stored in cell vacuoles, and

this might even be a preferred storage form due to the low energy cost involved (Schroeder 2006). Because of the limited mobility of NO3-in the phloem, the

persistence of detectable levels in kiwifruit at maturity could indicate that higher concentrations were present at earlier times during the fruit’s development

(e.g.,Walton and De Jong 1990). With the high transpiration rates of the fruit during early developmental stages, the supply of NO3-in the xylem flow would be increased

depending on its concentration in the xylem. The concentration of NO3-in the xylem

is increased with N fertilisation. Given the potential for NO3-to interact with vine and

fruit metabolism the effect of elevated NO3-uptake on fruit quality deserves further

study.

1.3.4.1 Nitrate reduction

Nitrate taken up by the plant can be stored in vacuoles of roots, shoots, and storage organs, or assimilated into organic compounds, in which case it must first be reduced to NH3 (Marschner 2002). Nitrate uptake, assimilation, and particularly NO3-

reduction (NR) are energy intensive processes. In barley 15% of the energy from root respiration is used for NR, compared to 5% for uptake and 3% for assimilation (Marschner 2002). In leaves, NR may alleviate the effects of excessive light (photo- inhibition and photo-oxidation). Although kiwifruit leaves require a high light level to be light-saturated and photosynthesis (Pn) is light limited for much of the season, photoinhibition could result from exposure of previously shaded leaves to high light (Buwalda and Smith 1990). However, NR also competes with the plant’s energy reserves with reduction of CO2in fruiting plants or low light situations so that

elevated NR can result in reduced carbohydrate synthesis.

Nitrate reduction can occur in roots or shoots depending on the level of NO3-supply,

and the age and species of the plant. At low levels of availability most reduction takes place in roots, but as supply increases, root capacity becomes saturated and NO3-is

translocated to shoots. However, subtropical and tropical perennials tend to reduce more in the shoots even at low external supply (Marschner 2002). Metge (1980) found the NO3-content of kiwifruit xylem sap ranged between 10 and 50% of the total N

(most reduction in shoots), kiwifruit was intermediate in the distribution of NR between roots and shoots.

For a given species, NR in roots increases with temperature and plant age (Marschner 2002). This suggests that NR in kiwifruit leaves may decrease during the summer relative to the spring and also decrease with older plants. Metge (1980) also found increased NR in roots with increasing air temperature but this effect was only found in one district of the two included in his investigation, and considered NR to occur mostly in the leaves of young kiwifruit plants based on the distribution of NR between different organs. The accompanying cation also influences the distribution of NO3-

reduction. For example, when K+is the accompanying cation, translocation of both K+ and NO3-to shoots is more rapid than if Ca+2or Na+is the cation (Marschner 2002).

Thus NR rates in leaves or N content within the vine generally is likely to be positively affected by high K fertiliser rates and availability.

Nitrogen fertilisation is likely to increase the ratio of NR shoot:root and this is confirmed by studies that show increased sap flux (Peuke 2000; Hubbard et al. 2004) and sap NO3-concentration in eucalyptus (Hubbard et al. 2004), broccoli (Belec et al.

2001), tomato (Anderson et al. 1999), and grapevine (Roubelakis-Angelakis and Kliewer 1979) in response to N fertilisation. No published research discusses the effect of increasing NO3-availability or N-fertilisation on kiwifruit xylem sap

composition or NR distribution or intensity. Measurements of the N composition of xylem sap in kiwifruit and NR have been done on vines receiving conservative N fertiliser rates compared to those sometimes found in New Zealand orchards (Metge 1980; Ferguson et al. 1983) (Table 1.3). Nevertheless, high NO3-concentrations and

NO3-:reduced-N ratio in xylem during late December (Ferguson et al. 1983; Metge

1980; Clark and Smith 1991), together with high transpiration rates at this time, supports the idea that NR could be at high levels during this critical period of fruit development (Beever and Hopkirk 1990). In the study by Metge (1980), NO3-content

of the xylem sap fell during the month between bud-break and leafing out, but then rose again coincident with canopy filling. This pattern, and that recorded later in the season, roughly coincides with the rainfall distribution and fertiliser applications for both sites used in the study. An increase in NO3-content of the xylem sap later in the

season is also consistent with increased root growth at this time (Buwalda and Hutton 1988).